The severe fluctuations of the feeder current would, if the

motor were shunt or compound wound, cause most serious sparking

and various other troubles, but in a series motor where the back

ampere turns on the armature that react on the field vary in

precisely the same proportion as the ampere turns in the field,

there exists at all times a tendency to balance the active forces

and produce satisfactory operation. If, however, the field magnet

cores be very large, they cannot so quickly respond, magnetically,

to changes in the strength of the current, and there is then

greater liability of the armature reaction momentarily weakening

the field and thereby producing temporary sparking.

=Ques. Are motor generators always composed of direct current sets?=

Ans. No.

=Ques. Describe conditions requiring a different combination.=

Ans. For purposes where for instance direct currents of widely

different voltages are to be obtained from an alternating current

circuit, and it is desired to install but one set, a motor generator

consisting of an alternating current motor such as an induction

motor, and a dynamo must necessarily be employed.

In such sets, it is common to find both motor and dynamo

armatures mounted on a common shaft, and the respective field

frames resting on a single base, although for connection on a

very high pressure alternating current circuit, separate armature

shafts insulated from each other but directly connected together,

and separate bases resting on a single foundation, are usually

employed to afford the highest degree of insulation between the

respective circuits of the two machines.

=Ques. What is the objection to a set composed of alternating current

motor and alternator?=

Ans. The commercial field that would be naturally covered by such a

set is better supplied by a transformer.

=Ques. Why?=

Ans. Because a transformer contains no moving parts, and is therefore

simpler in construction, cheaper in price, and less liable to get out

of order.

=Dynamotors.=--A dynamotor differs from a motor generator in that

the motor armature and the generator armature are combined into one,

thereby requiring but one field frame. Since the motor and generator

armature windings are mounted on a single core, the armature reaction

due to the one winding is neutralized by the reaction caused by the

other winding. There is, consequently, little or no tendency for

sparking to occur at the brushes, and they therefore need not be

shifted on this account for different loads.

=Ques. How is a dynamotor usually constructed?=

Ans. It is usually built with two pole pieces which are shunt wound.

=Ques. Why does the voltage developed fall off slightly under an

increase of load?=

Ans. Because a compound winding cannot be provided.

[Illustration: FIGS. 2,862 and 2,863.--Method of putting on belts

when the driver is in motion, and device used. The latter is called a

_belt slipper_, and consists, as shown in fig. 2,862, of a cone and

shield, which revolve upon the stem, B, thus yielding easily to the

pull of the belt. A staff or handle C of any convenient length can be

fastened to the socket. The mode of operation is illustrated in fig.

2,863, which is self explanatory.]

=Ques. Describe the armature construction and operation.=

Ans. It consists of two separate windings; one of which is joined to

a commutator mounted on one side of the armature for motor purposes,

and the other to the commutator on the other side of the armature for

generator purposes.

By means of two studs of brushes pressing on the motor

commutator, current from the service wires is fed into the winding

connected to this commutator, and since the shunt field winding

is also excited by the current from the service wires, there is

developed in the generator winding on the rotating armature a

direct voltage which is proportional to the speed of rotation of

the armature in revolutions per second, the number of conductors

in series which constitute the generator winding, and the total

strength of the field in which the armature revolves. This

pressure causes current to pass through the generator winding and

the distributing circuit when the distributing circuit to which

this winding is connected by means of its respective commutator,

brushes, etc., is closed.

[Illustration: FIGS. 2,864 to 2,866.--Converter connections; fig.

2,864 double delta connection; fig. 2,865 diametrical connection;

fig. 2,866 two circuit single phase connection. For six phase

synchronous converter, two different arrangements of the connections

are generally used. One is called the _double delta_, and the other

the _diametrical_ connection. Let the armature winding of the

converter be represented by a circle as in figs. 2,864 and 2,865,

and let the six equidistant points on the circumference represent

collector rings, then the secondary of the supply transformers can

be connected to the collector rings in a _double delta_ as in fig.

2,864, or across diametrical pairs of pointer as in fig. 2,865.

In the first instance, the voltage ratio is the same as for the

three phase synchronous converter and simply consists of two delta

systems. The transformers can also be connected in double star, and

in such a case the ratio between the three phase voltage between

the terminals of each star, and the direct voltage will be the same

as for double delta, while the voltage of each transformer coil,

or voltage to neutral, is 1 ÷ √3 times as much. With the

diametrical connection, the ratio is the same as for the two ring

single phase converter, it being analogous to three such systems.

_ _

Hence six phase double delta E_{1} = √3E ÷ 2√2 = .612E.

Six phase diametrical, E_{1} = E ÷ √2 = .707E. The ratio of the

virtual_voltage E_{0} between any collector ring and the neutral

point is always E_{0} = (E ÷ 2) √2 = .354E. For single phase

synchronous converters, consisting of a closed circuit armature

winding tapped at two equidistant points to the two collector rings

the virtual voltage is 1 ÷ √2 × the direct current voltage. While

such an arrangement of the single phase converter is the simplest,

requiring only two collector rings, it is undesirable, especially

for larger machines, on account of excessive heating of the armature

conductors. In fig. 2,866, which represents the armature winding of a

single phase converter, the supply circuits from two secondaries of

the step down transformers are connected to four collector rings, so

that the two circuits are in phase with each other, but each spreads

over an arc of 120 electrical degrees instead of over 180 degrees

as in the single phase circuit converter. To distinguish the two

types, it is generally called a two circuit single phase synchronous

converter. The virtual voltage E_{2} bears to the direct voltage the

same relation as in the three phase converter, that is single phase

two circuit, E_{1} = √3 ÷ 2√2 =.612E.]

=Ques. How is a dynamotor started?=

Ans. It is connected at its motor end and started in the same manner

as any shunt wound motor on a constant pressure circuit.

=Ques. What precautions should be taken in starting a dynamotor?=

Ans. The necessary precautions are, to have the poles strongly

magnetized before passing current through the motor winding on the

armature; to increase gradually the current through this winding, and

not to close the generating circuit until normal conditions regarding

speed, etc., are established in the motor circuit.

=Ques. How is the current developed in the machine regulated?=

Ans. It can be regulated by the introduction of resistance in one or

the other of the armature circuits, or by a shifting of the brushes

around the commutator.

=Ques. Are dynamotors less efficient than motor generators of a

similar type?=

Ans. No, they are more efficient.

=Ques. Why?=

Ans. Because they have only one field circuit and at least one

bearing less than a motor generator.

A motor generator has at least three bearings, and occasionally,

four, where the set consists of two independent machines directly

connected together.

=Rotary Converters.=--An important modification of the dynamotor

is the rotary converter. This machine forms, as it were, a link

between alternating and direct current systems, being in general a

combination of an alternating current motor and a dynamo.

[Illustration: FIG. 2,867.--Skeleton diagram showing wiring of

alternator, exciter, transformer and converter. The cut also shows

switchboard and connections.]

It has practically become a fixture in all large electric railway

systems and in other installations where heavy direct currents of

constant pressure are required at a considerable distance from the

generating plant. In such cases a rotary converter is installed

in the sub-station, and being simpler in construction, higher in

efficiency, more economical of floor space, and lower in price than

a motor generator set consisting of an alternating current motor and

a dynamo which might be used in its place, it has almost entirely

superseded the latter machine for the class of work mentioned.

=Ques. What is the objection to the single phase rotary converter?=

Ans. It is not self-starting.

=Ques. What feature of operation is inherent in a rotary converter?=

Ans. A rotary converter is a "reversible machine."

That is to say, if it be supplied with direct current of the

proper voltage at its commutator end, it will run as a direct

current motor and deliver alternating current to the collector

rings. While this feature is sometimes taken advantage of in

starting the converter from rest, the machine is not often used

permanently in this way, its commercial application being usually

the conversion of alternating currents into direct currents.

=Ques. How does a rotary converter operate when driven by direct

current?=

Ans. The same as a direct current motor, its speed of rotation

depending upon the relation existing between the strength of the

field and the direct current voltage applied.

If the field be weak with respect to the armature magnetism

resulting from the applied voltage, the armature will rotate at a

high speed, increasing until the conductors on the armature cut

the lines of force in the field so as to develop a voltage which

will be equal to that applied.

Again, if the field be strong with respect to the armature

magnetism, resulting from the applied voltage, the armature will

rotate at a low speed. If, therefore, it be desired to operate the

converter in this manner and maintain an alternating current of

constant frequency, the speed of rotation must be kept constant by

supplying a constant voltage not only to the brushes pressing on

the commutator, but also to the terminals of the field winding.

[Illustration: FIG. 2,868.--General Electric synchronous converter

with series booster. This type of converter generally consists of

an alternator with revolving field mounted on the same shaft as the

converter armature. The armature of the alternator, or booster, as it

is usually called, is stationary and connected electrically in series

between the supply circuit and the collector rings of the synchronous

converter. The booster field has the same number of pole as the

converter and is generally shunt wound. A change in the booster

voltage will correspondingly change the alternating voltage impressed

on the converter and this regulation can, of course, be made so as

to either increase or decrease the impressed voltage by means of

strengthening or weakening the booster field. The voltage variation

can be made either non-automatic or automatic, and in the latter

case, it becomes necessary to provide a motor operated rheostat

controlled by suitable relays, or the booster can be provided with

a series field. By means of a booster, it is possible to vary the

direct voltage of the converter with a constant alternating supply

voltage, and this voltage regulation is obtained without disturbance

of the power factor or wave shape of the system. Synchronous

converters are frequently installed in connection with Edison

systems, where three wire direct current is required. The three wire

feature is obtained either by providing extra collector rings and

compensator, as with ordinary direct current generators, or also by

connecting the neutral wire directly to the neutral point of the

secondary winding of step down transformers, if such be furnished.]

=Ques. How does it operate with alternating current drive?=

Ans. The same as a synchronous motor.

=Ques. What is the most troublesome part and why?=

Ans. The commutator, because of the many pieces of which it is

composed and the necessary lines along which it is constructed, its

peripheral speed must be kept within reasonable limits.

=Ques. What should be the limit of the commutator speed?=

Ans. The commutator speed, or tangential speed at the brushes should

not exceed 3,000 feet per minute.

[Illustration: FIG. 2,869.--Wiring diagram for General Electric

synchronous converter with series booster as illustrated in fig.

2,868.]

=Ques. Name another limitation necessary for satisfactory operation.=

Ans. The pressure between adjacent commutator bars should not exceed

eight or ten volts.

If the commutator bars be made narrow in order to obtain

the necessary number for the desired voltage with the minimum

circumference and therefore low commutator speed, the brushes

employed to collect the current are liable to require excessive

width in order to provide the proper cross section and yet not

cover more than two bars at once.

=Ques. How can the commutator speed be kept within reasonable limits,

other than by reducing the width of the commutator bars?=

Ans. By using alternating current of comparatively low frequency.

For a rotary converter delivering 500 volt direct current, the

proper frequency for the alternating current circuit has been

found to be 25 cycles per second.

=Ques. When a rotary converter is operated in this usual manner on an

alternating current circuit, how can the direct current be varied?=

Ans. It may be varied (from zero to a maximum) by changing the value

of the alternating pressure supplied to the machine, or it may be

altered within a limited range by moving the brushes around the

commutator, or in a compound wound converter by changing the amount

of compounding.

Under ordinary conditions, varying the voltage developed by

changing the voltage at the motor end is not practical, hence

the voltage developed can be varied only over a limited range.

In addition to this, the voltage developed at the direct current

end bears always a certain constant proportion to the alternating

current voltage applied at the motor end; this is due to the same

winding being used both for motor and generator purposes. In all

cases the proportion is such that the alternating current voltage

is the lower, being in the single phase and in the two phase

converters about .707 of the direct current voltage, and in the

three phase converter about .612 of the direct current voltage.

It is thus seen that whatever value of direct current voltage be

desired, the value of the applied alternating current voltage must

be lower, requiring in consequence the installation of step down

transformers at the sub-station for reducing the line wire voltage

to conform to the direct current pressure required.

=Ques. What is the efficiency of a rotary converter?=

Ans. It may be said to have approximately the same efficiency as that

in the average of the same output, although in reality the converter

is a trifle more efficient on account of affording a somewhat shorter

average path for the current in the armature, reducing in consequence

the resistance loss and the armature reaction.

=Ques. May a converter be overloaded more than a dynamo of the same

output, and why?=

Ans. Yes, because there is usually less resistance loss in the

armature of the converter than in the armature of the dynamo.

[Illustration: FIG. 2,870.--Wiring diagram for three wire synchronous

converter with delta-Y connected step down transformer with the

neutral brought out. It is evident that in this case each transformer

secondary receives ⅓ of the neutral current, and if this current

be not so small, as compared with the exciting current of the

transformer, it will cause an increase in the magnetic density.]

Thus, a two phase converter may be overloaded approximately 60

per cent., and a three phase converter may be overloaded about 30

per cent. above their respective outputs if operated as dynamos.

=Ques. Describe how a converter is started.=

Ans. There are several methods any one of which may be employed, the

choice in any given case depending upon which of them may best be

followed under the existing conditions.

If it be found advisable to start the converter with direct

current, the same connections would be made between the source of

the direct current and the armature terminals on the commutator

side of the converter as would be the case were a direct current

shunt motor of considerable size to be started; this naturally

means that a starting rheostat and a circuit breaker will be

introduced in the armature circuit.

The shunt field winding alone is used, and this part of the

wiring may be made permanent if, as is usually the case, the same

source of direct current is used normally for separate field

excitation.

[Illustration: FIG. 2,871.--Wiring diagram of three wire synchronous

converter with distributed Y secondary. This system eliminates the

flux distortion due to the unbalanced direct current in the neutral.

Two separate interconnected windings are used for each leg of the Y.

The unbalanced neutral current flowing in this system may be compared

in action to the effect of a magnetizing current in a transformer.

The effect of the main transformer currents in the primary and

secondary is balanced with regard to the flux in the transformer

core, which depends upon the magnetic current. When a direct current

is passed through the transformer, unless the fluxes produced by

the same neutralize one another, its effect on the transformer iron

varies as the magnetizing current. For example, assume a transformer

having a normal ampere capacity of 100 and, approximately, 6 amperes

magnetizing current, and assume that three such transformers are

used with Y connected secondaries for operating a synchronous

converter connected to a three wire Edison system. Allowing 25 per

cent. unbalancing, the current will divide equally among the three

legs giving 8.33 amperes per leg, which is more than the normal

magnetizing current. The loss due to this current is, however,

inappreciable, but the increased core losses may be considerable.

If a distributed winding be used, the direct current flows in the

opposite direction, around the halves of each core thus entirely

neutralizing the flux distortion. Whether the straight Y connection

is to be used is merely a question of balancing the increased core

loss of the straight Y connection against the increased copper loss

and the greater cost of the interconnected Y system. The straight

Y connection is much simpler, and it would be quite permissible to

use it for transformers of small capacities where the direct current

circulating in the neutral is less than 30 per cent. of the rated

transformer current.]

The direct current may be derived from a storage battery, from a

separate converter, or from a motor generator set installed in the

sub-station for the purpose.

An adjustable rheostat will, of course, be connected in the

field circuit for regulation. Before starting the converter,

however, it is necessary to do certain wiring between the

terminals on the collector side of the machine and the alternating

current supply wires, in order that the change over from direct

current motive power to alternating current motive power may be

made when the proper phase relations are established between

the alternating current in the supply wires and the alternating

current in the armature winding of the converter.

In order that proper phase relations exist, the armature

of the converter must rotate at such a speed that each coil

thereon passes its proper reversal point at the same time as the

alternating current reverses in the supply wires. This speed may

be calculated by doubling the frequency of the supply current and

then dividing by the number of pole pieces on the converter, but a

far more accurate method of judging when the converter is in step

or in synchronism with the supply current consists in employing

incandescent lamps as shown in fig. 2,872.

=Ques. How is a polyphase converter started with alternating current?=

Ans. This may be done by applying the alternating pressure directly

to the collector rings while the armature is at rest. There need

be no field excitation; in fact the field windings on the separate

pole pieces should be disconnected from each other before the

alternating voltage is applied to the armature, else a high voltage

will be induced in the field windings which may prove injurious to

their insulation. The passage of the alternating current through the

armature winding produces a magnetic field that rotates about the

armature core, and induces in the pole pieces eddy currents, which,

reacting on the armature, exert a sufficient torque to start the

converter from rest and cause it to speed up to synchronism.

=Ques. How much alternating current is required to start a polyphase

converter?=

Ans. About 100 per cent. more than that required for full load.

=Ques. How may this starting current be reduced?=

Ans. Transformers may be switched into circuit temporarily to reduce

the line wire voltage until the speed become normal.

[Illustration: FIG. 2,872.--Wiring diagram showing arrangement of

incandescent lamps for determining the proper phase relations in

starting a rotary converter. The alternating current side of a

three phase converter is shown at C. The three brushes, D, T and G

pressing on its collector rings are joined in order to the three

single pole switches H, L and B which can be made to connect with

the respective wires M, R, and V, of the alternating current supply

circuit. Across one of the outside switches, H, for example, a number

of incandescent lamps are joined in series as indicated at E, while

the three pole switch (not shown) in the main circuit, between the

alternator and the single pole switches is open. If then the main

switch just mentioned and the middle switch L be both closed, and

the armature of the alternator be brought up to normal speed by

running it as a direct current motor, the lamps at E will light up

and darken in rapid succession; the lighting and darkening of the

lamps will continue until, by a proper adjustment of the speed,

the correct phase relations be established between the alternating

current in the supply circuit and the alternating current developed

in the armature of the converter. As this condition is approached,

the intervals between the successive lighting up and darkening of

the lamps will increase until they remain perfectly dark. There is

then no difference of pressure between the supply circuit M R V and

the rotary converter armature circuit, so the source of the direct

current may at that instant be disconnected from the machine, and the

switches H and B, closed. If the change over has been accomplished

before the phase relations of the two circuits differed, the

converter will at once conform itself to the supply circuit and run

thereon as a synchronous motor without further trouble. The opening

of the direct current circuit and the closing of the alternating

current supply circuit may be done by hand, but preferably by

employing a device that will automatically trip the circuit breaker

in the direct current circuit at the instant the switches in the

alternating current circuit are closed.]

In conjunction with this method, the method of synchronizing

shown in fig. 2,872 may be used, thus, in starting, there is

an alternating current between the brushes which pulsates very

rapidly, but when synchronism is approached, the pulsations

become less rapid until finally with the converter in step with

the alternator the pulsations entirely disappear.

The light given by the lamps thus connected indicates accurately

the condition of affairs at any one time, varying from a rapidly

fluctuating light at the beginning to one of constant brilliancy

at synchronism.

[Illustration: FIG. 2,873.--Diagram of motor converter. This machine

which is only to be used for converting from alternating to direct

current, consists of an ordinary induction motor with phase wound

armature, and a dynamo. The revolving parts of both machines are

mounted on the same shaft and from the figure it is seen that the

armature of the motor and the armature of the dynamo are also

electrically connected. The motor converter is a synchronous machine,

but the dynamo receives the current from the armature of the motor at

a frequency much reduced from that impressed upon the field winding

of the motor. Assuming that the motor and the converter have the same

number of pole, the motor will rotate at a speed corresponding to

one-half the frequency of the supply circuit. The motor will operate

half as a motor and half as a transformer, and the converter, half

as a dynamo and half as a synchronous converter, in that one-half of

the electrical energy supplied to the motor will be converted into

mechanical power for driving the converter, while the other one-half

is transferred to the secondary motor windings and thereby to the

converter armature in the form of electrical power. The capacity of

the motor is theoretically only half what it would be if it were to

convert the whole of the electrical energy into mechanical power

because the rating depends upon the speed of the rotating field and

not on that of the rotor. If the two machines have a different number

of pole, or are connected to run at different speeds, the division of

power is at a different but constant ratio. The machine starts up as

an ordinary polyphase induction motor and the field of the converter

is built up as though it were an ordinary dynamo. Motor converters

are occasionally used on high frequency systems, as their commutating

component is of half frequency, and thus permits better commutator

design than a high frequency converter. The advantage of this type

of machine is that for phase control it requires no extra reactive

coils, the motor itself having sufficient reactance. It is, however,

larger than standard converters, but smaller than motor generators,

as half the power is converted in each machine. Its efficiency is

less than for synchronous converters, and the danger of reaching

double speed in case of a short circuit on the direct current side

is very great. It has been used abroad to some extent for 60 cycle

work, in preference to synchronous converters, but with the present

reliable design of 60 cycle converters, and the general use of 25

cycles, where severe service conditions are met, as in railroading,

motor converters should not be recommended.]

=Ques. If the armature of the starting motor have a starting

resistance, how must this be connected?=

Ans. It should be connected in series with the armature inductors

before the alternating voltage is applied.

As the motor increases in speed, the starting resistance is

gradually short circuited until it is entirely cut out of circuit.

[Illustration: FIG. 2,874.--Sectional view of General Electric

vertical synchronous converter. In this construction, the field frame

carrying the poles is mounted on cast iron pedestals and is split

vertically. This allows the two halves of the frame to be separated

for inspection or repairs of the armature. The armature, including

commutator and collector rings, is mounted on a vertical stationary

shaft, which is rigidly supported from the foundation. The thrust

of the armature is carried on a roller bearing attached to the top

of the shaft and upper side of the armature spider. The under side

of the lower plate of the roller bearing is made spherical and fits

into a corresponding spherical cup on the end of the shaft, making

the bearing self aligning. The armature spider has a babbitted sleeve

along the fit of the vertical shaft, which acts as a guide bearing

and has to take only the thrust due to the unbalancing effect of

the rotating parts. A circulating pump furnishes oil to the roller

bearing, the oil draining off through the guide bearing. A marked

advantage of this type of construction is the accessibility of the

commutator for adjustment of the brushes, etc., as there is no pit or

pedestal bearing to interfere.]

NOTE.--Some converters are provided

with a small induction motor for starting

mounted on an iron bracket cast in the converter

frame, and whose shaft is keyed to that of the

converter. Allowing for a certain amount of

slip in the induction motor, the field of this

machine must possess a less number of magnet

poles than the converter in order to enable the

latter machine to be brought to full synchronism.

To start the induction motor, it is simply

necessary to apply to its field terminals the

proper alternating voltage. The bracket, and

therefore the motor, is usually mounted outside

the armature bearing on the collector side of the

converter.

[Illustration: FIG. 2,875.--Resistance measurement by "drop" method.

The circuit whose resistance is to be measured, is connected in

series with an ammeter and an adjustable resistance to vary the

flow of current. A voltmeter is connected directly across the

terminals of the resistance to be measured, as shown in the figure.

According to Ohm's law I = E ÷ R, from which, R = E ÷ I. If then the

current flowing in the circuit through the unknown resistance be

measured, and also the drop or difference of pressure, the resistance

can be calculated by above formula. In order to secure accurate

determination of the resistance such value of current must be used

as will give large deflections of the needle on the instruments

employed. A number of independent readings should be taken with some

variation of the current and necessarily a corresponding variation

in voltage. The resistance should then be figured from each set

of readings and the average of all readings taken for the correct

resistance. Great care must be taken, however, in the readings, and

the instruments must be fairly accurate. For example, suppose that

the combined instrument error and the error of the reading in the

voltmeter should be 1 per cent., the reading being high, while the

corresponding error of the ammeter is 1 per cent. low. This would

cause an error of approximately 2 per cent. in the reading of the

resistance. In making careful measurements of the resistance, it is

also necessary to determine the temperature of the resistance being

measured, as the resistance of copper increases approximately .4

of 1 per cent. for each degree rise in temperature. Use is made of

this fact for determining the increase in temperature of a piece of

apparatus when operating under load. The resistance of the apparatus

at some known temperature is measured, this being called the cold

resistance of the apparatus. At the end of the temperature test the

hot resistance is taken. Assume the resistance has increased by 15

per cent. This would indicate a rise in temperature of 37½ degrees

above the original or cold temperature of the apparatus. Suppose then

that in measuring the cold resistance, results are obtained which are

2 per cent. low, and that in measuring the hot resistance, there be

2 per cent. error in the opposite direction. This would mean that a

total error of 4 per cent. had been made in the difference between

the hot and cold resistances, or an error of 10 degrees. The correct

rise in temperature is, therefore, about 27½ instead of 37½ degrees.

In other words, an error of 2 per cent. in measuring each resistance

has caused an error of approximately 36½ per cent. in the measurement

of the rise in temperature. The constant .4 which has been used above

is only approximate and should not be used for exact work. For detail

instructions of making calculations of resistance and temperature,

see "Standardization Rules of the A.I.E.E."]

=Ques. Describe the usual wiring for the installation of a rotary

converter in a sub-station.=

Ans. Commencing at the entrance of the high pressure cables, first

there is the wiring for the lightning arresters, then for the

connection in circuit of the high tension switching devices, from

which the conductors are led to bus bars, and thence to the step down

transformers.

[Illustration: Figs. 2,876 to 2,879.--How to connect instruments

for power measurement. There are several ways of connecting an

ammeter, voltmeter and wattmeter in the circuit for the measurement

of power. A few of the methods are discussed below. With some of the

connections it is necessary to correct the readings of the wattmeter

for the losses in the coil, or coils, of the wattmeter, or for losses

in ammeter or voltmeter. This is necessary since the instruments may

be so connected that the wattmeter not only measures the load but

includes in its indications some of the instrument losses. If the

load measured be small, or considerable accuracy is required, these

instrument losses may be calculated as follows: Loss in pressure

coils is E^{2} ÷ R, in which E is the voltage at the terminals of

the pressure coil and R is the resistance. Loss in current coil is

I^{2} R in which I is the current flowing and R the resistance of the

current coil. In general let E_{v} = voltage across terminals of the

voltmeter; E_{_w_} = voltage across the terminals of the pressure

coil of the wattmeter; I_{_w_} = current through current coil of

wattmeter; I_{_a_} = current through current coil of ammeter; R_{_v_}

= resistance of pressure coil of voltmeter; R_{_w_} = resistance of

pressure coil of wattmeter; R^{1}_{w} = resistance of current coil

of wattmeter; R_{_a_} = resistance of current coil of ammeter. Then

the losses in the various coils will be as follows: E^{2}_{_v_} ÷

R_{_v_} = loss in pressure coil of voltmeter. E^{2}_{_w_} ÷ R_{_w_}

= loss in pressure coil of wattmeter. I^{2}_{_w_} ÷ R_{_v_} = loss

in current coil of wattmeter. I^{2}_{_a_}R_{_a_} = loss in current

coil of ammeter. If connection be made as in fig. 2,876, the correct

power of the circuit will be the wattmeter reading W-(E^{2}_{_v_} ÷

R_{_v_} + E^{2}_{_w_} ÷ R_{_w_}) in which E_{_v_} = E_{_w_}. In fig.

2,877, the power is W-E^{2}_{_w_} ÷ R_{_w_}. In fig. 2,878, the power

is W-I^{2}_{_w_}R^{1}_{_w_}, or the correct power is the wattmeter

reading minus the loss in the current coil of the wattmeter. In fig.

2,879, the power is W-(E^{2}_{_w_} ÷ R_{_w_} + I^{2}_{_a_}R_{_a_})·

The usual method of connection is either as in fig. 2,876 or fig.

2,877. In either case the current reading is that of the load plus

the currents in the pressure coils of the voltmeter and wattmeter.

Unless the current being measured, however, is very small, or extreme

accuracy is desired, it is unnecessary to correct ammeter readings.

In fig. 2,877 a small error is introduced due to the fact that the

actual voltage applied to the load is that given by the voltmeter

minus the small drop in voltage through the current coil of the

wattmeter. If an accurate measure of the current in connection with

the power consumed by the load be required, the connections shown

in fig. 2,879 are used, and if extreme accuracy is required, the

wattmeter reading is reduced by the losses in the ammeter and in

the pressure coil of the wattmeter. The loss in the pressure coil

of a wattmeter or voltmeter may be as high as 12 or 15 watts at 220

volts. The loss in the current coil of a wattmeter with 10 amperes

flowing may be 6 or 8 watts. It can be easily seen that if the core

or copper losses of small transformers are being measured, it is

quite necessary to correct the wattmeter readings, for the instrument

losses. In measuring the losses of a 25 or 50 H.P. induction motor,

the instrument losses may be neglected. A careful study of the above

will show when it becomes necessary to correct for instrument losses

and the method of making these corrections. Connections are seldom

used which make it necessary to correct for the losses in the current

coils of either ammeter or wattmeter, as the losses vary with the

change in the current. On the other hand, the voltages generally

used are fairly constant at 110 or 220, and when the losses of the

pressure coils at these voltages have once been calculated, the

necessary instrument correction can be readily made.]

On a three phase system the transformers should be joined in

delta connection, as a considerable advantage is thereby gained

over the star connection, in that should one of the transformers

become defective, the remaining two will carry the load without

change except more or less additional heating. Between the

transformers and rotary converter the circuits should be as short

and simple as possible, switches, fuses, and other instruments

being entirely excluded. The direct current from the converter is

led to the direct current switchboard, and from there distributed

to the feeder circuits.

=WATTMETER ERROR FOR A LOAD OF 1,000 VOLT-AMPERES=

(For a lag of 1 degree in the pressure coil)

+------------+----------+-------+-------------------+

| | | |Error of indication|

|Power factor|True watts| Error | in per cent |

| | | | of true value |

+------------+----------+-------+-------------------+

| 1. | 1,000 | .3 | 0.03 |

| .9 | 900 | 7.6 | 0.85 |

| .8 | 800 | 10.5 | 1.31 |

| .7 | 700 | 12.5 | 1.78 |

| .6 | 600 | 13.9 | 2.32 |

| .5 | 500 | 15.1 | 3.02 |

| .4 | 400 | 15.9 | 3.98 |

| .3 | 300 | 16.6 | 5.54 |

| .2 | 200 | 17.1 | 8.55 |

| .1 | 100 | 17.3 | 17.30 |

+------------+----------+-------+-------------------+

NOTE.--In the iron vane type instrument when

used as a wattmeter, the current of the series

coil always remains in perfect phase with

the current of the circuit, provided series

transformers are not introduced. The error,

then, is entirely due to the lag of the current

in the pressure coil, and this error in high

power factor is exceedingly small, increasing as

the power factor decreases. In the above table

it should be noted that the value of the error

as distinguished from the per cent. of error,

instead of indefinitely increasing as the power

factor diminishes, rapidly attains a maximum

value which is less than 2 per cent. of the power

delivered under the same current and without

inductance. It should also be noted that the

above tabulation is on the assumption of a lag of

1 degree in the pressure coil. The actual lag in

Wagner instruments for instance, is approximately

.085 of a degree, and the error due to the lag

of the pressure coil in Wagner instruments is,

therefore, proportionally reduced from the

figures shown in the above tabulation.

=Ques. In large sub-stations containing several rotary converters how

are they operated?=

Ans. Frequently they are installed to receive their respective

currents from the same set of bus bars; that is, they may be operated

as alternating current motors in parallel. They are also frequently

operated independently from single bus bars, but very seldom in

series with each other.

[Illustration: FIG. 2,880.--Single phase motor test. In this method

of measuring the input of a single phase motor of any type, the

ammeter, voltmeter and wattmeter are connected as shown in the

illustration. The ammeter measures the current flowing through the

motor, the voltmeter, the pressure across the terminals of the motor,

and the wattmeter the total power which flows through the motor

circuit. With the connections as shown, the wattmeter would also

measure the slight losses in the voltmeter and the pressure coil of

the wattmeter, but for motors of ¼ H.P. and larger, this loss is so

small that it may be neglected. The power factor may be calculated by

dividing the true watts as indicated by the wattmeter, by the product

of the volts and amperes.]

=Ques. How may the direct current circuit be connected?=

Ans. In parallel.

NOTE.--In motor testing, by the methods

illustrated in the accompanying cuts, it is

assumed that the motor is loaded in the ordinary

way by belting or direct connecting the motor

to some form of load, and that the object is to

determine whether the motor is over or under

loaded, and approximately what per cent. of full

load it is carrying. All commercial motors have

name plates, giving the rating of the motor and

the full load current in amperes. Hence the

per cent. of load carried can be determined

approximately by measuring the current input

and the voltage. If an efficiency test of the

apparatus be required, it becomes necessary to

use some form of absorption by dynamometer, such

as a Prony or other form of brake. The output of

the motor can then be determined from the brake

readings. The scope of the present treatment

is, however, too limited to go into the subject

of different methods of measuring the output of

the apparatus, and is confined rather to methods

of measuring current input, voltage, and watts.

The accuracy of all tests is obviously dependent

upon the accuracy of the instruments employed.

Before accepting the result obtained by any test,

especially under light or no load, correction

should be made for wattmeter error. See table of

wattmeter error on page 2,075.

[Illustration: FIG. 2,881.--Three phase motor test; voltmeter and

ammeter method. If it be desired to determine the approximate load

on a three phase motor, this may be done by means of the connections

as shown in the figure, and the current through one of the three

lines and the voltage across the phase measured. If the voltage be

approximately the rated voltage of the motor and the amperes the

rated current of the motor (as noted on the name plate) it may be

assumed that the motor is carrying approximately full load. If, on

the other hand, the amperes show much in excess of full load rating,

the motor is carrying an overload. The heat generated in the copper

varies as the square of the current. That generated in the iron

varies anywhere from the 1.6 power, to the square. This method is

very convenient if a wattmeter be not available, although, it is, of

course, of no value for the determination of the efficiency or power

factor of the apparatus. This method gives fairly accurate results,

providing the load on the three phases of the motor be fairly well

balanced. If there be much difference, however, in the voltage of

the three phases, the ammeter should be switched from one circuit to

another, and the current measured in each phase. If the motor be very

lightly loaded and the voltage of the different phases vary by 2 or 3

per cent., the current in the three legs of the circuit will vary 20

to 30 per cent.]

=Ques. What provision should be made against interruption of service

in sub-stations?=

Ans. There should be one reserve rotary converter to every three or

four converters actually required.

=Ques. Why does a rotary converter operate with greater efficiency,

and require less attention than does a dynamo of the same output?=

Ans. There is less friction, and less armature resistance, the

latter because the alternating current at certain portions of each

revolution passes directly to the commutator bars without traversing

the entire armature winding as it does in a dynamo; there is no

distortion of the field and consequently no sparking, or shifting of

the brushes, since the armature reaction resulting from the current

fed into the machine and that due to the current generated in the

armature completely neutralizes each other.

[Illustration: FIG. 2,882.--Three phase motor test by the two

wattmeter method. If an accurate test of a three phase motor be

required, it is necessary to use the method here indicated. Assume

the motor to be loaded with a brake so that its output can be

determined. This method gives correct results even with considerable

unbalancing in the voltages of the three phases. With the connections

as shown, the sum of the two wattmeter readings gives the total power

in the circuit. Neither meter by itself measures the power in any

one of the three phases. In fact, with light load one of the meters

will probably give a negative reading, and it will then be necessary

to either reverse its current or pressure leads in order that the

deflection may be noted. In such cases the algebraic sums of the two

readings must be taken. In, other words, if one read plus 500 watts

and the other, minus 300 watts, the total power in the circuit will

be 500 minus 300, or 200 watts. As the load comes on, the readings of

the instrument which gave the negative deflection will decrease until

the reading drops to zero, and it will then be necessary to again

reverse the pressure leads on this wattmeter. Thereafter the readings

of both instruments will be positive, and the numerical sum of the

two should be taken as the measurement of the load. If one set of the

instruments be removed from the circuit, the reading of the remaining

wattmeter will have no meaning. As stated above, it will not indicate

the power under these conditions in any one phase of the circuit. The

power factor is obtained by dividing the actual watts input by the

product of the average of the voltmeter readings × the average of the

ampere readings × 1.73.]

=What electrical difficulty is experienced with a rotary converter?=

Ans. Regulation of the direct current voltage.

=Ques. How is this done?=

Ans. It can be maintained constant only by preserving uniform

conditions of inductance in the alternating current circuit, and

uniform conditions in the alternator.

While changes in either of these may be compensated to a certain

extent by adjustment of the field strength of the converter, they

cannot be entirely neutralized in this manner; it is therefore

necessary that both the line circuit and the alternator be

given attention if the best results are to be obtained from the

converter.

=Ques. What mechanical difficulty is experienced with rotary

converters?=

Ans. Hunting.

=Ques. What is the cause of this?=

Ans. It is due to a variation in frequency.

The inertia of the converter armature tends to maintain a

constant speed; variations in the frequency of the supply circuit

will cause a displacement of phase between the current in the

armature and that in the line wires, which displacement, however,

the synchronizing current strives to decrease. The synchronizing

current, although beneficial in remedying the trouble after it

occurs, exerts but little effort in preventing it, and many

attempts have been made to devise a plan to eliminate this trouble.

NOTE.--Three phase motor test; polyphase

wattmeter method. This is identical with the test

of fig. 2,882, except that the wattmeter itself

combines the movement of the two wattmeters.

Otherwise the method of making the measurements

is identical. If the power factor be known to be

less than 50 per cent., connect one movement so

as to give a positive deflection; then disconnect

movement one and connect movement two so as to

give a positive deflection. Then reverse either

the pressure or current leads of the movement,

giving the smaller deflection, leaving the

remaining movement with the original connections.

The readings now obtained will be the correct

total watts delivered to the motor. If the power

factor be known to be over 50 per cent., the same

methods should be employed, except that both

movements should be independently connected to

give positive readings. An unloaded induction

motor has a power factor of less than 50 per

cent., and may, therefore, be used as above

for determining the correct connections. For a

better understanding of the reasons for the above

method of procedure, the explanation of the two

wattmeter method, fig. 2,882, should be read. The

power factor may be calculated as explained under

fig. 2,882. Connect as shown in fig 2,882. The

following check on connection may be made. Let

the polyphase induction motor run idle, that is,

with no load. The motor will then operate with a

power factor less than 50 per cent. The polyphase

meter should give a positive indication, but

if each movement be tried separately one will

be found to give a negative reading, the other

movement will give a positive reading. This can

be done by disconnecting one of the pressure

leads from the binding post of one movement. When

the power factor is above 50 per cent. then both

movements will give positive deflection.

=Ques. What are the methods employed to prevent hunting?=

Ans. 1, the employment of a strongly magnetized field relative to

that developed by the armature; 2, a heavy flywheel effect in the

converter; 3, the increasing of the inductance of the armature by

sinking the windings thereon in deep slots in the core, the slots

being provided with extended heads; and 4, the employment of damping

devices or amortisseur winding on the pole pieces of the converter.

[Illustration: FIG. 2,883.--Three phase motor test; one wattmeter

method. This method is equivalent to the two wattmeter method with

the following difference. A single voltmeter (as shown above) with

a switch, A, can be used to connect the voltmeter across either one

of the two phases. Three switches, B, C and D, are employed for

changing the connection of the ammeter and wattmeter in either one

of the two lines. With the switches B and D in the position shown,

the ammeter and wattmeter series coils are connected in the left hand

line. The switch C must be closed under these conditions in order to

have the middle line closed. Another reading should then be taken

before any change of load has occurred, with switch A thrown to the

right, switch B closed, switch D thrown to the right and switch C

opened. The ammeter and the current coil of the wattmeter will then

be connected to the middle line of the motor. In order to prevent

any interruption of the circuit, the switches B, D and C should be

operated in the order given above. With very light load on the motor

the wattmeter will probably give a negative deflection in one phase

or the other, and it will be necessary to reverse its connections

before taking the readings. For this purpose a double pole, double

throw switch is sometimes inserted in the circuit of the pressure

coil of the wattmeter so that the indications can be reversed

without disturbing any of the connections. It is suggested, before

undertaking this test, that the instructions for test by the two

wattmeter and by the polyphase wattmeter methods be read.]

=Ques. What method is the best?=

Ans. The damping method.

The devices employed for the purpose are usually copper shields

placed between or around the pole pieces, although in some

converters the copper is embedded in the poles, and in others it

is made simply to surround a portion of the pole tips.

In any case its action is as follows: The armature rotating at a

variable speed has a field developed therein which is assumed to

be also rotating at a variable speed; the magnetism of this rotary

field induces currents in the copper which, however, react on the

armature and oppose any tendency toward a further shifting of the

magnetism in the armature and therefore prevent the development

of additional currents in the copper. Since copper is of low

resistance, the induced currents are sufficient in strength to

thus dampen any tendency toward phase displacement, and so exert a

steadying influence upon the installation as a whole.

[Illustration: FIG. 2,884.--Three phase motor, one wattmeter and Y

box method. This method is of service, only, provided the voltages of

the three phases are the same. A slight variation of the voltage of

the different phases may cause a very large error in the readings of

the wattmeter, and inasmuch as the voltage of all commercial three

phase circuits is more or less unbalanced, this method is not to be

recommended for motor testing. With balanced voltage in all three

phases, the power is that indicated by the wattmeter, multiplied by

three. Power factor may be calculated as before.]

=Electrical Measuring Instruments.=--In the manufacture of most

measuring instruments, the graduations of the scale are made at the

factory, by comparing the deflections of the pointer with voltages

as measured on standard apparatus. The voltmeters in most common

use have capacities of 5, 15, 75, 150, 300, 500 and 750 volts each,

although in the measurement of very low resistances such as those of

armatures, heavy cables, or bus bars, voltmeters having capacities as

low as .02 volt are employed.

[Illustration: FIG. 2,885.--Test of three phase motor with neutral

brought out; single wattmeter method. Some star connected motors

have the connection brought out from the neutral of the winding. In

this case the circuit may be connected, as here shown. The voltmeter

now measures voltage between the neutral and one of the lines, and

the wattmeter the power in one of the three phases of the motor.

Therefore, the total power taken by the motor will be three times the

wattmeter readings. By this method, just as accurate results can be

obtained as with the two wattmeter method. The power factor will be

the indicated watts divided by the product of the indicated amperes

and volts.]

The difference between the design of direct current voltmeters of

different capacities lies simply in the high resistance joined in

series with the fine wire coil. This resistance is usually about 100

ohms per volt capacity of the meter, and is composed of fine silk

covered copper wire wound non-inductively on a wooden spool.

In the operation of an instrument, if the pointer when deflected do

not readily come to a position of rest owing to friction in the

moving parts, it may be aided in this respect by gently tapping

the case of the instrument with the hand; this will often enable

the obstruction, if not of a serious nature, to be overcome and an

accurate reading to be obtained.

[Illustration: FIG. 2,886.--Temperature test of a large three

phase induction motor. Temperature tests are usually made on small

induction motors by belting the motor to a generator and loading the

generator with a lamp bank or resistance until the motor input is

equal to the full load. If, however, the motor be of considerable

size, such that the cost of power becomes a considerable item in

the cost of testing, the method here shown may be employed. For

this purpose, however, two motors, preferably of the same size and

type, are required. One is driven as a motor and runs slightly below

synchronism, due to its slip when operating with load. This motor is

belted to a second machine. If the pulley of the second machine be

smaller than the pulley of the first machine, the second machine will

then operate as an induction generator, and will return to the line

as much power as the first motor draws from the line, less the losses

of the second machine. By properly selecting the ratio of pulleys,

the first machine can be caused to draw full load current and full

load energy from the line. In this way, the total energy consumed

is equivalent to the total of the losses of both machines, which is

approximately twice the losses of a single machine. The figure shows

the connection of the wattmeters, without necessary switches, for

reading the total energy by two wattmeter method. Detailed connection

of the wattmeter is shown in fig. 2,883. It is usual, in making

temperature tests, to insert one or more thermometers in what is

supposed to be the hottest part of the winding, one on the surface

of the laminae and one in the air duct between the iron laminae. The

test should be continued until the difference in temperature between

any part of the motor and the air reaches a steady value. The motor

should then be stopped and the temperature of the armature also

measured. For the method of testing wound armature type induction

motors of very large size, see fig. 2,890. For the approved way

of taking temperature readings and interpreting results, see the

"Standardization Rules of the A.I.E.E."]

=Ques. Describe a two scale voltmeter.=

Ans. In this type of instrument, one scale is for low voltage

readings and the other for high voltage readings; on these scales

the values of the graduations for low voltages are usually marked

with red figures, while those for high voltages are marked with

black figures. A voltmeter carrying two scales must also contain two

resistances in place of one; a terminal from each of these coils must

be connected with a separate binding post, but the remaining terminal

of each resistance is joined to a wire which connects through the

fine wire coil with the third binding post of the meter. The two

first mentioned binding posts are usually mounted at the left hand

side of the meter and the last mentioned binding post and key at the

right hand side.

[Illustration: FIG. 2,887.--Alternator excitation or magnetization

curve test. The object of this test is to determine the change of

the armature voltage due to the variation of the field current when

the external circuit is kept open. As here shown, the field circuit

is connected with an ammeter and an adjustable resistance in series

with a direct current source of supply. The adjustable resistance is

varied, and readings of the voltmeter across the armature, and of

the ammeter, are recorded. The speed of the generator must be kept

constant, preferably at the speed which is given on the name plate.

The excitation or magnetization curve of the machine is obtained by

plotting the current and the voltage.]

The resistance corresponding to the high reading scale is

composed of copper wire having the same diameter as that

constituting the resistance for the low reading scale, but as the

capacity of the former scale is generally a whole number of times

greater than that of the latter scale, the resistances for the two

must bear the same proportion.

[Illustration: FIG. 2,888.--Three phase alternator synchronous

impedance test. In determining the regulation of an alternator, it

is necessary to obtain what is called the _synchronous impedance_ of

the machine. To obtain this, the field is connected, as shown above.

Voltmeters are removed and the armature short circuited with the

ammeters in circuit. The field current is then varied, the armature

driven at synchronous speed, and the armature current measured by

the ammeters in circuit. The relation between field and armature

amperes are then plotted. The combination of the results of this

test, with those obtained from the test shown in fig. 2,887, are used

in the determination of the regulation of an alternator. Engineers

differ widely in the application of the above to the determination

of regulation, and employ many empirical formulae and constants for

different lines of design.]

=Ques. How is a two scale voltmeter connected?=

Ans. In the connection of a two scale voltmeter in circuit, the

single binding post is always employed regardless of which scale is

desired. If, then, the voltage be such that it may be measured on the

low reading scale, the other binding post employed is that connected

to the lower of the two resistances contained within; if, however,

the pressure be higher than those recorded on the low reading scale,

the binding post connected to the higher of the two resistances

contained within is used.

NOTE.--Three phase alternator load test. By

means of the connection shown in fig. 2,888,

readings of armature current and field amperes

can be obtained with any desired load. The field

current can be varied also so as to maintain

constant armature voltage irrespective of load;

or the field current may be kept constant and

the armature voltage allowed to vary as the load

increases. The connections may also be used to

make a temperature test on the alternator by

loading it with an artificial load. In some cases

after the alternator is installed the connection

may be used to make a temperature test, using

the actual commercial load the alternator is

furnishing.

Inasmuch as the capacities of the scales are

usually marked on or near the corresponding

binding posts, there will generally be no

difficulty in selecting the proper one of the two

left hand binding posts.

[Illustration: FIG. 2,889.--Three phase alternator or synchronous

motor temperature test. In this test two alternators or synchronous

motors of same size and type are used, and are belted together, one

to be driven as a synchronous motor and the other as an alternator.

The method employed is to synchronize the synchronous motor with

the alternator or alternators on the three phase circuit, and then

connect to the line by means of a three pole single throw switch.

The alternator is then similarly synchronized with the alternator

of the three phase circuit and thrown onto the line. By varying the

field of the alternator it can be made to carry approximately full

load, and the motor will then be also approximately fully loaded.

The usual method is to have the motor carry slightly in excess of

full load, and the alternator slightly less than full load. Under

these conditions the motor will run a little warmer than it should

with normal load, while the alternator will run slightly cooler.

Temperature measurements are made in the same way as discussed

under three phase motors. The necessary ammeters, voltmeters and

wattmeters for adjusting the loads on the motors and generator are

shown in above figure. If pulleys be of sufficient size to transmit

the full load, with, say one per cent. slip, the pulley on the motor

should be one per cent. larger in diameter than the pulley on the

alternator, so as to enable the alternator to remain in synchronism

and at the same time deliver power to the circuit. With very large

machines under test, it is inadvisable to use the above method as

it is sometimes difficult to so adjust the pulleys and belt tension

that the belt slip will be just right to make up for the difference

in diameter of the pulleys, and very violent flapping of the belt

results. To meet such cases, various other methods have been devised.

One which gives consistent results is shown in fig. 2,890.]

=Ques. How is a two scale voltmeter connected when the binding posts

are not marked?=

Ans. If only an approximate idea is possessed of the voltage to be

measured, it is always advisable to connect to the binding post

corresponding to the high reading scale of the meter in order

to determine if the measurement may not be made safely and more

accurately on the low reading scale. In any case, some knowledge must

be had of the voltage at hand, else the high reading portion of the

instrument may be endangered.

[Illustration: FIG. 2,890.--Three phase alternator or synchronous

motor temperature test. Supply the field with normal field current.

The armature is connected in open delta as illustrated, and full load

current sent through it from an external source of direct current,

care being taken to ground one terminal of the dynamo so as to avoid

danger of shock due to the voltage on the armature winding. The field

is then driven at synchronous speed. If the armature be designed to

be connected star for 2,300 volts, the voltage generated in each leg

of the delta will be 1,330 volts, and unless one leg of the dynamo

were grounded, the tester might receive a severe shock by coming

in contact with the direct current circuit. The insulation of the

dynamo would also be subjected to abnormal strain unless one terminal

were grounded. By the above method the field is subjected to its

full copper loss and the armature to full copper loss and core loss.

Temperature readings are taken as per standardization rules of the

A.I.E.E. This method may also be used with satisfactory results on

large three phase motors of the wound rotor type. If the alternator

pressure be above 600 volts, a pressure transformer should be used in

connection with the voltmeter.]

_Too much care cannot be taken to observe these precautions_

whenever the voltmeter is used, for the burning out or charring

of the insulation either in the fine wire coil or in the high

resistance of the meter by an excessive current, is one of the

most serious accidents that can befall the instrument.

If a voltmeter has been subjected to a voltage higher than that

for which it was designed, yet not sufficiently high to injure the

insulation, but high enough to cause the pointer to pass rapidly

over the entire scale, damage has been done in another way. The

pointer being forced against the side of the case in this manner,

bends it more or less and so introduces an error in the readings

that are afterward taken.

The same damage will be done if the meter be connected in

circuit so the current does not pass through it in the proper

direction, although in this case the pointer is not liable to

be bent so much as when it is forced to the opposite side of

the meter by an abnormal current, since then it has gained

considerable momentum which causes a severer impact. The extent

of the damage may be ascertained by noting how far away from the

zero mark the pointer lies when no current is passing through

the instrument. If this distance be more than two-tenths of a

division, the metal case enclosing the working part should be

removed and the pointer straightened by the careful use of a pair

of pinchers.

[Illustration: FIG. 2,891.--Direct motor or dynamo magnetization

test. The object of this test is to determine the variation of

armature voltage without load, with the current flowing through the

field circuit. The armature should be driven at normal speed. The

adjustment resistance in the field circuit is varied and the voltage

across the armature measured. The curve obtained by plotting these

two figures is usually called magnetization curve of the dynamo. It

is usual to start with the higher resistance in the field circuit so

that very small current flows, gradually increasing this current by

cutting out the field resistance. When the highest no load voltage

required is reached, the field current is then diminished, and what

is called the descending (as opposed to the ascending) magnetization

curves are obtained. The difference in the two curves is due to the

lag of the magnetization behind the magnetizing current, and is

caused by the hysteresis of the iron of the armature core.]

=Ques. What should be noted with respect to location of instruments?=

Ans. If they be placed near conductors carrying large currents,

the magnetic field developed thereby will produce a change in the

magnetism of the instruments and so introduce an error in the

readings.

[Illustration: FIG. 2,892.--Shunt dynamo external characteristic

test. The external characteristic of a shunt dynamo is a curve

showing the relation between the current and voltage of the external

circuit. This is obtained by the connection as here shown. The shunt

field is so adjusted that the machine gives normal voltage when

the external circuit is open. The field current is then maintained

constant and the external current varied by varying the resistance

in the circuit. By plotting voltage along the vertical, against the

corresponding amperes represented along the horizontal, the external

characteristic is obtained.]

=Ques. How should portable instruments be wired?=

Ans. The wires must be firmly secured to the supports on which they

rest, so as to reduce the possibility of their being pulled by

accident, and so causing the instruments to fall.

A fall or a rough handling of the meter at once shows its effect

on the readings, for as much harm is done as would result from a

similar treatment of a watch.

The hardened steel pivots used in all high grade voltmeters are

ground and polished with extreme care so as to secure and maintain

a high degree of sensitiveness. The jewels on which the moving

parts revolve are of sapphire, and they too must necessarily be

made with skill and carefulness; if, therefore, the jewels become

cracked and the pivots dulled by careless handling, the meter at

once becomes useless as a measuring instrument.

=Ques. How should readings be taken?=

Ans. The deflection of the pointer should be read to tenths of a

division; this can be done with considerable accuracy, especially

after a little practice.

[Illustration: FIG. 2,893.--Load and speed test of direct current

shunt motor. The object of this test is to maintain the voltage

applied to the motor constant, and to vary the load by means of a

brake and find the corresponding variation in speed of the machine

and the current drawn from the circuit. If the motor be a constant

speed motor, the field resistance is maintained constant. The above

indicates the method of connecting instruments for the test alone;

for starting the machine the ordinary starting box, should, of

course, be inserted.]

For very accurate results, a temperature correction should be

applied to compensate the effect which the temperature of the

atmosphere has upon the resistance of the meter when measurements

are being taken. In ordinary station practice the temperature

correction is negligible, being for resistance corresponding to

the high scale in first class meters, less than one-quarter of 1

per cent. for a range of 35 degrees above or 35 degrees below 70

degrees Fahrenheit.

=Ques. What attachment is sometimes provided on station voltmeters

used for constant pressure service?=

Ans. A normal index.

[Illustration: FIG. 2,894.--Temperature test of direct current

motor or dynamo; loading back method. In making temperature tests

on a small dynamo it is usual to drive the dynamo with a motor and

load the dynamo by means of a lamp bank or resistance, the voltage

across the dynamo being maintained constant, and the current through

the external circuit adjusted to full load value. The temperatures

are then recorded, and when they reach a constant value above the

temperature of the atmosphere, the test is discontinued. Similarly,

in making a test on a small motor, the motor is loaded with a dynamo

and the load increased until the input current reaches the normal

full load value of the motor, the test being conducted as for a small

dynamo. When, however, the apparatus, either motor or dynamo, reaches

a certain size, it becomes necessary, in order to economize energy,

to use what is called the =loading back method=, as here illustrated.

The motor is started in the usual way, with the dynamo belted to it,

the circuit of the dynamo being open. The field of the dynamo is then

adjusted so that the dynamo voltage is equal to that of the line.

The dynamo is then connected to the circuit and its field resistance

varied until it carries normal full load current. Under these

conditions, if the motor and dynamo be of the same size and type,

the motor will carry slightly in excess of full load, the difference

being approximately twice the losses of the machines. Under these

conditions the total power drawn from the line is equal to twice the

loss of either machine. Temperature readings are taken as in other

temperature tests.]

=Ques. What precaution must be taken in connecting station

voltmeters?=

Ans. Care must be taken to guard against any short circuiting of the

voltmeter, which, would mean a short circuiting of the generator, and

as a result the probable burning out of its armature.

The high resistance of the voltmeter prevents any such

occurrence when it is connected in the proper way, but should one

side of the circuit be grounded to the metal case or frame of the

meter, a careless handling of the lead connected with the other

side of the circuit would produce the result just mentioned.

[Illustration: FIG. 2,895.--Compound dynamo external characteristics

test; adjustable load. The object of this test is to determine the

relation between armature voltage and armature current. Shunt field

is adjusted to give normal secondary voltage when the external

circuit is open. The load is then applied by means of an adjustable

resistance or lamp bank, and readings of external voltage and current

recorded. If the machine be normally compounded, the external voltage

will remain practically constant throughout the load range. If the

machine be under-compounded, the external voltage will drop with

load, while if over-compounded, there will be a rise in voltage with

increase in load.]

=Ques. Why do station voltmeters indicate a voltage slightly lower

than actually exists across the leads?=

Ans. Since they are usually connected permanently in circuit; a

certain amount of heat is developed in the wiring of the instrument.

[Illustration: FIGS. 2,896 and 2,897.--Transformer core loss and

leakage, or exciting current test. With the primary circuit open,

the ammeter indicates the exciting or no load current. It should

be noted that all instruments are inserted on the low voltage

side, for both safety of the operator and because the measurements

are more accurate. The no load primary current, if the ratio of

transformation be 10: 1, will be one-tenth of the measured secondary

current. The wattmeter connected, as shown, measures the sum of the

losses, in the transformer, in the pressure coil of the wattmeter,

and in the voltmeter. On all standard makes of portable instruments,

the resistance of the wattmeter pressure coil and of the voltmeter

is given, and the loss in either instrument is the square of the

voltage at its terminals, divided by its resistance. Subtracting

these losses from the total indicated upon the wattmeter, gives the

true core or iron loss. It should be noted that in this diagram is

shown an auxiliary transformer with a number of taps for obtaining

the exact rated voltage of the transformer under test. In fig. 2,897

is shown, in general, the same connections as in fig. 2,896, except

that the auto-transformer has been replaced by a resistance. If the

line voltage available be not much in excess of the rated voltage of

the transformer under test, very little error is introduced by the

use of the resistance method. However, if the difference be 10 per

cent. or more the auxiliary transformer shown in fig. 2,896 should be

used. Measurements made under the resistance method always give lower

results than those obtained with the auxiliary transformer.]

The effect of this heat increases the voltmeter resistance and

consequently reduces the current below that which otherwise would

pass through the meter; since the deflections of the pointer

are governed by the strength of the current, station voltmeters

invariably indicate a voltage slightly lower than that which

actually exists across their leads.

[Illustration: FIG. 2,898.--Diagram of connections for calibrating a

wattmeter. The calibration of a portable wattmeter is accomplished

with direct current of constant value which is passed through the

series winding by connecting the source thereof with the current

terminals. A direct current voltage which may be varied throughout

the range of the wattmeter is also applied to the instrument

between the middle and right hand pressure terminals A and E the

wiring in the meter between these terminals being such that its

differential winding is then cut out of circuit. The method of

procedure consists in comparing the deflections on the wattmeter at

five of six approximately equidistant points over its scale with the

corresponding products of volts and amperes used to obtain them. The

changes in the wattmeter deflections are effected by merely varying

the voltage, the value of the current being maintained constant at a

value which represents the full current capacity of the meter.]

NOTE.--=Checking up of a recording wattmeter.=

This may conveniently be done by noting the

deflections at short intervals on an ammeter

connected in circuit, and also the readings

on the dial of the recording wattmeter during

this period. If this test be continued for an

appreciable time, the product of the pressure

in volts, the current in amperes, and the time

in hours, should equal the number of watthours

recorded on the counters of the dial.

NOTE.--=Transformer testing.= In the early

days of transformer building, before the

commercial wattmeter had been perfected, leakage

or exciting current was the criterion of good

design. After the introduction of the wattmeter,

core loss became the all important factor, and

for a long time the question of leakage current

was lost sight of. With the introduction of

silicon steel, leakage or exciting current again

assumed prominence. Keeping in mind the fact

that all characteristics of a transformer are of

more or less importance, it is essential that

the user of such apparatus have at hand the

necessary facilities for making tests of all such

variable quantities. The tests which all users

of transformers should make, are given in this

chapter.

=Ques. Can direct current be measured by an alternating current

voltmeter?=

Ans. Yes.

[Illustration: FIG. 2,899.--Transformer copper loss by wattmeter

measurement and impedance. At first glance, this method would

seem better than the calculation of loss after measurement of the

resistance. However, it should be noted that the wattmeter is, in

itself, subject to considerable error under the low power factor

that will exist in this test. The secondary of the transformer is

short circuited, and a voltage applied to the primary which is just

sufficient to cause full load primary current. If full current pass

through the primary of the transformer with the secondary short

circuited, the secondary will also carry full load current. With

connections as shown, and with the full load current, the voltmeter

indicates the impedance volts of the transformer. This divided by

the rated voltage gives what is called the _per cent. impedance of

the transformer_. In a commercial transformer of 5 kw., this should

be approximately 3 per cent. The iron loss of the transformer under

approximately 3 per cent. of the normal voltage will be negligible,

and the losses measured will be the sum of the primary and secondary

copper losses. As in the discussion of the core loss measurements,

the wattmeter readings must be corrected for the loss in its pressure

coil, the method of correction being the same as that discussed under

the core loss measurement. If the impedance volts, as measured, be

divided by the primary current, the impedance of the transformer

is obtained. The reciprocal of this quantity is known by the term

"admittance." _When two or more transformers are connected in

parallel they divide the load in proportion to their admittance._

It is, therefore, important that the users of transformers know the

impedance of the apparatus used, in order to determine whether two

or more transformers will operate satisfactorily in parallel. For

discussion of wattmeter error on low power factor, see note on page

2,075. For accurate measurement of impedance, the voltmeter should

be connected directly across the terminals of the transformer rather

than as shown in the diagram.]

NOTE.--=Transformer copper loss test.= The

usual and best method of obtaining copper

losses is to separately measure the primary and

secondary resistance and calculate from these

the primary and secondary copper losses. For

general diagram of connections and discussion

of the drop method, see fig. 2,875. The current

should be kept well within the load current of

the transformer to avoid temperature rise during

the test. In other words, the resistance of the

coil is the voltage across its terminals divided

by the current. The resistance of the primary

coil can be measured similarly. The copper loss

in watts in each coil will then be the product

of the resistance and the square of the rated

current for that coil. The total copper loss will

be the sum.

=Ques. What would be the effect of placing a direct voltmeter across

an alternating current circuit, and why?=

Ans. There would be no deflection of the pointer owing to the rapid

reversals of the alternating current.

=Ques. What are the usual capacities of alternating current

voltmeters?=

Ans. They are 3, 7.5, 10, 12, 15, 20, 60, 75, 120, 150, 300 and 600

volts, but these capacities may each be increased by the use of a

multiplier.

[Illustration: FIG. 2,900.--Temperature test of transformer with

non-inductive load. The figure shows the simplest way of making the

test. Connect the primary of the transformer to the line as shown,

and carry normal secondary load by means of a bank of lamps or other

suitable resistance, until full load secondary current is shown by

the ammeter in the secondary circuit. The transformer should then be

allowed to run at its rated load for the desired interval of time,

temperature readings being made of the oil in its hottest part, and

also of the surrounding air. Where temperatures of the coil rather

than temperatures of the oil are desired, it is necessary to use the

resistance method. This is obtained by first carefully measuring the

resistance of both primary and secondary coils at the temperature

of the room, and then, after the transformer has been under heat

test for the desired time, disconnect it from the circuit and again

measure the resistance of primary and secondary. For proper method

of calculating the temperature rise from resistance measurements,

the reader is referred to the standardization rules of the A.I.E.E.

In making resistance measurements of large transformers by the drop

method care should be taken to allow both ammeter and voltmeter

indications to settle down to steady values before readings are

taken. This may require several minutes. Each time the current is

changed it is necessary in order to obtain check values on resistance

measurements, to wait until the current is again settled to its

permanent value before taking readings. All resistance measurements

must be taken with great care, as small errors in the measurement of

the resistance may make very large errors in the determination of

the temperature rise. The method above described is satisfactory for

small transformers. Where large units are to be tested, the cost of

current for testing becomes an important item. The "bucking test" as

in fig. 2,901, is more economical.]

=Ques. How are station voltmeters usually attached to the

switchboard?=

Ans. They are usually bolted to the switchboard by means of four

iron supports mounted on the back of the instrument; two of these are

fastened near each side of the case.

Under certain conditions, however, as in paralleling of

alternators, it is convenient to have the alternating current

voltmeter mounted on a swinging bracket at the side of the

switchboard. The voltmeter may then be swung around in any desired

direction so as to enable the attendant to keep informed of the

voltage while switching in each additional alternator.

[Illustration: FIG. 2,901.--Transformer temperature "bucking test."

For this purpose two transformers of the same size and ratio are

required. The connections are as shown. Full secondary voltage is

applied, and rheostats or auxiliary auto-transformers are inserted

in the circuit to properly regulate the voltage. The primaries

are connected with one bucking the other, and a voltage equal to

twice the impedance voltage of either transformer inserted in the

primary circuit. It should be noted that when the secondaries are

subjected to the full secondary voltage, a full primary voltage

exists across either primary, but with the primaries connected so

that the voltage of one is bucked against the voltage of the other,

the resultant voltage in the circuit will be zero. By applying to the

primary circuit twice the impedance voltage of either transformer,

full primary and secondary current will circulate through both

transformers. On the other hand, by subjecting the secondaries to

the full secondary voltage, the iron of the transformer will be

magnetized as under its regular operating conditions, and the full

iron loss of the transformer introduced. This method permits the

operation of two transformers under temperature test with their

full losses, without taking energy from the line equal to the rated

capacity. Measurements of temperature are taken in exactly the

same way as above. This method is successfully employed for making

temperature tests on transformers of all sizes.]

=Ques. How should an ammeter be operated to get accurate readings,

and why?=

Ans. It should be cut out of circuit except while taking a reading,

because of the error introduced by the heating effect of the current.

[Illustration: FIG. 2,902.--Transformer insulation test. In applying

a 10,000 volt insulation test between the primary and secondary of

a transformer, the testing leads should be disconnected from the

transformer under test, and a spark gap introduced as shown, with the

test needle set at a proper sparking distance for 10,000 volts. A

high resistance should be connected in the secondary before closing

its circuit, and the voltage gradually increased by cutting out

this secondary resistance until a spark jumps across the spark gap.

When the spark jumps across the spark gap, the voltmeter reading

should be recorded and the testing transformer disconnected. The

spark gap should then be increased about 10 per cent. and the high

tension leads connected to the transformer under test as indicated

in the diagram. In order to equalize the insulation strains, all

primary leads should be connected together, all secondary leads not

only connected together, but to the core as well. All resistance

in the rheostat in the low tension circuit should then be inserted

and the switch closed. Gradually cut out secondary resistance until

the voltmeter shows the same voltage as was recorded previously

when the spark jumped across the gap, and apply this voltage to the

transformer for one minute. Insulation tests for a period of over

one minute are very unadvisable, as transformers with excellent

insulation may be seriously damaged by prolonged insulation tests.

The longer the strain to which any insulation is subjected, the

shorter the subsequent life of the insulation. Also the greater the

applied voltage above the actual operating voltage of the apparatus,

the shorter the subsequent life of the insulation. In testing small

transformers, the spark gap may be omitted and the voltage of the low

pressure coil of the testing transformer measured. This multiplied by

the ratio of transformation gives the testing voltage.]

In an ammeter having a capacity of 50 amperes, the error thus

introduced will be less than 1 per cent. if connected continuously

in circuit with a current not exceeding three-quarters this

capacity.

An ammeter of 100 amperes capacity may be used indefinitely

in circuit with less than 1 per cent. error up to one-half its

capacity, and for five minutes at three-quarters capacity without

exceeding the 1 per cent. limit.

[Illustration: FIG. 2,903.--Transformer insulation test as made

when a special high tension transformer be not available. In this

method a number of standard transformers, connected as shown may be

employed, but great care should be taken to have such transformer

cases thoroughly insulated from the ground and from one another, in

order to minimize the insulation strains in the testing transformers.

Care should be taken to insert in the circuit of each testing

transformer a fuse, not in excess of the transformer capacity, which

will blow, in case of a break down in the apparatus under test.

In testing insulation between secondary and core, disconnect the

primary entirely, apply one terminal of the testing transformer to

the secondary terminals of the transformer under test, and the other

terminal of the testing transformer to the core of the transformer

under test. This test should also not be in excess of one minute.]

The 150 scale ammeter may be left in circuit for an indefinite

length of time at one-third its full capacity, and for three

minutes at one-half its full capacity, with a negligible error.

Ammeters of 200 and of 300 ampere capacities must not

continuously carry more than one-quarter of these loads

respectively if the readings are to have an accuracy within 1 per

cent. nor more than one-half these respective number of amperes

for three minutes if the same degree of accuracy be desired.

In order to cut or shunt the ammeter out of circuit when not

in use, it is customary when wiring the instrument in place, to

introduce a switch as a shunt across it; this switch is kept

closed except when a measurement is being taken.

When currents larger than 300 amperes have to be measured,

ammeter shunts are generally employed, although ammeters up to

500 amperes capacity are manufactured.

[Illustration: FIG. 2,904.--Transformer internal insulation test,

sometimes called double normal voltage test, from the fact that most

transformers are tested with double normal voltage across their

terminals. If either the primary or secondary of the transformer

be connected to some source of current with voltage double that of

the voltage of the transformer under test, the insulation between

adjacent turns, and also the insulation between adjacent layers

will be subjected to twice the normal operating voltage. It is good

practice to employ high frequency for this test in order to prevent

an abnormal current from passing through the transformer. Sixty

cycle transformers are usually tested on 133 cycles, and 25 cycle

transformers on 60 cycle circuits for this double normal voltage

test. It is necessary to insert the resistance in the circuit of the

transformer and bring the voltage up gradually, the same as applying

other high insulation tests in order to prevent abnormal rises in

pressure at the instant of closing the circuit.]

=Ques. What is used in place of instrument shunts for high pressure

alternating current measurements?=

Ans. Instrument transformers.

=Ques. What important attention should be periodically given to

measuring instruments?=

Ans. They should be frequently tested by comparison with standards

that are known to be correct.

Electrical measuring instruments, owing to the nature of their

construction and the conditions under which they must necessarily

be used, are subject to variations in accuracy. This feature is an

annoying one on account of the difficulty of detecting it; a meter

may, as far as appearances go, be in excellent working order and

yet give readings which are not to be relied upon.

Ridiculous as it may appear, the average station attendant

may frequently be seen straining his eyes to read to tenths of

a division on the scale of a meter which, if subjected to test,

would show an inaccuracy of over 2 per cent.

In testing a meter, by comparing it with a standard, in order to

obtain the best results there should be one man at each meter so

that simultaneous readings may be taken on both instruments, and

the man at the standard meter should maintain the voltage constant

while a reading is being taken, by means of a rheostat in the

field circuit of the generator supplying the current.

[Illustration: FIG. 2,905.--Transformer insulation resistance

test. The insulation, besides being able to resist puncture, due

to increased voltage, must also have sufficient resistance to

prevent any appreciable amount of current flowing between primary

and secondary coils. It is, therefore, sometimes important that the

insulation resistance between primary and secondary be measured. This

can be done, as here shown. Great care should be taken to have all

wires thoroughly insulated from the ground, and to have an ammeter

placed as near as possible to the terminals of the transformer under

test, in order that current leaking from one side of the line to the

other, external to the transformer, may not be measured. Great care

is required in making this measurement, in order to obtain consistent

results.]

Each meter should be checked or calibrated at five or six

approximately equidistant points over its scale; the adjustable

resistance being varied each time to give a deflection on the

standard meter of an even number of divisions and the deflection

on the other meter recorded at whatever it may be. Having obtained

the necessary readings, the calculation of the constant or

multiplying factor of the meter undergoing test is next in order.

This may best be shown by taking an actual case in which a 150

scale voltmeter is being tested to determine its accuracy. The

data and calculations are as follows:

Readings on Readings on Constant

standard meter meter tested

150 149.2 150 ÷ 149.2 = 1.005

125 125.0 125 ÷ 125.0 = 1.000

100 98.9 100 ÷ 98.9 = 1.011

75 73.6 75 ÷ 73.6 = 1.019

50 50.0 50 ÷ 50.0 = 1.000

25 24.8 25 ÷ 24.8 = 1.008

------

6.043

Average constant for six readings, 6.043 ÷ 6 = 1.007.

[Illustration: FIG. 2,906.--Transformer winding or ratio test. The

object of this test is to check the ratio between the primary and

the secondary windings. For this purpose a transformer of known

ratio is used as a standard. Connect the transformer under test with

a standard transformer as shown. Leave switch S_{2} open. With the

single pole double throw switch in position S_{1}B, the voltmeter

is thrown across the terminals of the standard transformer. With

the switch in position S_{1}A, the voltmeter is thrown across the

terminals of the transformer under test. The voltmeter should be read

with the switch in each position. If the winding ratio be the same as

that of the standard transformer, the two voltmeter readings will be

identical.]

It may be stated in general that before taking the readings for

this test, the zero position of the pointer on the meter tested

should be noted, and if it be more than two-tenths of a division

off the zero mark, the case of the meter should be removed and the

pointer straightened.

Furthermore, it will be noticed from the readings here recorded

that the test is started at the high reading end of the scale;

this is done in order that the pointer may gradually be brought

up to this spot, by slowly cutting out of circuit the adjustable

resistance, and thus show whether or not the pointer has a

tendency to stick at any part of the scale. If the meter seem to

be defective in this respect, it should be remedied either by

bending the pointer or scale, or by renewing one or both of the

jewels, before the comparison with the standard is commenced.

It is obvious from the readings recorded for the 150 scale

voltmeter, that as compared with the corresponding deflections of

the standard, the former are a trifle low.

In order to determine for each observation how much too low

they are, it is necessary to divide each reading on the standard

by the corresponding reading on the meter tested. The result is

the amount by which a deflection of this size on the meter tested

must be multiplied in order to obtain the exact reading. This

multiplier is called a constant, and as shown, a constant is

determined for each of the six observations.

The average constant for the six readings is then found, and

this is taken as the constant for the meter as a whole; that is,

whenever this 150-scale voltmeter is used, each reading taken

thereon must be multiplied by 1.007 in order to correct for its

inaccuracy.

The most convenient and systematic way of registering the

constant of a meter is to write it, together with the number of

the meter and the date of its calibration, in ink on a cardboard

tag and loop the same by means of a string to the handle or some

other convenient part of the meter.

NOTE.--=Transformer polarity test.= A test of

importance in the manufacture of transformers,

and sometimes necessary for the user, is the

so called _banking_ or _polarity_ test. The

transformers from any particular manufacturer

have the leads brought out in such a manner that

a transformer of any size can be connected to

primary and secondary lines in a given order

without danger of blowing the fuses due to

incorrect connections. All manufacturers of

transformers, however, do not bank transformers

in the same way, so that it is necessary in

placing transformers of different makes to test

for polarity. This is done as shown in fig.

2,906. One transformer is selected as a standard

and the leads of the second transformer connected

as indicated in the diagram. If the transformers

be 1,100-2,200 volts to 110-220, two 110 volt

lamps are connected in the secondaries of the

transformers as indicated, while the primary of

the transformer is connected across the line.

In transformers built for two primary and two

secondary voltages, it is necessary to test each

primary and each secondary. The diagram shows the

method of connecting one 2,200 volt coil and one

110 volt coil to the transformer to be tested.

When the primary circuit of the transformer under

test is closed, and if the secondary leads of

the 110 volt coil under test be brought out of

the case properly, the two 110 volt lamps should

be brightly illuminated. If, on the other hand,

the two 110 volt terminals have been reversed,

no current will flow through the lamps. If

these two terminals be found to be brought out

correctly, transfer the secondary leads of the

transformer under test to the second 110 volt

coil. Upon closing the primary circuit, the lamp

should again be brightly illuminated. Repeat this

process with each of the secondary coils and the

other primary coil, and if the lamps show up

brightly in every case on closing the primary

circuit, all leads have been properly brought

out. If on any tests the lamps do not light up

brightly, the leads on the transformer must be so

changed as to produce the proper banking.

=Ques. What are the usual remedies applied to a voltmeter to correct

a 3 or 4 per cent. error?=

Ans. They consist of straightening the pointer, varying the tension

of the spiral springs, renewing the jewels in the bearings, altering

the value of the high resistance, and, in the case of a direct

current instrument, strengthening the permanent magnet.

=Ques. How is the permanent magnet strengthened?=

Ans. After detaching it from the instrument, wrap around several

turns of insulated wire, and pass through this wire for a short time

3 or 4 amperes of direct current in such a direction as to reinforce

the magnet magnetism.

=Ques. How may the value of the high resistance of a voltmeter be

altered?=

Ans. Determine the resistance of the voltmeter and add or subtract,

according as the reading is high or low, a certain length of wire

whose resistance is in per cent. of the voltmeter resistance the same

as the per cent. of error.

NOTE.--The complete calibration of a two

scale voltmeter does not, as might be supposed,

necessitate that the readings on both scales be

checked with standards, for since the resistance

corresponding to the one scale is always some

multiple of the resistance of the other, the

constants of the two scales are proportional. For

instance, if S = the reading at the end of the

high scale of the voltmeter; S^{1} = the reading

at the end of the low scale of the voltmeter; R =

the resistance in the meter corresponding to the

high scale; R^{1} = the resistance in the meter

corresponding to the low scale; K = the constant

for the high scale, and K^{1} = the constant for

the low scale. Then

SK ÷ R = S^{1}K^{1} ÷ R^{1}

from which

K^{1} = SKR ÷ S^{1}R

That is to say, if the respective resistances corresponding to

the two scales be known, and the constant of the high scale be

determined by comparison with a standard, then by aid of these

known values and the maximum readings on the two scales, the

constant of the low scale may be calculated. It is also possible

to calculate the constant of the high scale if the constant of the

low scale be known, together with the values of the resistances

corresponding to the two scales; for from the equation previously

given.

K = RS^{1}K^{1} ÷ R^{1}S

=Ques. What is a frequent cause of error in an alternating current

meter, and why?=

Ans. The deterioration of its insulation, which permits the working

parts of the instrument coming in contact with the surrounding metal

case.

A convenient method of testing for deterioration of insulation

is shown in fig. 2,905.

[Illustration: FIG. 2,907.--Diagrams showing various synchronous

converter transformer connections. The diametrical connection is

used most frequently as it requires only one secondary coil on each

transformer, this being connected to diametrically opposite points on

the armature winding. The middle points can be connected together and

a neutral obtained the unbalanced three wire direct current having no

distorting effect. With diametrical secondaries, the primaries should

preferably be connected delta, except with regulating pole converters

where they must be connected Y. Diametrical secondaries with delta

primaries should not be used with regulating pole converters. Double

star connection of secondaries may, however, be used with delta

primaries, and is free from the trouble of the triple harmonic of the

transformer appearing in the primary. In this case, however, the two

secondary neutrals must not be connected with each other.]

=How to Test Generators.=--In the operation of electrical stations,

many problems dealing with the generators installed therein can be

readily solved by the aid of characteristic curves, which bear a

relation to the generators similarly as do indicator diagrams to

steam engines.

[Illustration: FIG. 2,908.--General form of characteristic curves

for a series dynamo. The general curve that may be expected is OA.

It is obtained practically in the same manner as for the shunt

characteristic curve, except that no field rheostat is employed.

Commencing with no load or amperes, there will probably be a

small deflection noticeable on the voltmeter, due to the residual

magnetism. The other readings are taken with successive reductions

of main current resistance. The curve OA thus obtained for a certain

series generator is practically a straight line at the beginning,

representing thereby a proportional increase of voltage with

increase of current, but after a certain current is reached (about

20 amperes in this case) the curve flattens and takes a downward

direction. The turning point occurs in the characteristic curves

of all series generators, and it denotes the stage at which the

iron magnet cores become so saturated with lines of magnetic force

that they will not readily allow more to pass through them; this

turning point is technically known as the point of saturation, and

the current corresponding (20 amperes in this case) is called the

critical current of the dynamo. The point of saturation in any given

series machine is governed by the amount of iron in the magnetic

circuit; its position in the curve therefore varies according to

the design of the generator as does also the critical current. The

value of the latter is important inasmuch as the valuable features

of a series generator assert themselves only when the machine is

supplying a greater number of amperes than that of the critical

current, for if the series generator be worked along that part M A of

the curve to the right of the point of saturation it becomes nearly

self-regulating as regards current, because as the current increases

the voltage drops. In the diagram in addition to the characteristic

curve O A, which may more definitely be called an external

characteristic curve on account of representing the conditions

external to the generator, there is shown a total characteristic

curve, O C B. The latter curve represents the relation between the

current and the total voltage developed in the armature, and may be

plotted from the external characteristic curve if the resistance of

the armature between brushes and the resistance of the series field

winding be known. For example, assume these combined resistances

amount to .6 ohm. At 30 amperes there would be required 30 × .6 = 18

volts to force this current through the armature and field windings.

At 30 amperes the external pressure is 65 volts, as shown by the

curve O A; the total voltage developed for 30 amperes is, therefore,

the external voltage plus the internal voltage or 65 + 18 = 83

volts. Plotting 83 volts for 30 amperes will give one point for the

external characteristic curve of this machine, and by determining in

like manner the total voltages developed for six or eight different

currents over the scale, sufficient data will be at hand for plotting

and drawing in the curve O C B.]

In steam engineering, a man who did not fully understand the method

of taking an indicator diagram would be considered not in touch with

his profession, and in electrical engineering the same would be true

of one ignorant of the method of obtaining characteristic curves.

The necessary arrangement or connection of the generator from

which it is desired to obtain a characteristic curve, consists in

providing a constant motive power so that the machine may be run

at a uniform speed, and when the field magnets of the generator

are separately excited the field current from the outside source

must also be maintained constant, preferably by a rheostat

connected in the field of the auxiliary exciting machine. It is

also necessary in every case that means be provided for varying

the main current of the generator step by step from zero to

maximum. This may best be done by employing a water rheostat, as

shown in fig. 2,909.

=Ques. What instruments are needed in making a test of dynamo

characteristics?=

Ans. A voltmeter, ammeter, speed indicator, the usual switches and

rheostats.

=Ques. How is the apparatus connected?=

Ans. It is connected as shown in fig. 2,910.

=Ques. Describe the test.=

Ans. Having completed the preliminaries as in fig. 2,910, the test

should be started with the main circuit of the generator open. Then,

in the case of the shunt machine, the speed should be made normal and

the field rheostat adjusted until the voltmeter reading indicates

the rated voltage of the machine at no load and readings taken. The

electrodes of the water rheostat should be adjusted for maximum

resistance and main circuit closed, and a second set of readings

taken. Several sets of readings are taken, with successive reductions

of water rheostat resistance. The results are then plotted on

coordinate paper giving the characteristic curve shown in fig. 2,908.

[Illustration: FIG. 2,909.--Water rheostat. It consists essentially

of a tank of suitable size containing salt water into which are

placed two electrodes having means of adjustment of the distance

separating them. The solution depends on the voltage. Pure water is

seldom used for pressures under 1,000 volts. The size of the tank is

determined by the size of the electrodes, and roughly the size of the

latter equal the number of amperes. With a current density of one

ampere per square inch, a water solution gives a drop of 2,500 to

3,000 volts per inch distance between the plates. Where high voltage

is used, the water must be circulated through and from the tank by

rubber hose allowing for 2,500 volts, a length of 15 to 20 feet of

1 inch hose to prevent grounding. In place of the arrangement shown

above, a barrel may be used for the tank, and for the electrodes,

coils of galvanized iron wire. This is the simplest form and is

satisfactory.]

=Ques. What does the characteristic curve (fig. 2,911) show?=

Ans. An examination of the curve shows that the highest point of

the curve occurs at no load or 0 amperes; that as the current is

increased, the voltage drops, first slightly to the point B and then

rapidly until the point E is reached, when any further lowering of

resistance in the main circuit to increase the current causes not

only a rapid decline in the voltage but also of the current until

both voltage and current become approximately zero.

[Illustration: FIG. 2,910.--Connections for test of dynamo. During

the test, one man should be assigned to the tachometer, another man

to the water rheostat, and there should preferably be one man at

each of the electrical measuring instruments. In order to enable the

man at the tachometer to keep the speed constant, he should be in

communication either directly or indirectly with the source of the

driving power, and the man at the water rheostat should be in plain

view of the man reading the ammeter so that the latter party may

signal him for the proper adjustment of the rheostat in order that

the desired increase of current be obtained for each set of readings.]

In some generators, a very slight current results even when the

terminals of the machine are actually short circuited; that is,

due to residual magnetism in the pole pieces, the lower portion of

the curve often terminates, not exactly at zero, but at a point

some distance along the current line.

The working portion of the curve is from A to C, at which time

the machine is supplying a fairly constant voltage. From C to E

shows a critical condition of affairs, while the straight portion

D O represents the unstable part of the curve caused by the field

current being below its proper value.

The position of the point C determines the maximum power the

machine is capable of developing, being in this case (47.5 × 25) ÷

746 = 1.59 horse power.

=Ques. How may the commercial efficiency of a generator be

determined?=

Ans. To obtain the commercial efficiency, the _input_ and _output_

must be found for different loads.

The input may be found by running the generator as a motor

at its rated speed, loading it by means of a Prony brake. The

generator must be stripped of all belting or other mechanical

connections, supplied with its normal voltage and full load

current, and the pressure of the Prony brake upon its armature

shaft or pulley adjusted until the rated speed of the armature is

obtained. The data thus obtained is substituted in the formula.

2π L W R

input in brake horse power = ---------- (1)

33,000

in which

L = length of Prony brake lever;

W = pounds pull at end of lever;

R = revolutions per minute.

The output or electrical horse power for the same load is easily

calculated from the formula

amperes × volts

output in electrical horse power = --------------- (2)

746

After obtaining value for (1) and (2) the commercial efficiency for

the load taken is obtained from the formula

output

commercial efficiency = ------ (3)

input

Having obtained the commercial efficiency, the difference between the

ideal 100 per cent. and the efficiency found will be due to certain

losses in the generator. These losses may be classified as

1. Mechanical.

2. Electrical.

The mechanical losses are the friction of the bearings and

brushes, and air friction. The electrical losses consist of the

eddy current loss, hysteresis loss, armature resistance loss, and

field resistance loss.

In testing for these losses, the generator to be tested should

be belted to a calibrated motor which latter machine should

preferably be of the constant pressure, shunt wound type.

The friction of the bearings and belt of the generator are

determined together by raising the brushes off its commutator and

running it at the rated speed by means of the calibrated motor.

[Illustration: FIG. 2,911.--Characteristic curve of shunt dynamo.

Suppose in making the test, the deflections on the meters for the

first readings be 63 volts and 0 amperes, the plotting of these

values will give the first point on the curve. Similarly, the second

readings with main circuit closed and maximum resistance in the water

rheostat may be assumed to be 62.5 volts and 7.5 amperes, which

plotted gives the second point B. A still further lowering of the

plate will permit a stronger current in the main circuit, and the

value of this together with its corresponding voltage will give a

third point for the curve. Neither for this reading, however, nor

for the following readings of the test should the field rheostat be

altered. When six or eight points ranging from zero to a maximum

current have been obtained and plotted, a curved line should be drawn

through them such as shown through ABCDEFG0, the _characteristic

curve_ of the dynamo. While the curve may be sketched in free hand,

it should preferably be drawn by the aid of French curves. In case

the French curve cannot be exactly made to coincide with all the

points as for instance C and D, it should be run in between giving an

average result, and smoothing out irregularities, or small errors due

to the "personal equation." The meter of course must be correct or

calibrated and the readings corrected by the calibration coefficient.]

The amount of power as ascertained from the calibration curve of

the motor for the voltage and current used therein when driving

the generator as just explained, is a measure of these two losses.

The power thus used is practically constant at all loads and is

about 2 per cent. of that necessary to drive the generator at full

load.

[Illustration: FIG. 2,912.--Characteristic curves for a compound

dynamo. If the machine be over compounded, the characteristic curve

has the form of the curve A B, which curve was obtained from a

machine over-compounded from 118 to 123 volts, and designed to give

203 amperes at full load. The preliminary arrangements for testing

a compound dynamo are similar to those for a shunt generator, and

if the shunt across the series field winding be already made up and

in position, the readings are taken precisely in the same manner.

It is generally considered sufficient if observations be recorded

at zero, ¼, ½, ¾ and full load. If it be desired to ascertain the

effect which residual magnetism has upon the field magnets the

current is decreased after the full load point is reached without

opening the circuit, and readings are taken in succession at ¾,

½, ¼ and zero load giving in this case the curve B C D E S. It is

thus seen that residual magnetism exerts no small effect upon the

voltage obtained at the different loads, for had there been no

residual magnetism in the field magnets the curve B C D E S would

have coincided with the curve A B. The curve A B, and the straight

line A X drawn through the points A and B, are almost identical,

and as A X represents the theoretical characteristic curve for the

machine, it is seen that compounding is practically perfect. In

order to insure such accurate results being obtained, providing

the machinery be correctly designed, requires considerable care in

taking the readings; for example, each step or load on the ascending

curve should not be exceeded before the corresponding deflection is

taken, else the residual magnetism will cause the pressure reading

to be higher than it actually should be, and the following pressure

readings will also be affected in the same manner. In case the shunt

to be employed across the series field has not been made up, it is

advisable to perform a trial test before taking the readings for the

curve as previously described. The trial test consists in taking

two readings,--one at no load and the other at full load, the shunt

being so adjusted as to length and section that the desired amount

of compounding will be obtained in the latter reading with normal

voltage at no load. If the first trial fail to produce the desired

result by giving too low a voltage at full load, the length of the

shunt across the series field should be increased, or its section

should be reduced by employing a less number of strips in its makeup;

again, if the voltage at full load be higher than that desired, there

must be made a decrease in length or an increase of section in the

shunt employed.]

The friction of the brushes can very conveniently be determined

next by lowering them on the commutator and giving them the proper

tension.

The increase in power resulting from the greater current that

will now be taken by the motor to run the dynamo at its rated

speed, will be a measure of this loss. In general, its value will

be about .5 per cent. of the total power required to drive the

dynamo at full load, and this also will remain constant at all

loads.

The friction of the air upon the moving armature of the dynamo

cannot be determined experimentally, but theoretically this loss

is small and may be estimated as .5 per cent.; it is also constant

at all loads.

The core loss may be determined experimentally by exciting

the field magnets of the dynamo with the normal full load field

current through the magnet coils, and noting the increase of power

required by the motor to maintain the rated speed of the dynamo

thus excited under no load, over that necessary under the same

conditions with no field excitation. This increase of power will

be the value of the core loss. The core loss is approximately 3

per cent. of the power required to operate the dynamo at full

load, and it is constant at varying loads. If it be desired to

divide the core loss into its component parts, it is necessary

also to run the dynamo under the same conditions as before with

field excitation but at half its rated speed. If, then,

H = the power lost in hysteresis at rated speed,

E = the power lost in eddy currents at rated speed,

T = the power lost in hysteresis and eddy currents at rated speed,

S = the power lost in hysteresis and eddy currents at half speed.

there may be formed the two following equations:

H E

T = H + E, and S = --- + ---,

2 2

from which the elimination of H will give E = 2T - 4S.

The value of the eddy current loss thus found will be about 1½

per cent., and constant at all loads.

Having previously ascertained the power lost in both eddy

currents and hysteresis, and knowing now the power lost in eddy

currents alone, it is easy to find that lost in hysteresis by

simply subtracting the latter known value from the former. The

value of the hysteresis loss is therefore approximately 1½ per

cent., and it is constant at different loads.

There yet remains to be determined the armature resistance

loss and the field resistance loss. As for the calibrated motor,

this may be disconnected from the dynamo, as it need not be used

further in the test.

The armature resistance is the resistance of the armature

winding of the dynamo, between the commutator bars upon which

press the positive and negative brushes. Assume that the value

of the armature resistance be known, call this value R ohms,

together with that of the full load armature current, which is

also known and which call I amperes, this is sufficient data for

calculating the armature resistance loss at full load. It is

evident that to force the full load current I through the armature

resistance R will require a pressure of R volts, and that the

watts lost in doing so will be the voltage multiplied by the

current. The armature resistance is consequently

IR × I = I^{2}R watts

or, expressed in horse power it is

I^{2}R ÷ 746

At full load it is usually about 2 per cent. of the total

power required to drive the generator fully loaded. The armature

resistance loss varies in proportion to the load, in fact, as the

last expression shows, it increases as the square of the armature

current.

The field resistance loss is calculated in the same manner as

just explained for the armature resistance loss, it being equal

in horse power to the square of the full load field current

multiplied by the resistance of the field winding and divided by

746. In a shunt dynamo it is practically constant at 2 per cent.

of the total power at full load, but in a series or in a compound

generator it will vary in proportion to the load.

HAWKINS PRACTICAL LIBRARY OF ELECTRICITY

IN HANDY POCKET FORM PRICE $1 EACH

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=ELECTRICAL GUIDE, NO. 1=

Containing the principles of Elementary Electricity, Magnetism,

Induction, Experiments, Dynamos, Electric Machinery.

=ELECTRICAL GUIDE, NO. 2=

The construction of Dynamos, Motors, Armatures, Armature

Windings, Installing of Dynamos.

=ELECTRICAL GUIDE, NO. 3=

Electrical Instruments, Testing, Practical Management of Dynamos

and Motors.

=ELECTRICAL GUIDE, NO. 4=

Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,

Storage Batteries.

=ELECTRICAL GUIDE, NO. 5=

Principles of Alternating Currents and Alternators.

=ELECTRICAL GUIDE, NO. 6=

Alternating Current Motors, Transformers, Converters, Rectifiers.

=ELECTRICAL GUIDE, NO. 7=

Alternating Current Systems, Circuit Breakers, Measuring

Instruments.

=ELECTRICAL GUIDE, NO. 8=

Alternating Current Switch Boards, Wiring, Power Stations,

Installation and Operation.

=ELECTRICAL GUIDE, NO. 9=

Telephone, Telegraph, Wireless, Bells, Lighting, Railways.

=ELECTRICAL GUIDE, NO. 10=

Modern Practical Applications of Electricity and Ready Reference

Index of the 10 Numbers.

=Theo. Audel & Co., Publishers. 72 FIFTH AVENUE=,

=NEW YORK.=