CONVERTERS
The alternating current must change to a direct current in many cases
as in railroad work because the induction motor is not so satisfactory
as the direct current series motor and the alternating current series
motor is slow in coming into general use.
In all kinds of electrolytic work, transformation must be made, and
in many cities where the direct current system was started, it is
still continued for local distribution, but the large main stations
generating alternating currents and frequently located some distance
away from the center of distribution have replaced a number of small
central stations.
Transformation may be made by any of the following methods:
1. Rotary converters;
2. Motor generator sets;
3. [A]Mercury vapor rectifiers;
4. [9]Electrolytic rectifiers.
[9] NOTE.--Rectifiers are explained in detail in Chapter LIV.
Strictly speaking, _a converter is a revolving apparatus for converting
alternating current into direct current or vice versa_; it is usually
called a rotary converter and is to be distinguished from the other
methods mentioned above.
Broadly, however, a converter may be considered as _any species of
apparatus for changing electrical energy from one form into another_.
According to the standardization rules of the A. I. E. E. converters
may be classified as:
1. Direct current converters;
2. Synchronous converters;
3. Motor converters;
4. Frequency converters;
5. Rotary phase converters.
[Illustration: FIGS. 2,032 and 2,033.--Gramme ring dynamo and
alternator armatures illustrating converter operation. The current
generated by the dynamo is assumed to be 100 amperes. Now, suppose, an
armature similar to fig. 2,032 to be revolving in a similar field, but
let its windings be connected at two diametrically opposite points to
two slip rings on the axis, as in fig. 2,032. If driven by power, it
will generate an alternating current. As the maximum voltage between
the points that are connected to the slip rings will be 100 volts, and
the virtual volts (as measured by a voltmeter) between the rings will
be 70.7 (= 100 ÷ √2̅), if the power applied in turning this armature
is to be 10 kilowatts, and if the circuit be non-inductive, the output
in virtual amperes will be 10,000 ÷ 70.7 = 141.4. If the resistances
of each of the armatures be negligibly small, and if there be no
frictional or other losses, the power given out by the armature which
serves as motor will just suffice to drive the armature which serves as
generator. If both armatures be mounted on the same shaft and placed in
equal fields, the combination is a ~motor dynamo~. In actual machines
the various losses are met by an increase of current to the motor.
Since the armatures are identical, and as the similarly placed windings
are passed through identical magnetic fields, one winding with proper
connections to the slip rings and commutator will do for both. In this
case only one field is needed; such a machine is called a ~converter~.]
~A direct current converter~ converts from a direct current to
a direct current.
~A synchronous converter~ (commonly called a _rotary
converter_) converts from an alternating current to a direct
current.
~A motor converter~ is a combination of an induction motor with
a synchronous converter, the secondary of the former feeding
the armature of the latter with current at some frequency other
than the impressed frequency; that is, it is a synchronous
converter in combination with an induction motor.
~A Frequency Converter~ (preferably called a _frequency
changer_) converts alternating current at one frequency into
alternating current of another frequency with or without a
change in the number of phases or voltages.
~A Rotary Phase Converter~ changes alternating current of one
or more phases into alternating current of a different number
of phases, but of the same frequency.
[Illustration: FIG. 2,034.--Diagram of ring wound single phase rotary
converter. It is a combination of a synchronous motor and a dynamo. The
winding is connected to the commutators in the usual way, and divided
into two halves by leads connecting segments 180° apart to collector
rings. A bipolar field is shown for simplicity; in practice the field
is multipolar and energized by direct current.]
~Rotary Converters.~--The synchronous or rotary converter consists of
a synchronous motor and a direct current generator combined in one
machine. It resembles a direct current generator with an unusually
large commutator and an auxiliary set of collector rings.
~Ques. In general, how does a rotary converter operate?~
Ans. On the collector ring side it operates as a synchronous motor,
while on the commutator side, as a dynamo.
Its design in certain respects is a compromise between
alternating current and direct current practice most noticeably
with respect to the number of poles and speed.
~Ques. Upon what does the speed depend?~
Ans. Since the input side consists of a synchronous motor, the speed is
governed by the frequency of the alternating current supplied, and the
number of poles.
[Illustration: FIG. 2,035.--Diagram of two phase rotary converter. This
is identical with the single phase machine with the exception that
another pair of collector rings are added, and connected to points
on the winding at right angles to the first, giving four brushes on
the alternating side for the two phase current. The pressure will be
the same for each phase as in the single phase rotary. Neglecting
losses the current for each phase will be equal to the direct
current × 1 ÷ / √2̅ = direct current × .707.]
Fig. 2,034 is a diagram of a ring wound rotary converter. This
style winding is shown to simplify the explanation. In practice
drum wound armatures are used, the operation, however, is the
same.
With this simple machine the following principles can be
demonstrated:
1. If the coil be rotated, alternating currents can be taken
from the collector rings and it is called an alternator.
2. By connecting up the wires from the commutator segments,
a direct current will flow in the external circuit making a
dynamo.
3. Two separate currents can be taken from the armature, one
supplying alternating current and the other direct current;
such a machine is called a _double current generator_.
4. If a direct current be sent in the armature coil through the
commutator, the coil will begin to rotate as in a motor and an
alternating current can be taken out of the collector rings.
Such an arrangement is called an _inverted rotary converter_.
5. If the machine be brought up to synchronous speed by
external means and then supplied with alternating current at
the collector rings, then if the direction of the current
through the armature coil and the pole piece have the proper
magnetic relation, the coil will continue to rotate in
synchronism with the current. A direct current can be taken
from the commutator, and when used thus, the machine is called
simply a _rotary converter_.
[Illustration: FIG. 2,036.--Diagram of three phase rotary converter. In
this type, the winding is tapped at three points 120° distant from each
other, and leads connected with the corresponding commutator segments.]
[Illustration: FIGS. 2,037 to 2,041.--Various rotary converter and
transformer connections. Fig. 2,037 two phase connections; fig. 2,038
three phase delta connections; fig. 2,039 three phase ~Y~ or star
connections; fig. 2,040 six phase delta connections; fig. 2,041 six
phase ~Y~ connections.]
~Ques. What is the relation between the impressed alternating pressure
and the direct pressure at the commutator?~
Ans. The ratio between the impressed alternating pressure and the
direct current pressure given out is theoretically constant, therefore,
the direct pressure will always be as 1 to .707 for single phase
converters or if the pressure of the machine used above indicate 100
volts at the direct current end, it will indicate 70.7 volts at the
alternating current side of the circuit.
~Ques. Name two different classes of converter.~
Ans. Single phase and polyphase.
~Ques. What is the advantage of polyphase converters?~
Ans. In the majority of cases two or three phase converters are used on
account of economy of copper in the transmission line.
~Ques. How is the armature of a polyphase converter connected?~
Ans. Similar to that of an alternator with either delta or ~Y~
connections.
Figs. 2,037 to 2,041 show various converter connections between
the collector rings and commutator.
Fig. 2,037 indicates how the armature is tapped for two phase
connections.
Fig. 2,038 shows three phase delta connections, and fig. 2,039
the three phase ~Y~ or star connections.
Six phase delta and ~Y~ connections are frequently used as
shown in fig. 2,040 and fig. 2,041, both of which require
two secondary coils in the transformer, one set of which is
reversed, so as to supply the current in the proper direction.
~Ques. With respect to the wave, what is the relation between the
direct and alternating pressures?~
Ans. The direct current voltage will be equal to the crest of the
pressure wave while the alternating voltage will depend on the virtual
value of the maximum voltage of the wave according to the connections
employed.
Table of
Alternating Current and Voltage in Terms of Direct Current
(According to Steinmetz)
------------------------------------------------------------------------
| | DIRECT| SINGLE | TWO | THREE |
| |CURRENT| PHASE | PHASE | PHASE |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
|COLLECTOR RING | 1 | | | |
|AND NEUTRAL POINT| |1/(2√2̅)=.354|1/(2√2̅)=.354| 1/(2√2̅)=.354 |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
| ADJACENT | 1 | 1/√2̅=.707 |½=.5 | √3̅/(2√2̅)=.612|
|COLLECTOR RINGS | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
| AMPERES | 1 | √2̅=1.414 | 1/√2̅=.707 | (2√2̅)/3=.943 |
| PER LINE | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
|AMPERES BETWEEN | 1 | √2̅=1.414 |½=.5 |(2√2̅)/(3√3̅)=.545|
|ADJACENT LINES | | | | |
------------------------------------------------------------------------
-------------------------------------------------------------
| | SIX | TWELVE | n |
| | PHASE | PHASE | PHASE |
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
|COLLECTOR RING | | | |
|AND NEUTRAL POINT|1/(2√2̅)=.354|1/(2√2̅)=.354|1/(2√2̅)=.354|
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
| ADJACENT |1/(2√2̅)=.354| .183 | (SIN(π/n))/√2̅|
|COLLECTOR RINGS | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
| AMPERES | √2̅/3=.472 | .236 |(2√2)̅/n |
| PER LINE | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
|AMPERES BETWEEN | √2̅/3=.472 | .455 |(√2̅SIN(π/n))/n|
|ADJACENT LINES | | | |
-------------------------------------------------------------
In a single phase rotary, the value of the direct pressure is 1
to .707, therefore a rotary which must supply 600 volts direct
current must be supplied by 600 × .707 = 424 volts alternating
current. For three phase rotaries the ratio is 1 to .612, or
in order to produce 600 volts direct current, 600 × .612 = 367
volts on the alternating current side of the rotary is required.
[Illustration: FIG. 2,042.--Westinghouse rotary converter armature
coils. These are wound from bar copper and are interchangeable. The
armature coils are heavily insulated to withstand the tests specified
in the standardization rules of the American Institute of Electrical
Engineers.]
Fig. 2,034 shows a complete diagram of the electrical
connections. A single phase rotary is illustrated so as to
simplify the wiring.
The table of Steinmetz on page 1,464 gives the values of the
alternating volts and amperes in units of direct current.
~Ques. How is the voltage of a rotary varied on the direct current
side?~
Ans. Pressure or potential regulators are put in the high tension
alternating current circuit and may be regulated by small motors
operated from the main switchboard or operated by hand.
~Ques. What is the advantage of unity power factor for rotary
converters?~
Ans. It prevents overheating when the rotary is delivering its full
load in watts.
~Ques. What greatly influences the power factor of the high tension
line?~
Ans. The strength of the magnetic field.
[Illustration: FIG. 2,043.--Westinghouse rotary converter armature
spider. It is made of cast iron or cast steel. The dovetail grooves
are machined in the feet or ends of the arms and in these slots the
laminations forming the armature coil engage.]
~Ques. Does variation of the field strength materially affect the
voltage?~
Ans. No.
Since variation of the field strength does not materially
affect the voltage, by adjusting the resistance in series with
the magnetic circuit, the strength of the field can be changed
and the power factor kept 1 or nearly 1 as different loads are
thrown on and off the rotary.
~Ques. What is the effect of a field too strong or too weak?~
Ans. If too strong, a leading current is produced, and if too weak,
the current lags, both of which reduce the power factor and are
objectionable.
Usually there is a power factor meter connected up in the main
generating station and one also in the rotary substation, and
it is the duty of the attendant at the substation to maintain
the proper power factor.
~Ques. What is the ordinary range of sizes of rotaries~?
Ans. From 3 kw. to 3,000 kw.
[Illustration: FIG. 2,044.--Equalizer connections of Westinghouse
rotary converter. The armature coils are cross connected at points
of equal voltage and taps are led out from the winding at suitable
points to the slip rings. This construction insures a uniform armature
saturation below each pole piece and eliminates one cause of sparking
at the commutator.]
~Ques. What is the general construction of a rotary converter?~
Ans. It is built similar to a dynamo with the addition of suitable
collector rings connected to the armature windings at points having the
proper phase relations.
Standard rotary converters have been developed for 25 and 60
cycles. The standard railway machines are compound wound, the
series field being designed for a compounding of 600 volts at
no load and full load when supplied from a source of constant
pressure with not more than 10 per cent. resistance drop and
with 20 to 30 per cent. reactance in the circuit. The large
size machines are usually wound for six phase operation.
[Illustration: FIGS. 2,045 and 2,046.--Westinghouse pole construction
for converters. Fig. 2,045, pole without windings; fig 2,046, pole with
windings. Poles are built up of steel laminations held together with
rivets. Projections on the inner ends of the poles form seats for the
field coils and hold them in position. Copper dampers set in slots in
the pole faces insure stable operation. Rotary converters for railway
service are almost invariably compound wound. The series windings are
formed of bare copper strap. The shunt windings are of insulated copper
strap or wire. Spaces between coil turns and sections are provided for
ventilation.]
~Compounding of Rotary Converters.~--Compounding is desirable where the
load is variable, such as is the case with interurban railway systems.
The purpose of the compounding is to compensate automatically for the
drop due to line, transformer, and converter impedance.
On account of the low power factor caused by over compounding, and
the fact that substations are customarily connected to the trolley at
its nearest point without feeder resistance, over compounding is not
recommended. An adjustable shunt to the series field is provided with
each machine.
Shunt wound converters are satisfactory for substations in large
cities and similar installations where due to the larger number of car
units demanding power, the load is more nearly constant.
~Ratio of Conversion.~--The relation between the alternating and
direct current voltages varies slightly in different machines, due to
differences in design. The best operating conditions exist when the
desired direct current voltage is obtained with unity power factor at
the converter terminals when loaded.
[Illustration: FIG. 2,047.--Westinghouse rotary converter brush rigging
showing method of bracing the brushes. The brushes are supported by
a rigid cast iron rocker ring which fits accurately in the frame.
A handwheel worm and screw arrangement for shifting the brushes is
provided. Cast iron arms bolted to, but insulated from the rings, carry
the rods on which the brush holders are mounted. Brush holders are of
brass cast in one piece, of the sliding type and have braided copper
shunts. Brush tension is adjustable.]
[Illustration: FIG. 2,048.--Westinghouse commutating pole rotary
converter. The construction details are substantially the same as for
the railway converter, with exception of the commutating poles. The
application of commutating pole converters is particularly desirable
where special requirements such as great overload capacity or large
capacity and low voltage enable them to show to the greatest advantage.
Commutating poles as applied to rotary converters fulfill the same
functions as in the more familiar applications to dynamos and motors.
That is, the commutating pole insures sparkless commutation from no
load to heavy overloads with a fixed brush position. Brush shifting
devices are not furnished on commutating pole converters. Commutating
pole rotary converters for railway service are normally arranged for
automatic compounding which is effected by the proper combination of
series excitation and inductance between the generator and the rotary
converter. This inductance is normally included in the transformer
but in special cases may be partly in a transformer and partly in a
separate reactance. It is possible to produce by this means a slight
increase in the direct current voltage provided the voltage drop
in the alternating current line be not excessive. Usually it is so
arranged that the compounding that can be obtained is just sufficient
to overcome the alternating current line voltage drop. The standard
Westinghouse method of starting is alternating current self-starting.
With this method of self-starting, the brushes of a commutating pole
rotary converter must be lifted from the commutator during the starting
operation to prevent sparking. A mechanical device, as shown in fig.
2,050, is provided which accomplishes this. With direct current or
motor starting a brush lifting device is not necessary.]
~Ques. Upon what does the ratio of conversion depend?~
Ans. Upon the number of phases and method of connecting the windings.
For single phase or two phase machines it is 1 to .7; for three
phase, 1 to .612, or six phase, 1 to .7 or 1 to .613 depending
upon the kind of connection used for the transformer.
For example, a two phase rotary receiving alternating current
at 426 volts will deliver direct current at 600 volts, while a
three phase rotary receiving alternating current at 367 volts
will deliver direct current at 600 volts.
[Illustration: FIG. 2,049.--Commutating pole of Westinghouse
commutating pole rotary converter. The commutating poles are similar in
general construction to the main poles. The coils are of bare copper
strap wound on edge. Ventilating spaces are provided between the pole
and coil and between turns. The copper winding is bare except for a few
turns at each end. Insulating bolts retain the turns in their proper
position.]
~Ques. What difficulty would be encountered if other ratios of
conversion than those given above were required?~
Ans. An armature with a single winding could not be used.
It would be necessary to use a machine with two distinct
armature windings or else a motor generator set.
~Ques. What change in voltage is necessary between a converter and the
alternator which furnishes the current?~
Ans. The voltage must be reduced to the proper value by a step down
transformer.
~Voltage Regulation.~--As the ratio of the alternating to the direct
current voltage of a converter is practically constant, means must be
provided to compensate for voltage variation due to changes of load in
order to maintain the direct current pressure constant.
[Illustration: FIG. 2,050.--Westinghouse brush lifting device for
commutating pole rotary converter. A rack is attached to each brush as
shown. Into this rack the spring hinged lifting hook of the raising
device engages only when the lifting lever is shifted toward the
raised position. The lifting arrangement is independent of the brushes
during normal running, so it can in no way affect the operation of the
machine. Each brush is merely raised and lowered within its own holder
so the brush position or commutation is not altered.]
There are several methods of doing this, as by:
1. Shifting the brushes (objectionable);
2. Split pole method;
3. Regulating pole method;
4. Reactance method;
5. "Multi-tap" transformer method;
6. Synchronous regulator.
~Shifting the Brushes.~--Were it not for the difficulties encountered,
this would be a most convenient method of voltage regulation, since by
this procedure the direct current voltage may be varied from maximum to
zero. It is, however, not practical because of the excessive sparking
produced when the brushes are shifted out of the neutral plane.
[Illustration: FIGS. 2,051 to 2,053.--Woodbridge split pole rotary
converter. Each pole is split into three sections and provided with
windings as indicated in fig. 2,051. When excited as in fig. 2,052, the
commutator voltage is at its highest value; when excited as in fig.
2,053, the commutator voltage is low. The change in commutator voltage
for constant collector ring voltage is in virtue of the property of
rotary converters that the ratio of these two voltages is a function of
the width of the pole arc.]
~Split Pole Method.~--In order to overcome the difficulty encountered
in shifting the brushes the split pole method was devised by Woodbridge
in which each field pole is split into two or three parts.
The effect of this is the same as shifting the brushes except that no
sparking results.
The other part is arranged so that its excitation may be varied, thus
shifting the resultant plane of the field with respect to the direct
current brushes.
One of these parts is permanently excited and it produces near its edge
the fringe of field necessary for sparkless commutation.
~Regulating Pole Method.~--As applied to the rotary converter
regulating poles fulfill the same functions as commutating or
interpoles (see page 385) on motors and dynamos, that is, they insure
sparkless commutation from no load to heavy overloads with a fixed
brush position.
[Illustration: FIG. 2,054.--General Electric regulating pole rotary
converter. The field structure is divided into two parts, a main pole
and a regulating pole. The ratio between the voltages on the direct
current and alternating current sides may be readily varied by varying
the excitation of the regulating poles, the only auxiliary apparatus
required being a field rheostat for controlling the exciting current.
Where automatic regulation is required, machines may be provided with
compound windings, or automatic field regulators may be used responsive
to either voltage or current. These converters are adapted for a
variety of purposes where a variable conversion ratio is required,
either to maintain constant D. C. voltage with varying A. C. voltage
or to vary the D. C. voltage as required. Converters may be operated
inverted where it is required to furnish constant or variable A. C.
voltage from a D. C. source. Where converter and inverted converter
operation are desired, an opposite direction of rotation is required
for the inverted operation. Converters of this type are built in
capacities from 300 kw. up to 3,000 kw., and constructed to give a
voltage range between 240 and 300 volts, to cover the usual lighting
circuit requirements. In design, they are similar to standard rotary
converters, with the exception that the regulating poles are located
next to the main pole pieces and a slightly different form of pole
piece bridge is used for the main poles, in order to allow the
auxiliary poles to be readily removed or assembled.]
The regulating poles are used in order to vary the ratio between the
alternating current collector rings and the direct current side without
the use of auxiliary apparatus such as induction regulators or dial
switches which involve complicated connections and many additional
wires. The regulating poles are arranged with suitable connection so
that the current through them can be raised, lowered or reversed.
[Illustration: FIG. 2,055.--Detail of Westinghouse commutating pole
rotary converter brush, showing rack. The brush lifting mechanism and
its operation is explained in fig. 2,050.]
The characteristics of the regulating pole converter being novel, a
detailed explanation of the principles involved is given to facilitate
a clear understanding of its operation.
Consider a machine with a field structure as shown in fig.
2,056 resembling in appearance a machine with commutating
poles, but with the brushes so set that one of the regulating
poles adds its flux to that of one main pole, cutting the
inductors between two direct current brushes. The regulating
pole is shown with a width equal to 20 per cent. of that of the
main pole.
To obtain definite figures, it will be assumed that the machine
at normal speed, with the main poles excited to normal density,
but with no excitation on the regulating poles, gives 250 volts
direct current pressure. Then with each regulating pole excited
to the same density as the main poles, and with a polarity
corresponding to that of the main pole in the same section
between brushes, the direct current pressure will rise to 300
volts at the same speed, since the total flux cutting the
inductors in one direction between brushes has been increased
20 per cent.
If, on the other hand, the excitation of the regulating poles
be reversed and increased to the same density as that of the
main poles, the direct current pressure will fall to 200
volts, since in this case the regulating poles give a reverse
pressure, that is, a pressure opposing that generated by the
main poles.
[Illustration: FIG. 2,056.--Diagram of field of regulating pole
converter illustrating principles explained in the accompanying text.]
Now, if the machine be equipped with collector rings, that is,
if it be a converter, this method of varying the direct current
voltage from 200 to 300 volts does not give nearly as great
a variation of the alternating current voltage; in fact, the
latter voltage will be the same when delivering 200 volts as
when delivering 300 volts direct current pressure, if the field
excitation be the same.
This may be seen by reference to fig. 2,057, which is a diagram
of the alternating current voltage developed in the armature
windings by the two sets of poles.[10]
[10] NOTE.--In the Burnham split pole rotary converter, each pole is
divided into only two sections, one larger than the other. A main shunt
winding is arranged on the large sections, and a winding for providing
the voltage regulation is placed on the other section. When the current
is sent through this latter winding in one direction the voltage is
raised, when in the other direction the voltage is lowered.
The horizontal line OA represents the alternating current
voltage generated by the main poles, alone, with the
regulating poles unexcited, that is, when delivering 250 volts
direct current pressure.
For a six phase converter OA measures about 180 volts
diametrically, that is, between electrically opposite collector
rings.
If now the regulating poles be excited to full strength,
to bring the direct current pressure up to 300 volts, the
alternating current voltage generated by the regulating poles
will be 90 degrees out of phase with that generated by the main
poles (since they are placed midway between the main poles),
and will be about 40 volts as shown by the line AB.
The resultant alternating current volts across the collector
rings will be represented by the line OB with a value equal to
184.
[Illustration: FIG. 2,057.--Voltage diagram for regulating pole
converter illustrating principles explained in the accompanying text.]
Again, if the regulating poles be reversed at full strength,
to cut the direct current pressure down to 200 volts, the
alternating current voltage of the main and regulating poles
will be OA and AC respectively, giving the resultant OC equal
to OB with a value of 184 volts. Accordingly, the direct
current pressure may be either 200 or 300 volts with the same
alternating current pressure, and if the main field be kept
constant, the direct current pressure may range between 200 or
300 volts, while the alternating current pressure varies only
between 180 and 184 volts.
The alternating current pressure can be kept constant through
the full range of direct current voltage by changing the main
field so as always to give an equal and opposite flux change to
that of the regulating field. A constant total flux may thus
be obtained equal to the radius of the arc BC, fig. 2,057. In
this case the line OA, representing the main field strength,
will equal OB when the regulating field is not excited, and 250
volts can only be obtained at this adjustment.
This method of operation gives unity power factor with a
constant impressed pressure of 184 volts alternating current
with a range of direct current voltage from 200 to 300 volts.
[Illustration: FIGS. 2,058 and 2,059.--Diagrams illustrating the effect
on the alternating current voltage due to varying the regulating field
strength (of a machine proportioned according to fig. 2,060), from a
density equal to that in the main poles to the same density reversed,
the main field strength remaining constant. The D. C. voltage in this
case varies from 30 per cent. above that produced by the main field
alone to 30 per cent. below, or from 325 to 175 volts, while the A. C.
voltage varies only from 200 to 175 volts. To keep the A. C. voltage
constant with such a machine the main field must be strengthened as the
regulating field is weakened or reversed to reduce the D. C. voltage.
This strengthening increases the core loss particularly on low direct
current voltages, which however, are rarely required, hence a machine
proportioned as in fig. 2,060, would not be operated through so wide a
range as 175 to 325 volts. Assume that the range is 240 to 300 volts,
and that at the highest voltage, both main and regulating fields have
the same density, presenting to the armature practically one continuous
pole face of uniform flux intensity. The diagram of A. C. component
voltages to give constant A. C. resultant voltage across the rings for
the case, is shown in fig. 2,059. At 300 volts D. C., the main field
produces an A. C. voltage OA, and the regulating field, a voltage AB,
with a resultant OB, equal to about 200 volts A. C. At 270 volts D. C.,
the main field produces an A. C. voltage OA, and a regulating field
voltage AB, giving a resultant A. C. voltage OB, equal to 200 volts.
Similarly, at 240 volts D. C., the main field produces an A. C. voltage
OA, and the regulating field (now reversed) produces the reverse
voltage AB, giving the resultant OB again equal to 200 volts. It will
be noted that, theoretically the main field strength must be increased
about 15 per cent. above its value at 300 volts D. C. in order to keep
the D. C. voltage at 250 volts.]
~Ques. Where should the regulating poles be located for best results?~
Ans. A better construction is obtained by placing them closer to the
corresponding main pole, as in fig. 2,060, than when spaced midway
between the main poles as in fig. 2,056.
~Ques. When the regulating poles are spaced as in fig. 2,060, what is
the effect on the direct current voltage?~
Ans. The effect is the same as for the midway position (fig. 2,056)
except for magnetic leakage from the main poles to the regulating poles
when the latter is opposed to the former, that is, when the direct
current voltage is being depressed.
[Illustration: FIG. 2,060.--Diagram illustrating placement of
regulating poles. In practice machines are not built as indicated
diagrammatically in fig. 2,056, that is, with regulating poles spaced
midway between the main poles, because a better construction is
obtained by placing the regulating pole closer to the corresponding
main pole, as shown above.]
~Ques. What is the effect on the alternating current voltage?~
Ans. It is somewhat altered as explained in figs. 2,058 and 2,059.
~Reactance Method.~--This consists in inserting inductance in the
supply circuit and running the load current through a few turns around
the field cores. This method is sometimes called _compounding_, and
as it is automatic it is generally used where there is a rapidly
fluctuating load.
[Illustration: FIG. 2,061.--Westinghouse 300 kw., 1,500 volt, three
phase, 25 cycle, commutating pole rotary converter. The illustration
shows clearly the commutating, and main poles and the relative sizes,
also arrangement of the terminal connections.]
If a lagging current be passed through an inductance, the collector
ring voltage will be lowered, but will be raised in case of a leading
current. The degree of excitation governs the change in the phase of
the current to the converter, the excitation, in turn, being regulated
by the load current. Accordingly with series inductance, the effect of
the series coils on the field of the converter is quite similar to that
of the compounding of the ordinary railway dynamo.
~Multi-tap Transformer Method.~--The employment of a variable ratio
step down transformer for voltage regulation is a non-automatic method
of control and, accordingly, is not desirable except in cases where the
load is fairly constant over considerable periods of time. It requires
no special explanation.
[Illustration: FIG. 2,062.--Mechanical oscillator and speed limit
device of Westinghouse commutating pole rotary converter. It
automatically prevents the armature of the converter remaining in
one position and thus not allowing brushes to wear grooves in both
commutator and collector rings. The oscillator is a self-contained
device carried at one end of the shaft. The operating parts consist
of a hardened steel ball and a steel plate with a circular ball
race, backed by a spring. The machine is so installed with a slight
inclination toward the end carrying the oscillator. As the armature
revolves the ball is carried upward and owing to the convergence of
the steel race and shaft face, the spring is compressed. The reaction
of the spring forces the armature away from its natural position and
allows the ball to drop back to the lowest point of the race.]
~Synchronous Booster Method.~--This consists of combining with the
converter a revolving armature alternator having the same number of
poles.
~Ques. How is the winding of the booster alternator armature connected?~
Ans. It is connected in series with the input circuits on the converter.
[Illustration: FIG. 2,063.--Westinghouse 2,000 kw., 270 volt, direct
current, 6 phase, 167 R.P.M., synchronous booster rotary converter,
having a voltage range from 230 to 310 volts. It consists of a standard
rotary converter in combination with a revolving armature alternator
mounted on the same shaft with the rotary converter and having the same
number of poles. By varying the field excitation of the alternator,
the alternating current voltage impressed on the rotary converter
can be increased or decreased as desired. The direct current voltage
delivered by the converter is thereby varied accordingly. The principle
of operation of the booster converter is therefore very simple and
easily understood. It is simply a combination of two standard pieces
of electrical apparatus, accordingly there are incorporated in it no
details of construction essentially different from those encountered
in standard rotary converters and alternators. The only novelty is in
their combination. The frames may be supported either from the rotary
converter frame, as in the small units, or from the bed plate, as in
the larger ones. A synchronous booster converter can be built, if
necessary, with a vertical shaft to satisfy special floor space and
head room requirements.]
~Ques. How are the field windings connected?~
Ans. They are either fed with current regulated by means of a motor
operated field circuit rheostat, or joined in series with the
commutator leads of the converter.
[Illustration: FIG. 2,064.--Armature of Westinghouse synchronous
booster converter. Heavy cast yokes form the frames. They are
proportioned to rigidly support the laminated steel field poles. The
poles are fastened to the frame with through bolts. A lifting hook is
provided on all frames. The bed plates are in one piece for the smaller
machines but two piece bed plates are used for the larger ones. The
bearings are ring oiling and have babbitt wearing surfaces that are
renewable.]
[Illustration: FIG. 2,065.--Westinghouse field frames and bearings for
synchronous booster rotary converter. The frames consist of heavy cast
yokes. The poles are fastened to the frames with through bolts. The bed
plate is in one piece for the small machines and in two pieces for the
large ones.]
~Ques. For what service is the synchronous booster method desirable?~
Ans. For any application where a relatively wide variation in direct
current voltage is necessary.
It is particularly desirable for serving incandescent lighting
systems where considerable voltage variation is required for
the compensation of drop in long feeders, for operation in
parallel with storage batteries and for electrolytic work where
extreme variations in voltage are required by changes in the
resistance of the electrolytic cells.
[Illustration: FIG. 2,066.--General Electric motor generator set
consisting of 2,300 volt synchronous motor and 550 volt dynamo.]
~Motor Generator Sets.~--The ordinary rotary converter is the most
economical machine for converting alternating currents into direct
currents, and where slight variations in the direct current voltage is
necessary, they are mostly used on account of their high efficiency,
and because they are compact.
[Illustration: FIG. 2,067.--General Electric motor generator set
consisting of 230 volt induction motor and 125 volt dynamo.]
In many central stations where they supply a great variety of
apparatus, the motor generator sets are employed as the generator is
independent of the alternating current line voltage and any degree of
voltage regulation can be performed.
~Motor Generator Combinations.~--The following combinations of motor
generators are made and used to suit local conditions:
Synchronous motor dynamo
Induction motor dynamo
Direct current motor dynamo
Direct current motor alternator
Synchronous motor alternator
Induction motor alternator
[Illustration: FIG. 2,068.--General Electric generator set, as
installed for the Cleveland Electric Illuminating Company, Cleveland,
Ohio. It consists of 11,431 volt motor and 275 volt generator. Speed,
360 revolutions per minute.]
Standard practice has adopted high tension alternating current for
transmission systems, but direct current distribution is very
frequently used. This is particularly true where alternating current
apparatus has been introduced in old direct current lighting systems.
The synchronous motor or the induction motor connected to a generator
stands next in importance to the rotary converter because it is easy
to operate and the pressure may be changed by a rheostat placed in the
field circuit of the generator.
The line wires carrying full voltage can usually be connected direct
to the motor and thus do away with the necessary step-down transformer
required by the rotary.
~Ques. What is the behavior of a rotary converter when hunting?~
Ans. It is liable to flash over at the direct current brushes, which is
common in high frequency converters where there are a great number of
poles and the brushes are necessarily spaced close together around the
commutator.
~Ques. Is this fault so pronounced with motor generator sets?~
Ans. The motor generator operating on a high frequency circuit, the
generator can be designed with a few poles and the brushes set far
apart which will greatly reduce the chance of flashing over.
A synchronous motor will drive a generator at a constant speed
during changes in load on it, and by having a field regulating
resistance it can be used to improve the power factor of the
system.
When an induction motor is used its speed drops off slowly
as the load comes on the generator, and it is necessary to
regulate the voltage of the generator by means of a field
rheostat, or compound wound machines may be used.
While an induction motor requires no separate excitation of the
field magnets like the synchronous motor, its effect on the
power factor of the system is undesirable.
Although it is seldom necessary to convert direct current to
alternating, such an arrangement of a direct current motor
driving an alternator is often justified in place of an
inverted rotary converter, as in this case the alternating
current voltage can be changed independent of the direct
current voltage.
The racing of an inverted rotary under a heavy inductive load
or short circuit does not take place in motor generator set
mentioned above.
[Illustration: FIG. 2,069.--General Electric frequency changer set,
consisting of a 11,000 volt synchronous motor with direct connected
exciter and a 2,300 volt alternator.]
~Frequency Changing Sets.~--A frequency of 25 cycles is generally used
on railway work and in large cities using the Edison three wire system,
and as a 25 cycle current is not desirable for electric lighting it is
necessary to change it to 60 cycles by means of a frequency changer
shown in fig. 2,069 for distribution in the outlying districts.
The two machines in this combination are of the same construction, only
the synchronous motor would have eight poles and have the 25 cycle
current passing through it, while the generator would have 20 poles
and produce 62½ cycles per second at 300 revolutions per minute. By
supplying the motor with 24 cycles, the generator would produce 60
cycles.
[Illustration: FIG. 2,070.--General Electric four unit frequency
changer set consisting of a 11,000 volt synchronous motor, 13,200 volt
alternator, 250 volt exciter, and 440 volt starting induction motor.
Where parallel operation is required between synchronous motor driven
frequency changers, a mechanical adjustment is necessary between
the fields or armatures of the alternator and motor to obtain equal
division of the load. The adjustment can be obtained by shifting the
keyway, or by special cradle construction. In the latter method, one
machine is bolted to a cradle fastened to the base. By taking out the
bolts, the frame can be turned around through a small angle relative to
the cradle and therefore to the armature frame of the other machine,
when the bolts can be replaced.]
It will be seen from the figure that the separate exciter is fastened
on the base plate and has its armature directly connected to the shaft.
~Parallel Operation of Frequency Changers.~--It is very difficult to
construct two or more frequency changers and join them to synchronous
motors so that the current wave of one machine will be in phase with
the other, since the speed of the motor will depend on the frequency of
the line and be independent of the load thrown on it.
When alternators are run in parallel, if one machine lag behind, the
other carries the load with the result that the lightly loaded machine
will speed up and get in step with the other, or in other words a
synchronizing current will flow between the two alternators and tend to
keep them in proper relation with respect to phase and load.
[Illustration: FIG. 2,071.--Diagram of "Cascade" motor generator
set or motor converter, as it is called in England where it is used
extensively for electric railway work. In the diagram of motor armature
winding, some of the connections are omitted for simplicity. The
windings are ~Y~ connected, and as they are fed by wires joined to the
slip rings at the right and center, the rest of the power passes to the
converter windings back to rotor winding and out to the slip rings so
that part of the power enters the rotor and part through the converter.]
~Cascade Converter.~--This piece of apparatus was introduced by Arnold
and La Cour. Briefly, it consists of a combination of an induction
motor having a wound armature and a dynamo, the armatures being placed
on the same shaft. The windings are joined in cascade, that is, in
series with those of the armature of the induction motor. The line
supplies three phase currents at high voltage direct to the field of
the induction motor and drives it, generating in it currents at a lower
voltage depending on the ratio of the windings.
[Illustration: FIG. 2,072.--General Electric shunt wound booster set.
Sets of this class are used in railway stations to raise the pressure
of the feeders extending to distant points of the system, for storage
battery charging and regulation, and in connection with the Edison
three wire lighting system. The design of the various sets is closely
dependent upon their application. Booster sets are constructed in
either series or shunt wound types and they may be arranged for either
automatic or hand regulation, depending on the nature of the service
required. Where there are a number of lighting feeders connected and
run at full load for only a short time each day it will generally be
economical to install boosters rather than to invest in additional
feeder copper. It is important, however, to consider each case where
the question of installing a booster arises, as a separate problem, and
to determine if the value of the power lost represents an amount lower
than the interest charge on the extra copper necessary to deliver the
same voltage without the use of a booster.]
Part of the current thus generated in the armature passes into the
armature of the dynamo and is _converted_ by the commutator into
direct current as in a rotary converter, but is also increased by the
current induced in the winding of the dynamo armature.
[Illustration: FIG. 2,073.--General Electric 9.5 kw. 1,000 R.P.M.
balancer set. This is a form of direct current compensator, the units
of which it is composed may be alternately motor or generator, and the
secondary circuit is interconnected with the primary. Balancer sets
are widely used to provide the neutral of Edison three wire lighting
systems. They are also installed for power service in connection with
the use of 250 volt motors on a 500 volt service or 125 volt motors
on a 250 volt service. Although the balancer set may be manufactured
for more than one intermediate voltage, the General Electric Company
recommends the three wire system as it is simpler. The motors may
be more equally divided and the reserve margin of the motor is more
conservative for that class of work requiring increase in torque
with decrease in speed. The voltage of the system being given, the
capacity of a three wire balancer set is fixed by the maximum current
the neutral wire is required to carry. This figure is a more definite
specification of capacity than a statement in per cent. of unbalanced
load. As designed for power work and generally for lighting service,
the brushes of each machine are set at the neutral point in order to
get the best results for operating alternately either as a generator or
motor. Where the changes of balance are so gradual as to permit of hand
adjustment if desired, a considerable increase in output is obtainable.]
~Ques. At what speed does the machine run?~
Ans. Assuming equal numbers of pole, the armatures rotate at a speed
corresponding to one half the circuit frequency.
[Illustration: FIGS. 2,074 and 2,075.--General Electric charging sets.
Fig. 2,074 set consists of dynamo and direct current motor; fig. 2,075
set consists of a dynamo and alternating current motor. Fig. 2,074 set
is equipped with 125 volt, shunt wound dynamo and 230 or 550 volt motor
and range in capacity from .125 kw. to 13 kw., the speed varying from
2,250 R.P.M. in the .125 kw. set to 925 R.P.M. in the 13 kw. set with
230 volt motor. Both motor and dynamo have the same type and size of
frame; these are bolted together and form a compact and symmetrical
outfit, no base being necessary. Sets of the type shown in fig. 2,075
range in capacity from .2 kw. to 10 kw. and are equipped with 125 volt
shunt wound dynamo and 110, 220, 440 or 550 volt two or three phase
motors. If desired they can be furnished with single phase motors
wound for 110 or 220 volts. The speed of this type is 1,800 R.P.M.
When a motor generator set is used to charge only one battery, the
insertion of a resistance between the charging dynamo and the battery
is not necessary, inasmuch as all adjustments of voltage can be made
by varying the field strength of the dynamo, and, therefore, there are
no large losses due to resistance since the loss in the dynamo field
rheostat is very small. When a motor generator set is used to charge
two or more batteries of different capacities, or voltages, or which
are in different conditions of charge, it is necessary to insert a
resistance in series with each battery, in order that the current may
be properly adjusted for each particular battery.]
Thus if the motor have six poles and the frequency be 50, the
rotary field revolves at 50 × 60 ÷ 3 = 1,000 R.P.M. and the
motor will revolve at one-half that speed or 1,000 ÷ 2 = 500
R.P.M.
Since the connections are so arranged that these currents
tend to set up in the armature a revolving field, rotating at
half speed in a sense opposite to that in which the shaft is
rotating at half speed, it follows that by the super-position
of this revolving field upon the revolutions of the machine,
the magnetic effect is equivalent to a rotation of the armature
at whole speed, so that it operates in synchronism, as does the
armature of a rotary converter.
Half the electric input into the motor part is, therefore,
turned into mechanical energy to drive the shaft, the other
half acts inductively on the armature winding, generating
currents therein.
As to the dynamo part it is half generator, receiving
mechanical power by transmission along the shaft to furnish
half its output, and it is half converter, turning the currents
received from the armature into direct current delivered at the
brushes.
~Ques. What action takes place in the motor armature winding?~
Ans. Since it runs at one-half synchronous speed, it generates
alternating current of half the supply current frequency, delivering
these to the armature of the dynamo.
~Ques. What claim is made for this type of apparatus?~
Ans. The cost is said to be less than a motor generator set, and it is
claimed to be self-synchronizing and to require no special starting
gear, also to be 2.5 per cent. more efficient than a motor generator.
~Ques. How is the machine started from the high pressure side?~
Ans. The field winding is connected directly to the high pressure
leads. The three slip ring brushes are connected with external
resistances which are used while starting, the external resistances
being gradually cut out of the circuit as the machine comes up to speed
(the same as with an ordinary slip ring motor).
~Ques. How does a cascade converter compare with a synchronous
converter?~
Ans. It is about equally expensive as the synchronous converter with
its necessary bank of transformers, but is about one per cent. less
efficient. It is claimed to be more desirable for frequencies above 40
on account of the improved commutation at the low frequency used in
the dynamo member. For lower frequencies the synchronous converter is
preferable.