REGULATING DEVICES
~Regulation of Alternators.~--Practically all the methods employed
for regulating the voltage of direct current dynamos and circuits,
are applicable to alternators and alternating current circuits. For
example: in order that they shall automatically maintain a constant or
rising voltage with increase of load, alternators are provided with
composite winding similar to the compound winding of direct current
dynamos, but since the alternating current cannot be used directly
for exciting the field magnets, an accessory apparatus is required to
rectify it or change it into direct current before it is used for that
purpose.
It is a fact, however, that composite wound alternators do not regulate
properly for inductive as well as non-inductive loads.
In order to overcome this defect compensated field alternators have
been designed which automatically adjust the voltage for all variations
of load and lag. These machines have already been described.
~Alternating Current Feeder Regulation.~--With slight modification, the
various methods of feeder regulation employed with direct current, may
be applied to alternating current distribution circuits. For instance,
if a non-inductive resistance be introduced in any electric circuit,
the consequent drop in voltage will be equal to the current multiplied
by the resistance. Therefore, feeder regulation by means of rheostats
is practically the same in the case of alternating current as in that
of direct current. In the case of the former, however, the effect of
self-induction may also be utilized to produce a drop in voltage. In
practice, this is accomplished by the use of self-induction coils which
are commonly known as reactance coils.
[Illustration: FIG. 2,414.--Diagram _illustrating the principle of_
~induction voltage regulators. The primary coil~ P, _consisting of many
turns of fine wire_, is connected across the main conductors C and D,
coming from the alternator. ~The secondary coil~ S, _consisting of a
few turns of heavy wire_, is connected in series with the conductor
D. The laminated iron core E, mounted within the coils, is capable of
being turned into the position shown by the dotted lines. When the
core is vertical, the magnetic lines of force produced in it by the
primary coil, induces a pressure in the secondary coil which aids the
voltage; when turned to the position indicated by the dotted lines, the
direction of the magnetic lines of force are reversed with respect to
the secondary coil and an opposing pressure will be produced therein.
Thus, by turning the core, the pressure difference between the line
wires G and H, can be varied so as to be higher or lower than that of
the main conductors C and D. Regulators operating on this principle may
be used for _theatre dimmers_, as controllers for series lighting, and
also to adjust the voltage or the branches of unbalanced three wire
single phase and polyphase systems.]
~Application of Induction Type Regulators.~--In supplying lighting
systems, where the load and consequently the pressure drop in the
line increases or decreases, it becomes necessary to raise or lower
the voltage of an alternating current, in order to regulate the
voltage delivered at the distant ends of the system. This is usually
accomplished by means of _alternating current regulators or induction
regulators_. A device of this kind is essentially a transformer, the
primary of which is excited by being connected directly across the
circuit, while the secondary is in series with the circuit as shown in
fig. 2,414. By this method the circuit receives the voltage generated
in the secondary.
[Illustration: FIG. 2,415.--Diagram of induction regulator ~raising the
voltage~ 10%. In the diagram an alternator is supplying 100 amperes
at 2,200 volts. The regulator raises the feeder pressure to 2,420
volts, the current being correspondingly reduced to 91 amperes, the
other 9 amperes flowing from the alternator through the primary of the
regulator, back to the alternator.]
[Illustration: FIG. 2,416.--Diagram of induction regulator ~lowering
the voltage~ 10%. The diagram shows the regulator lowering the feeder
pressure to 1,980 volts with an increase of the secondary current to
111 amperes, the additional 11 amperes flowing from the feeder, through
the primary back to the feeder.]
~Ques. Name two types of pressure regulator.~
Ans. The induction regulator, and the variable ratio transformer
regulator.
~Ques. Of what does an induction regulator consist?~
Ans. It consists of a primary winding or exciting coil, a secondary
winding which carries the entire load current.
[Illustration: FIG. 2,417.--Moving element or ~primary~ of Westinghouse
motor operated single phase induction regulator. It consists of a core
of punchings built up directly on the primary shaft and carrying the
primary winding, which is divided into four coils. The primary coils
are machine wound and the layers of the winding are separated from each
other by heavy insulating material in addition to the cotton covering
of the inductors. The complete coils are insulated and impregnated with
insulating compound before being placed in the slots. The coils are
held in position by fibre wedges.]
The primary is wound for the full transmission voltage, and is
connected across the line, while the secondary is connected in
series with the line.
~Ques. What is its principle of operation?~
Ans. When the primary coil is turned to various positions the magnetic
flux sent through the secondary coil varies in value, thereby causing
corresponding variation in the secondary voltage, the character of
which depends upon the value and direction of the flux.
~Ques. What is the effect of turning the secondary coil to a position
at right angles with the primary coil?~
Ans. The primary will not induce any voltage in the secondary, and
accordingly it has no effect on the feeder voltage.
~Ques. What is this position called?~
Ans. The neutral position.
~Ques. What are the effects of revolving the primary coil from the
neutral position first in one direction then in the other?~
Ans. Turning the primary in one direction increases the voltage induced
in the secondary, thus increasing the feeder voltage like the action of
a booster on a direct current circuit while turning the primary in the
opposite direction from the neutral position, correspondingly decreases
the feeder voltage.
[Illustration: FIG. 2,418.--Moving element or ~primary~ of Westinghouse
motor operated polyphase induction regulator.]
~Ques. It was stated that for neutral position the primary had no
effect on the secondary; does the secondary have any effect on the
feeder voltage?~
Ans. The secondary tends to create a magnetic field of its own
self-induction, and has the effect of a choke coil.
~Ques. How is this tendency overcome?~
Ans. The primary is provided with a short circuited winding, placed at
right angles to the exciting winding. In the neutral position of the
regulator, this short circuited winding acts like the short circuited
secondary of a series transformer, thus preventing a choking effect in
the secondary of the regulator.
~Ques. What would be the effect if the short circuited winding were not
employed?~
[Illustration: FIG. 2,419.--Top end of stationary element or
~secondary~ of Westinghouse polyphase induction regulator; view showing
leads. The secondary is built up in a short skeleton frame with
brackets for the rotor bearings bolted to the frame and the top cover
bolted to the top brackets. In assembling the secondary, the punchings
are stacked loosely in the skeleton frame and an expanding building
mandrel placed inside the punchings and expanded, thereby truing up the
latter before they are finally compressed and the end plates keyed in
position. Then, prior to removing the mandrel a finishing cut is taken
on the surface of the frame to which the bearing brackets are attached,
and as the top cover and brackets are also accurately machined the
alignment of the primary with the secondary is almost perfect, thus
reducing to a minimum the tendency to develop vibration and noise.]
Ans. The voltage required to face the full load current through the
secondary would increase as the primary is turned away from either the
position of maximum or minimum regulation, reaching its highest value
at the neutral position.
The short circuited winding so cuts down this voltage of
self-induction that the voltage necessary to force the full
load current through the secondary when the regulator is in the
neutral position is very little more than that necessary to
overcome the ohmic resistance of the secondary.
~Ques. What effect is noticeable in the operation of a single phase
induction regulator?~
Ans. It has a tendency to vibrate similar to that of a single phase
magnet or transformer.
[Illustration: FIG. 2,420.--Bottom end of stationary element or
~secondary~ of Westinghouse polyphase induction regulator.]
~Ques. Why?~
Ans. It is due to the action of the magnetizing field varying in
strength from zero to maximum value with each alteration of the
exciting current, thus causing a pulsating force to act across the air
gap, which tends to cause vibration when the moving part is not in
perfect alignment.
~Ques. Explain the effect produced by bad alignment?~
Ans. If the bearings of the primary be not in perfect alignment with
the bore of the secondary, thereby making the air gap on one side
smaller than that on the other, the crowding over of the flux to the
smaller air gap will cause an intermittent pull in that direction,
which will develop vibration unless the primary bearings are tight and
the shaft sufficiently stiff to withstand the pull.
~Ques. Upon what does the regulator capacity for any given service
depend?~
[Illustration: FIG. 2,421.--Westinghouse two kw., hand operated, air
cooled induction regulator for testing purposes.]
Ans. It depends upon the range of regulation required and the total
load on the feeder.
~Ques. How is the capacity stated?~
Ans. In percentage of the full load of the feeder.
For instance, on a 100 kilowatt circuit, a 10 kw. regulator
will give 10 per cent. regulation, and a 5 kw. regulator, 5 per
cent. regulation.
~Polyphase Induction Regulators.~--The polyphase induction regulator
is similar to the single phase regulator except that both the primary
and secondary elements are wound with as many sets of coil as there are
phases in the circuit.
In construction these windings are distributed throughout the complete
circumference of the stationary and moving elements and closely
resemble the windings of an induction motor.
[Illustration: FIG. 2,422.--Westinghouse polyphase motor operated
induction regulator showing operating mechanism. The primary shaft is
turned by means of a bronze worm wheel engaging a forged steel worm,
provided with a ball bearing end thrust. This worm gear is housed in a
separate casting bolted to the cover. The casting is made separate in
order to permit close adjustment between the worm wheel and the worm
to aid in counteracting the tendency to vibration. Finished surfaces
on the worm gear casting are provided for mounting the motor and the
brake. On the automatic regulator, the worm shaft is connected to the
motor through a spur gear and pinion, which constitutes a compact
driving device having very little friction. Provision is made for
either alternating current or direct current motor drive. When a
motor driven regulator is operated by hand, the brake must be held in
the release position, otherwise it will be impossible to operate the
regulator. In the hand operated regulator the spur gear is replaced by
a hand wheel and the regulator is driven directly from the worm shaft.]
Polyphase regulators have but little tendency to vibrate because the
field across the air gap is the resultant of two or more single phase
fields and is of a constant value at all times. This field rotates
at a rate depending upon the number of poles and the frequency of
the circuit. This produces a mechanical pull of constant value which
rotates with the magnetic field varying its position from instant to
instant.
It is evident that this pull is of an entirely different character from
that produced by the single phase field and that there is no tendency
to set up the vibration that the mechanical pull of the single phase
regulator tends to establish.
[Illustration: FIG. 2,423.--General Electric ~adjustable compensation
shunt~. ~It is used~ as the compensating shunt for direct current
voltage regulators. ~In operation,~ the shunt may be adjusted so as
to compensate for any desired line drop up to 15 per cent. It is
preferably placed in the principal lighting feeder but may be connected
to the bus bars so that it will take the total current. The latter
method is sometimes undesirable, as large fluctuating power loads on
separate feeders might disturb the regulation of the lighting feeders.
~Adjustment is made~ _by sliding the movable contact_ shown at the
center of the shunt. This contact may be clamped at any desired point
and it determines the pressure across the compensating winding of the
regulator's control magnet. Where pressure wires are run back to the
central station from the center of distribution, they may be connected
directly to the pressure winding of the main control magnet, and it is
unnecessary to use the compensating shunt.]
There is, however, considerable torque developed, and the device for
revolving the moving element must be liberally designed so as to
withstand the excess torque caused by temporary overloads or short
circuits.
~Ques. In what respects do polyphase induction regulators differ in
principle from single phase regulators?~
Ans. The induced voltage in the secondary has a constant value, and
the regulation is effected by varying the phase relation between the
line voltage and the regulator voltage.
~Ques. How is the primary wound?~
Ans. It is wound with as many separate windings as there are phases in
the circuit, and these primary or shunt windings are connected to the
corresponding phases of the feeder.
[Illustration: FIG. 2,424.--General Electric ~direct current~ (form
S) ~voltage regulator. It consists of~ _a main control magnet, relay,
condenser and reversing switch_, as shown in the diagram fig. 2,428.
This regulator cannot be used for compensation of line drop as the
current coil is omitted; it is not a switchboard instrument, but is
designed for inexpensive installations such as for regulating the
voltage of motor generator sets when the current is taken from a
trolley line or some other fluctuating source. ~The regulating outfit
comprises,~ besides the regulator, one or more condenser sections
according to field discharge, set of iron brackets when regulator
cannot be mounted on front of switchboard, one compensating shunt,
when it is desired to compensate for line drop. Field rheostats having
sufficient resistance to reduce the voltage the proper amount must
be used with voltage regulator installations. To prevent undue decay
at the relay contacts, allow one section for each 15 kw. capacity of
dynamo with laminated poles, and one for each 22 kw. capacity with
solid steel poles.]
~Ques. What kind of magnetizing flux is produced by the primary
windings?~
Ans. A practically constant flux which varies in direction.
~Ques. How is the secondary wound?~
Ans. There is a separate winding for each phase.
~Ques. Why is the voltage induced in the secondary constant?~
Ans. Because of the constant magnetizing flux.
~Ques. How is the line voltage varied by a polyphase regulator?~
Ans. When the regulator is in the position of maximum boost, the line
AB, fig. 2,425 represents the normal busbar voltage, BC the regulator
voltage, and AC the resultant feeder voltage. When the regulator
voltage is displaced 180 degrees from this position, the regulator is
in the position to deliver minimum voltage to the feeder, the regulator
voltage being then represented by BD, and the resultant feeder voltage
by AD. When the regulator voltage is displaced angularly in the
direction BF, so that the resultant feeder voltage AF becomes equal to
the normal busbar voltage AB, the regulator is in the neutral position.
Intermediate resultant voltages for compensating the voltage variations
in the feeders may be obtained by rotating the moving element or
primary in either direction from the neutral position. For example, by
rotating the primary through the angle FBE, the resultant voltage may
be made equal to AE or AJ, thereby increasing the feeder voltage by
an amount BJ; or by rotating it in the opposite direction through the
angle FBG, the feeder voltage may be reduced by an amount BH.
[Illustration: FIG. 2,425.--Diagram illustrating operation of polyphase
induction regulator.]
~Ques. How are induction regulators operated?~
Ans. By hand or automatically.
~Ques. How is automatic operation secured?~
Ans. By means of a small motor, controlled by voltage regulating relays.
~Ques. How is the control apparatus arranged?~
Ans. Two relays are employed with each regulator, a primary relay
connected to the feeder circuit and operating under changes of voltage
therein, and a secondary relay connected between the primary relay and
the motor, and operated by the contacts of the former, for starting,
stopping and reversing the motor in accordance with changes in the
feeder voltage, thereby causing the regulator to maintain that voltage
at its predetermined normal value.
[Illustration: FIG. 2,426.--Westinghouse voltage regulating ~primary~
relay; view of mechanism with case removed. This relay is practically a
voltmeter arranged for making and breaking contacts with fluctuations
of voltage. As shown in the figure, it consists essentially of a
solenoid and a balance beam carrying two movable contact points on one
end and attached to the solenoid core at the other. The oscillation of
the core causes the contact carrying end of the beam to move between
two stationary contact points connected to the auxiliary or secondary
relay circuit. The stationary contact points are fitted with adjusting
screws for either increasing or decreasing the distance between them,
to the amount of change in the voltage required for making or breaking
contact; in other words, for varying the sensitiveness of the relay.
Means for varying the normal voltage which it is desired to maintain
are provided in the spring attached to the balance beam and controlled
by the micrometer adjusting screw. Increasing the tension of the spring
results in lowering the normal voltage position. The relay is wound
for a normal voltage of 110 volts, and has a range of adjustment from
90 to 130 volts. The total energy required for its operation is about
50 watts at normal voltage. Voltage transformers having at least 50
watts capacity are, therefore, required. The parts are: A, solenoid;
B, solenoid core; C, end of balance beam; D, pivots, bearings; E,
movable contact arm; F, upper stationary contact point; G, lower
stationary contact point; H, adjusting screw; K, adjusting spring; L,
feeder binding posts; M, auxiliary circuit and secondary relay binding
posts.]
[Illustration: FIG. 2,427.--Westinghouse voltage regulating ~secondary~
relay; view showing relay removed from oil tank. The secondary relay
is practically a motor starting switch of the double pole double throw
type, electrically operated through the contacts of the primary relay.
It is provided with contact points of one-half inch rod. The relay is
suitably connected for starting, stopping and reversing the motor and
for properly operating the motor brake. The parts are: A, solenoid; B,
laminated field; C, movable contact arm; D, stationary contact arms; E,
removable brass contact points; F, terminal block; G, terminals.]
~Ques. Why are two relays used?~
Ans. For the reason that a primary relay, of sufficient accuracy and
freedom from errors due to temperature and frequency variations, could
not be made sufficiently powerful to carry the relatively large current
required for operating the motor.
~Ques. What names are given to the relays?~
Ans. Primary and secondary.
~Ques. What difficulties were encountered in the operation of relays? ~
[Illustration: FIG. 2,428.--Diagram of connections of General Electric
~direct current~ (form S) ~voltage regulator,~ for 125, 250, and 550
volts. ~The range of voltage~ is given in the following table:
-----------------------------------
|Regulator|Range of voltage |
| |------------------------
| |16 |17 |18 |19 |20 |
-----------------------------------
|125 |105 |110 |115 |120 |125|
|250 |210 |220 |230 |240 |250|
|550 |550 | | | | |
-----------------------------------]
Ans. Vibration or chattering at the contacts of both relays and
tendency of the movable contact arm of the primary relay to hug closer
to one of the stationary contact points than to the other, thereby
operating too often.
~Ques. What causes vibration or chattering at the contacts?~
Ans. This is due to the voltage frequently approximating the value
required for closing a contact, thereby causing the contact points to
barely touch and make several poor contacts in succession.
~Ques. What objectionable action is produced by vibration at the
contacts?~
Ans. Arcing, burning and pitting of the contacts, even when alloys of
the rarer metals are used, such as those of the platinum group, having
extreme hardness and high melting points.
[Illustration: FIG. 2,429.--Diagram of connections of automatic
induction regulator and auxiliary apparatus on single phase circuit.]
~Ques. What effect is produced by poor contact of the primary relay?~
Ans. It causes chattering in the secondary relay; which burns out and
wears away its contact points, increasing the heating of the motor,
creating objectionable noise and entailing wear and tear on the whole
outfit.
~Ques. Why does the movable contact arm of the primary relay tend to
remain nearer one of the stationary contact points than the other?~
Ans. This is due to the tendency of the relay to open the contact
whenever the voltage equals that at which the contact closes.
[Illustration: FIG. 2,430.--Diagram of connections of automatic
induction regulator and accessory apparatus on three phase feeder
circuit.]
~Ques. What provision is made in the primary relay to prevent vibration
or chattering?~
Ans. Two auxiliary windings are provided: one in series with each of
the stationary contact points and so arranged as to assist in making
the contact by increasing the pressure on the contact points at the
instant of closure.
The best effect of the compounding action of the auxiliary
coils is obtainable when arranged for ¾ per cent. of the torque
of the main coil.
[Illustration: FIG. 2,431.--Westinghouse drum type variable transformer
voltage regulator. ~It consists of~ a drum and finger type switch. A
preventive resistance is used between the different contacts, making
it unnecessary to open the circuit when moving from one tap of the
regulating transformer to the next tap. A spring actuated, quick
moving, central stopping mechanism is used to prevent burning the
resistances. The regulator is arranged to give 40 points of regulation.
In many cases this large number of points is not absolutely necessary,
but it is desirable to use them because the voltage per step is thus
reduced to a small value, and a corresponding increase in the life
of the contacts results because of the reduced sparking at the lower
voltage. ~Two drums~ are employed. The ~first drum~ has ten contacts
and a corresponding number of fingers, the latter being mounted upon
an insulated bar. These fingers are connected to the floating coils
of the regulating transformer, and as the drum is rotated, the finger
connected to the line is brought into contact successively with each of
the ten taps. The ~second drum~ is of similar construction and consists
of a changing and reversing switch. It connects the two floating coils
to the various taps on the main secondary coil of the regulating
transformer at the proper time, and also reverses the transformer so
that the total winding can be used for either raising or lowering the
voltage. All the points of regulation are obtained by a continuous
motion of the handle, the various connections produced in the manner
are shown in the diagram, fig. 2,433. The top and base of the regulator
are made of cast iron and the top is supported by steel bars, two of
which are insulated, and used to support the metallic bases finger
to which the cable leads are attached. The drums consist of metal
castings mounted upon insulated shafts. The first drum, which is the
only one upon which arcing can take place, is provided with removable
copper contact tips. The main castings are made of aluminum to secure
low inertia of the drum. A sheet iron cover is used to enclose the
regulator, and the leads are brought out through the bottom of the
controller.]
A non-inductive resistance placed in parallel with each coil
of the secondary relay, takes current approximately in phase
with the current in the main coil of the primary relay, and
of proper strength to make the number of ampere turns in the
auxiliary coil three-fourths per cent. of the number in the
main coil. The resistances have the additional effect of
absorbing the "discharge" from the main coils of the secondary
relay when the contacts are broken, thereby obviating sparking
at the primary contact points.
[Illustration: FIG. 2,432.--Diagram showing connections of the
Stillwell regulator.]
[Illustration: FIG. 2,433.--Diagram showing ~position of the floating
coil~ on different steps of Westinghouse ~drum type variable ratio
transformer regulator.~ _The upper half of the diagram_ shows the
connections of the various coils for each position of the regulator
handle. This arrangement applies to a regulator used in connection
with an independent regulating transformer. When regulators are
used in connection with large power transformers, the regulating
transformer can be omitted and auxiliary coils can be placed on the
main transformer to provide the necessary taps for regulating purposes.
_The lower half of the diagram_ shows the connections used when
auxiliary coils are added to a large transformer. ~The diagram shows~
connections for a ~single phase regulator~. Where polyphase regulators
are required, the connections consist essentially of two sets of single
phase connection, and the controller is extended in length so as to
contain double sets of drum and contact.]
~Variable Ratio Transformer Voltage Regulators.~--The principle of
operation of this class of regulator is virtually the same as that
of the induction type regulator; that is to say, both consist of
regulating transformers, but ~in the variable ratio method~ _the
primary or series coil is divided into a number of sections which may
be successively cut in or out of the circuit to be regulated_, instead
of varying the flux through the entire coil, as in the induction type.
There are two distinct mechanical forms of variable ratio regulator:
1. Drum type;
2. Dial type.
~Drum Type Regulators.~--This form of variable ratio transformer
consists essentially of a drum and finger type switch, similar to a
railway controller.
There are many contacts, giving a large number of points of regulation,
obtained by the use of changing switches and floating coils.
The floating coil is a part of the secondary winding of the regulating
transformer which is insulated from the main portion of the winding,
and is sub-divided by taps into a number of equal sections.
The sub-divisions of the main secondary winding are much larger, each
one being equivalent to the whole of the floating coil.
[Illustration: FIG. 2,434.--Diagram of connections of General Electric
~high voltage cut out relay~ (form A) for voltage regulators. ~Its use~
in connection with the regulator protects the system from any sudden
rise in voltage due to some accident to the regulator which might
cause the relay contacts to stick, thus producing _full field_ on the
exciter. ~In construction,~ _the control magnet is connected in series
with the alternating current control magnet on the regulator and the
contacts are connected in series with the rheostat shunt circuit._
Then, ~should the voltage rise beyond a certain value,~ predetermined
by the setting of the thumb screw supporting the plunger of the control
magnet, _the contacts of the relay are tripped open which, by inserting
all the resistance in the exciter field, reduces the exciter voltage
which in turn reduces the alternating current voltage_. This relay has
to be reset by hand.]
~Ques. Describe the operation of a drum regulator.~
Ans. The floating coil and main windings are first connected in series
with each other and with the line to be regulated. The floating coil
is then cut out of the circuit step by step. When entirely cut out
it is transferred to the next lower tap on the main winding, after
which it is again cut out step by step and then transferred again. By
continuing this process a large number of steps are provided with but
comparatively few actual taps on the transformer.
~Ques. How many floating coils are used and why?~
Ans. Two floating coils are included in each regulator so that one can
be transferred while the other is supplying the current to the line.
~Dial Type Regulators.~--This form of variable ratio transformer
regulator consists of a regulating transformer and a dial type switch
as shown in the accompanying illustrations. The regulating transformer
is similar to a standard transformer except that the secondary winding
is provided with a number of taps leading to the contact of the dial
switch as shown in the diagram fig. 2,437.
[Illustration: FIG. 2,435.--Dial of Westinghouse dial type variable
ratio voltage regulator. The dial consists of a marble slab, upon which
the contacts are mounted in a circle as shown. The contact arm is
arranged to move from contact to contact. The alternate small contacts
are dummies, serving to prevent the contact arm springing down between
contacts when moving from one to another. The panel contains a changing
switch which makes it possible to double the range of a regulator,
since the transformer connections can be changed to both raise and
lower to an extent equal to the full range of the transformer. The
total range in voltage from a certain per cent. below to a certain per
cent. above the line voltage can be obtained in a number of steps equal
to twice the number of divisions into which the secondary winding of
the transformer is divided.]
~Ques. What modification is made to adapt dial regulators for heavy
current?~
Ans. A dial with a series transformer, and a shunt or auto-transformer
are employed as shown in fig. 2,436.
~Ques. Why is such modification desirable?~
Ans. Because, the additional cost of a series transformer is small
in comparison with the cost of building a dial with a large current
carrying capacity, and the cost of bringing out a number of heavy leads
from a small transformer.
[Illustration: FIG. 2,436.--Diagram of connections for Westinghouse
~11 point dial, series transformer and auto-transformer~. The
auto-transformer has a number of taps connected across the line, the
series transformer is placed in series with one side of the line, and
connected to a dial, as shown.]
~Ques. How are dial regulators modified for high voltage?~
Ans. Standard dials may be used with series and shunt transformers
similar to the method used for heavy current circuits.
~Ques. Describe the connections.~
Ans. The primary of the shunt transformer is connected across the line
and the secondary has a number of taps which are connected to contacts
on the dial. The primary of the series transformer is connected in
series with the line and two leads from the secondary winding are
connected to the dial.
The connections are similar to those shown in fig. 2,437, except that
shunt transformers are used instead of auto-transformers.
[Illustration: FIG. 2,437.--Diagram of connections for Westinghouse
~dial type variable ratio voltage transformer. In construction~ the
secondary winding of the transformer is divided into 10, 14, or
20 parts giving 11, 15, or 21 taps which are brought out from the
secondary winding and connected to the various points of the dial.
The diagram shows connections for an 11 point dial and regulating
transformer. ~Since there is a difference of voltage~ between adjacent
contacts, _the contact arm must not touch the contact toward which
it is moving until after it has left the contact upon which it was
resting_. Moreover, it is undesirable to open the circuit each time in
moving from one contact to the next. These conflicting requirements
are met by the use of arcing tips which are placed on the contact arm
so that a very close adjustment can be obtained, and so arranged that
the contacts are not short circuited but always have a gap of from
one-sixteenth to one-eighth inch in the circuit during the time of
changing from one contact to the next. The air gaps form a "preventive
resistance." ~A quick moving mechanism~ is used to accelerate the
movement from one contact to the next, a very quick movement being
necessary to avoid undue arcing. ~The capacity of the regulator~ is
200 amperes at 2,200 volts, being arranged to give a maximum increase
in voltage of 400 volts. The maximum pressure between contacts is 25
volts.]
[Illustration: FIG. 2,438.--Diagram of connections of General Electric
pole type regulator. The operation of the regulator is obtained by
means of a small single phase motor which is in continuous operation,
and which by mechanical means may be connected to the regulator shaft.
The control of the mechanism is obtained by means of a voltage relay.
The operating motor, which is of the drawn shell type, is provided
with a starting clutch and will consequently start up with full load.
Under actual operating conditions it will, of course, be comparatively
seldom that the motor will be called upon to start up. ~A non-inductive
resistance~, made up from standard units, is connected in series with
the relay winding and several taps are provided, so that the relay
can be adjusted for any voltage from 10 per cent. below normal. In
order to readily dissipate the heat developed in the resistance, it
has been mounted in a pocket on the back of the tank, openings being
provided for natural air ventilation. ~The relay plunger~ is hinged
to one end of a balance arm, which arm is provided with two trip pins
to control the mechanism. ~An adjustable helical spring~ is attached
to the other end of the arm to assist the magnetic pull of the coil
in balancing the plunger and also for adjustment. The relay is not
provided with series winding for line drop compensation, but it may
be used with a standard line drop compensator, which then has to be
installed outside of the regulator. The voltage relay must be connected
to the feeder side of the regulator, the necessary low voltage to be
obtained from a distributing transformer, or if this should not be
available in the immediate vicinity, a 200 watt step down transformer
will be satisfactory. ~The motor is designed to operate~ in parallel
with the relay, the normal connections being as shown. The speed of
the motor and the ratio of the gearing is such that it requires about
90 seconds to operate the regulator from limit to limit, but, as this
regulator is not intended to take care of sudden voltage fluctuations,
the comparatively long time of operation will not be objectionable.]
[Illustration: FIGS. 2,439 to 2,443.--General Electric ~pole type
regulator~ removed from tank. ~It consists essentially~ _of a primary
and secondary coil, operating motor, and voltage relay mechanism_. The
regulator and mechanism is suspended in a cast iron tank, the lower
part, containing the regulator core and coils, being filled with oil.
The leads for the regulator are brought out at the upper part of the
tank. The outgoing leads are compressed into bushings and connected
to the leads of the regulator by means of terminals, the arrangements
being such that the regulator with mechanism can be removed from the
tank without difficulty. Besides the cover, the tank is also provided
with a hinged door on the front side so as to give access to the
mechanism. The door is provided with a gasket and the construction is
practically rain and dust proof. However as there is always danger of
the door not being clamped down perfectly, thus making it possible for
water to enter the tank, a pocket has been provided inside the tank
and underneath the door to collect the water. Capacity up to 2.3 kw.,
to control 2,300 volts, 60 cycle, 10 ampere feeders, and for a voltage
range of 10 per cent. above or below normal, the operating motor and
relay being designed for 110 or 220 volts. No provision is made for
line drop compensation, although this can be obtained by installing
a current transformer and a line drop compensator externally to the
regulator.]
It will be seen that the circuit comprising the dial, the secondary
of the shunt, transformer and the secondary of the series transformer
form a circuit which is not electrically connected to the main circuit.
It can therefore be grounded without disturbing the main circuit as
a safeguard to render it impossible for the pressure of the dial to
be higher above the ground than the secondary voltage of the shunt
transformer.
[Illustration: FIGS. 2,444 to 2,446.--Sectional views of General
Electric pole type regulator winding and core. The secondary core has
only two slots containing a single coil, while the rotor or primary
core has four slots. Two of these slots are occupied by a single
primary coil, and the two circular slots in quadrature thereto contain
the compensating or short circuit winding. This winding also serves
to hold the primary punchings together, and it consists of two copper
rods riveted to the two cast brass flanges. The secondary coil is
form wound, while the primary coil is wound directly on the core. The
rotor flanges, both top and bottom, are provided with discs which are
turned in alignment with the punchings, and these discs bear against
the top and bottom flanges between which the secondary punchings are
clamped. These secondary flanges are also turned in alignment with the
secondary punchings, so that an even air gap between the primary and
the secondary is assured. The secondary coil is wound with an opening
in the upper horizontal part which affords passage for the operating
shaft of the rotor. A bearing for this shaft is provided in the table
which supports the mechanism and from which the regulator is suspended.
Flexible leads are brought out from the rotor and twisted around the
shaft as in standard regulator practice. The regulator being two pole,
the rotor is turned through an angle of 180 deg. to obtain the full
range of the regulator.]
~Small Feeder Voltage Regulators.~--In some generating stations the
voltage is maintained constant at the busbars and the line drop
compensated by automatically operated regulators connected in the main
feeders. It is possible in this way to obtain constant voltage at all
loads at the various distribution centers, that is, at those points on
the feeders where the lines of the majority of consumers are connected
as shown in fig. 2,447.
[Illustration: FIGS. 2,447 and 2,448.--Systems of distribution
illustrating use of ~small feeder~ or ~pole type voltage regulators~.]
It is evident, however, that, while the voltage at the center of
distribution can be maintained constant, no account can be taken of the
drop in the lines between this center and the consumers. This drop is
generally negligible, except in some particularly long lines, as, for
example, consumer _B_ in fig. 2,447.
In order to obtain perfect regulation at B, it would be necessary
to install a separate regulator in that line, this regulator to be
installed either at the center C or preferably at B.
In a great many cases the power distribution is not as ideal as
indicated in fig. 2,447, but rather as shown in fig. 2,448, that is,
the consumers are connected all along the feeder. In this case there
is no definite center of distribution, and the automatic regulator
installed in the station can be adjusted to give only approximately
constant voltage at an imaginary center of distribution C; that is, the
voltage cannot be held constant at any definite point during changes of
load distribution.
[Illustration: FIG. 2,449.--General Electric pole type regulator in
service; its construction is shown in fig. 2,450.]
The majority of the consumers may, however, obtain sufficiently good
voltage while a few may have reason for criticism. To overcome this
difficulty it is necessary either to increase the copper in the feeder
or else to install small automatic regulators.
There are also cases where a small amount of power is transmitted a
long distance through a feeder direct from the station.
The amount of copper required to reduce the line drop is usually too
great to be considered and the cost of the ordinary automatic regulator
is also comparatively high. In such cases small pole type regulators as
shown in fig. 2,449 are desirable.
~Ques. Describe the operation of the regulator mechanism shown in fig.
2,450.~
Ans. Assuming the voltage to be normal, the balance arm of the relay
will be held horizontal, the trips F will not engage with the triggers
E, and no movement is therefore transmitted to the ratchet wheel C. If
the voltage drops below normal, the left hand trip will descend until
it finally gets in the way of the left hand trigger just before it
reaches the limit of its counterclockwise travel. This trigger will
therefore release the left pawl D, which will engage with the ratchet
wheel and will consequently turn it clockwise until the rocker arm
reaches its right hand limit. Before the rocker arm reaches the left
hand limit, the released pawl must be locked by its trigger, so that
if the voltage has reached its normal value, further movement of the
ratchet wheel will not take place, whereas if the voltage be still too
low, the trigger will again release the pawl by striking the trip of
the relay.
[Illustration: FIG. 2,450.--Mechanism of General Electric pole type
regulator. The operating motor (described in fig. 2,438) is direct
connected to a worm and gear, the shaft of which is provided with a
bell crank. A rod A, connects the crank with the rocker arm B, which
thus may be caused to oscillate over a ratchet wheel C. The rocker arm
is provided with two pawls D, which can engage with the teeth of the
ratchet wheel, so that this wheel can be rotated one way or the other.
The ratchet wheel is mounted on the same shaft as a worm, which engages
with the gear segment carried by the regulator shaft, so that the
movement of the ratchet wheel is directly transmitted to the regulator.
Besides the two large pawls D, the rocker arm also carries two smaller
ones E, called the triggers. These triggers usually hold the pawls
locked in such positions as not to engage with the ratchet wheel, but
the pawls will be released when the triggers strike the trips F of the
relay arm. A limiting device for the movement of the ratchet wheel and
the regulator segment is provided, as shown. This device consists of
two cams K, mounted on a common arm, which can turn on the shaft of the
ratchet wheel. Normally these cams are not within reach of the pawls,
but through a lever arrangement, controlled by the regulator segment,
the arm holding the cams may be rotated so that, if the trigger has
been raised, so as to release the pawl, the tip of the pawl will bear
on the cam of the limiting device, and before the pawl can engage with
the ratchet wheel it has already been locked by its trigger. A further
movement of the ratchet wheel in that particular direction is therefore
impossible, while it is free to be moved the other way. A positive
stop for the gear segment is also provided. The motor is provided with
oil ring bearings, and the gear for the motor worm runs in oil, the
supporting casting forming a well therefor.]
[Illustration: FIG. 2,451.--Diagram of connections of General Electric
~direct current voltage regulator~ (form T) with two dynamos and
one exciter. In cases where several shunt or compound wound direct
current machines are operating in parallel, either on two wire or
three wire systems, a good arrangement for voltage regulation and line
drop compensation is obtained by using this regulator and a separate
exciter. The compensating shunt as well as pressure wires can be used
to maintain a constant pressure at the center of distribution.]
~Ques. How is this automatic locking of the pawl obtained?~
Ans. By having a lip G on the under side of the pawl strike a finger H
fastened to the bearings in front of the ratchet wheel.
The pawl is thus raised just before it reaches the limit of its
clockwise travel sufficient to be locked by its trigger.
[Illustration: FIG. 2,452.--~Condenser sections~ and method of
assembling same with tripod. ~The tripod bolts~ are made of extra
length to accommodate the addition of extra condenser sections if
necessary. The illustration shows three sections in position.]
~Ques. How does the mechanism operate when the voltage rises above
normal?~
Ans. As described above, with the exception that the right hand trip
causes a rotation of the regulator in the opposite direction.
~Ques. How is adjustment made for various voltages?~
Ans. Taps are provided on the resistance in series with the relay, and
finer adjustment can be obtained by means of the helical spring on the
right hand end of the balance arm.
In order to adjust the sensitiveness of regulation, the bearing
for the balance arm can be raised or lowered by means of a stud
J, fig. 2,450, connecting this bearing with the bearing of the
operating shaft, and the regulator can be made to maintain the
voltage within 1 per cent. above or below normal.
[Illustration: FIG. 2,453.--Westinghouse unit switch type pressure
regulator, designed for handling heavy currents where a variable ratio
transformer type of regulator is desired. The regulator consists of
a number of electrically operated switches controlled from a master
switch. These switches are arranged to perform practically the
same cycle of operation as previously described for the drum type
regulators. The transformer windings are divided into sections, and
two floating coils are provided which are connected to various taps
on the main auto-transformer. These floating coils have intermediate
steps, and the successive operation of the switches connects the
floating coils in proper sequence to the main auto-transformer, and
transfers the line connection from one point of the floating coil to
the next. In this way a 23 point regulator with sixteen switches, and
a 71 point regulator with 21 switches may be supplied. The master
switches are arranged with an automatic lock to prevent their being
operated too rapidly. The magnet switches themselves are so interlocked
that the proper sequence of operation is insured. The electrically
operated switches may be of the open type, mounted on a slate or marble
switchboard, when the whole control outfit is placed in a room which
is comparatively free from dust or dirt of any kind, and where there
is no danger of employees coming in contact with the switches. The
other type of switch is entirely enclosed, the main contacts being
oil immersed. The frames of these switches are grounded and the whole
design is arranged to operate under ordinary dirty conditions. All
of these switches, however, should receive the necessary inspection
and attention. The contacts have a long life and are easily renewed.
Regulators of this type are adapted for metallurgical purposes where
the regulation is effected in the primary circuit and the secondary
circuit is of very low voltage but large current capacity and is used
for supplying power to the furnaces. These regulators have been built
in capacities up to 800 amperes at 3,300 volts.]
~Ques. What provision is made for convenient inspection?~
Ans. A snap switch is provided by means of which the power to the motor
and relay can be disconnected.
[Illustration: FIGS. 2,454 and 2,455.--Front and rear views of General
Electric automatic voltage regulator. The regulator has _a_ ~direct
current~ _control magnet_, _an_ ~alternating current~ _control magnet,
and a relay_. The ~direct current control magnet~ is connected to the
exciter bus bars. This magnet has a fixed stop core in the bottom and
a movable core in the top which is attached to a pivoted lever having
at the opposite end a flexible contact pulled downward by four spiral
springs. For clearness, however, only one spring is shown in the
figure. Opposite the direct current control magnet is ~the alternating
current control magnet~ which has a pressure winding connected by means
of a pressure transformer to the alternator or bus bars. ~There is an
adjustable compensating winding~ on the alternating current magnet
connected through a current transformer to the principal lighting
feeder. The object of this winding is to raise the voltage of the
alternating current bus bars as the load increases. The alternating
current control magnet has a movable core and a lever and contacts
similar to those of the direct current control magnet, and the two
combined produce what is known as the "floating main contacts." ~The
relay~ _consists of a_ ~U~ _shaped magnet core having a differential
winding and a pivoted armature controlling the contacts which open and
close the shunt circuit across the exciter field rheostat_. ~One of the
differential windings~ of the relay is permanently connected across the
exciter bus bars and tends to keep the contacts open; the other winding
is connected to the exciter bus bars through the floating main contacts
and when the latter are closed, neutralizes the effect of the first
winding and allows the relay contacts to short circuit the exciter
field rheostat. ~Condensers~ are connected across the relay contacts to
prevent severe arcing and possible injury.]
~Automatic Voltage Regulators for Alternators.~--The accurate
regulation of voltage on any alternating current system is of
importance. The desired voltage may be maintained constant at the
alternator terminals by rapidly opening and closing a shunt circuit
across the exciter field rheostat.
[Illustration: FIG. 2,456.--Diagram of connections of General Electric
~_contact making_ ammeter~ for operating on alternating current
circuits. _The instrument is designed to indicate_ with the aid of
a current transformer, _certain values of current in an alternating
current system_. This value depends upon the setting of the regulating
rheostat in parallel with the pressure coil of the ammeter. It is also
possible with this instrument, together with the necessary control
apparatus, to _hold_ certain values of current. ~By using a different
magnet coil~ this meter may be connected to a shunt instead of a
current transformer and used on a direct current system.]
~Ques. Describe in more detail this method of regulation.~
Ans. The rheostat is first turned in until the exciter voltage is
greatly reduced and the regulator circuit is then closed. This short
circuits the rheostat through contacts in the regulator and the voltage
of the exciter and alternator immediately rise. At a predetermined
point, the regulator contacts are automatically opened and the field
current of the exciter must again pass through the rheostat. The
resulting reduction in voltage is arrested at once by the closing of
the regulator contacts which continue to vibrate in this manner and
keep the generator voltage within the desired limits. The connections
are shown in fig. 2,457.
[Illustration: FIG. 2,457.--Diagram of General Electric automatic
voltage regulator connections with alternator and exciter. ~In
operation~, the circuit shunting the exciter field rheostat through the
relay contacts is opened by means of a single pole switch at the bottom
of the regulator panel and the rheostat turned in until the alternating
current voltage is reduced 65 per cent. below normal. This weakens both
of the control magnets and the floating main contacts are closed. This
closes the relay circuit and demagnetizes the relay magnet, releasing
the relay armature, and the spring closes the relay contacts. The
single pole switch is then closed and as the exciter field rheostat is
short circuited, the exciter voltage will at once rise and bring up
the voltage of the alternator. This will strengthen the alternating
current and direct current control magnets, and at the voltage for
which the counterweight has been previously adjusted, the main contacts
will open. The relay magnet will then attract its armature and by
opening the shunt circuit at the relay contacts will throw the full
resistance into the exciter field circuit tending to lower the exciter
and alternator voltage. The main contacts will then be again closed,
the exciter field rheostat short circuited through the relay contacts
and the cycle repeated. This operation is continued at a high rate of
vibration due to the sensitiveness of the control magnets and maintains
a steady exciter voltage.]
~Line Drop Compensators.~--In order that the actual voltage at a
distant point on a distribution system may be read at the station some
provision must be made to _compensate_ for the line drop, that is
to say, for the difference in voltage between the alternator and the
center of distribution.
In order to do this a device which is known as a "line drop
compensator" is placed in the voltmeter circuit as shown in the
diagram, fig. 2,458.
~Ques. What are the essential parts of a line drop compensator?~
Ans. The elements of a line drop compensator are a variable resistance,
and a variable inductance.
~Ques. Describe the connections.~
Ans. The secondary of a pressure transformer is connected in series
with the compensator inductance and resistance, and the secondary of a
current transformer as shown in the diagram, fig. 2,458.
[Illustration: FIG. 2,458--Diagram showing ~essential parts and
connections for a line drop compensator~. _The compensator corrects
the voltmeter indication at the supply end of a feeder for the ohmic
and inductive drop in pressure between that point and the point of
consumption_, so that the reading of the station voltmeter corresponds
with the actual voltage at the point of consumption, independent of
the power factor and current. ~It is especially useful~ for adjusting
pressure regulators.]
~Ques. How are the inductance and resistance wound?~
Ans. They are wound so that any proportion of the winding of either can
be put in or out of the voltmeter circuit.
[Illustration: FIG. 2,459.--General Electric line drop compensator. It
has two dial switches with many taps to the resistance and reactance in
the box so that it can be adjusted to compensate accurately for line
losses with loads of varying power factor. Dial R changes resistance,
and dial X, reactance.]
[Illustration: FIG. 2,460.--General Electric line drop compensator.
This compensator contains besides resistance and inductance, a current
transformer, the secondary of the transformer being connected in series
with the resistance and inductance; the primary of the contained
current transformer is connected to an external current transformer.
The reactance and resistance are both so wound that any proportion
of the winding can be cut in or out of the voltmeter circuit. Both
elements have 12 points of adjustment of one volt each, giving a total
combined drop at maximum setting of about 17 volts.]
~Ques. How can the voltmeter indicate the pressure at the center of
distribution?~
Ans. If the amount of inductance and resistance be properly adjusted,
there will be produced a local circuit corresponding exactly in all
its characteristics to the main circuit. Hence, any change in the main
circuit produces a corresponding change in the local circuit, and
causes the voltmeter to always indicate the pressure at the end of the
line or center of distribution or at any point for which the adjustment
is made.
[Illustration: FIG. 2,461.--Westinghouse line drop compensator. For
single phase circuits, one compensator and one series transformer,
that is the instrument as listed with transformers, will give correct
indications for a single phase circuit. The same voltage transformer
serves for both voltmeter and compensator. For balanced two phase
circuits one compensator and one transformer connected in one of the
phases is sufficient. Two single phase compensators should be used for
unbalanced two phase circuits. For three phase circuits the compensator
should be connected by means of two series transformers.]
~Ques. How should the adjustment be made?~
Ans. It is advisable to calculate the ohmic drop for full load and
set the resistance arm at the point which will give the required
compensation and then adjust the inductance arm until the voltmeter
reading corresponds to the voltage at the point on the line selected
for normal voltage.
[Illustration: FIG. 2,462.--Diagram of ~automatic voltage regulator~,
using ~line drop compensator~. For ordinary installations the
compensating winding on the alternating current control magnet
is connected to a current transformer in the main feeder. A dial
switch is provided by which the strength of the alternating current
control magnet can be varied and the regulator made to compensate
for any desired line drop up to 15 per cent. according to the line
requirements. Where the power factor of the load has a wide range of
variation, a special line drop compensator, such as shown in fig.
2,459, adapted to the regulator would be desirable. The connections are
readily understood by the diagram. The number of condenser sections
which will prevent undue arcing at the relay contacts depends on
the characteristics of the exciter. They may be roughly estimated
by allowing one section for each 15 kw. capacity for exciters with
laminated poles, and one for each 22 kw. capacity for exciters with
solid steel poles. It is necessary though to have one condenser section
for each pair of relay contacts, and at times it becomes necessary to
apply a double section for each pair of contacts. In the lower part of
the figure the line drop compensation and connections is reproduced in
more detail on a larger scale.[2]]
[2] NOTE.--It is desirable, in any system of distribution, to read the
active voltage at the point of distribution, by means of the voltmeters
in the station. A compensator proper consists of a variable resistance
and a variable inductance, and sometimes a current transformer. In
wiring, the voltmeter, instead of being connected directly across the
secondaries of a pressure transformer, has inserted in series with it,
portions of the resistance and inductance of the compensator. These are
so connected that the drop in pressure across them will be combined
with that of the pressure transformer, so that the voltmeter reading
indicates the pressure at the center of distribution or end of the
line.
~Starting Compensators.~--These are used for starting induction motors
and consist of inductive windings (one for each phase) with a number of
taps connecting with switch contacts as shown in fig. 2,463. A starting
compensator is similar to a rheostat except that inductive windings are
used in place of the resistance grids.
[Illustration: FIG. 2,463.--Diagram of connections of General Electric
two phase starting compensator with no voltage release and fuses.]
~Ques. Describe the inductive windings.~
Ans. The compensator winding consists of an inductive coil in each
phase with each coil placed on a separate leg of a laminated iron core.
Each coil is provided with several taps so located that a number of
sub-voltages may be obtained.
~Ques. Are starting compensators necessary for small motors? Why?~
Ans. No, because the full voltage starting current taken, although
equal to several times the load current, is nevertheless so small,
compared with the capacity of the station alternators or feeders, that
it does not materially affect the regulation of the circuit.
[Illustration: FIG. 2,464.--Diagram of connections of General Electric
three phase starting compensator with low voltage release and fuses.]
[Illustration: FIG. 2,465.--Diagram of connections of General Electric
two phase starting compensator with no voltage release and overload
relays for 1,040 to 2,500 volt circuits.]
Motors larger than about 7 horse power cause an objectionably
heavy rush of current if thrown directly on the line. Starting
compensators obviate such sudden variations of line load and
are accordingly recommended for motors above 7 horse power
except in cases where voltage variations and excessive starting
currents are not objectionable.
[Illustration: FIGS. 2,466 and 2,467.--General Electric three ~phase
hand operated starting compensator~. Fig. 2,466, compensator in case;
fig. 2,467, compensator with case removed. ~The compensator consists
of~ _a core and windings, a cable clamp, and a switch, assembled in
a substantial metal case with external operating handle and release
lever_. ~The windings~ consist of coils wound on separate legs of
a laminated core, and tapped at several points, the connections
terminating at the switch contacts. The shaft of the switch extends
through the sides of the compensator case, and is operated by a lever
at the right, being held in the running position by a lever at the
left. It is provided with wiping contacts. The switch is immersed
in oil, and is intended to be used as a line switch as well as for
starting the motor. ~The lever has three positions:~ "off," "starting,"
and "running." ~In the _off_ position~, _both compensator and motor
windings are disconnected from the line_. ~In the starting position~,
_the switch connects the line to the ends and the motor to the taps
of the compensator winding_ without overload relays or fuses in
circuit. ~In the running position~, _the compensation winding is cut
out and the motor is connected to the line through suitable fuses or
overload relays_ mounted directly above the compensator. To prevent the
attendant throwing the motor directly on the line, and thereby causing
a rush of current which it is the object of the compensator to avoid,
an automatic latch is provided and so arranged that the lever at _off_
position can be thrown only into the _starting position_ (backward);
and can be thrown thence into the _running position_ (forward) only by
a quick throw of the lever, whereby any appreciable drop in speed and
consequent increase in current in passing from the starting into the
running position is avoided.]
~Ques. What should be noted with respect to the compensator winding
taps?~
Ans. The choice of a tap giving so low a voltage as to require over one
minute for starting should be avoided so as to prevent the overheating
to which starting compensators, in common with other motor starting
devices, are liable if left in circuit unnecessarily long, or if the
motor be started several times in rapid succession.
[Illustration: FIG. 2,468.--Diagram of connections of General Electric
three phase starting compensator with no voltage release and overload
relays.]
It should also be noted that the starting current diminishes
rapidly as full speed is approached. It is, therefore,
important that the switch be kept in the starting position
until the motor has finished accelerating to prevent any
unnecessary rush of current when the switch is thrown to the
running position.
[Illustration: FIG. 2,469.--General Electric starting compensator with
low voltage release and overload relays. On the switch shaft there are
mounted two levers, held together with a strong spring which operates
in either direction and prevents the switch being left on the starting
position. On the running side it is held by the external low voltage
release lever until released either by hand or by the action of a low
voltage relay. The low voltage release consists of a cast iron frame
open at the bottom and totally enclosing the coil. A laminated plunger
is used to hold the tripping lever, the latter engaging with the lever
mounted on the switch shaft. The compensator cannot be thrown into the
running position without first going to the starting position and it
cannot be left on the starting position.]
~Ques. What is the usual arrangement of starting compensators for large
motors?~
Ans. Starting compensators may be wound for any voltage or current for
which it is practicable to build motors. For very large motors the
switching device is generally separate from, the compensator itself and
consists of triple and four pole switches for three phase and two phase
motors respectively. One double throw switch or two interlocked single
throw switches are required for the motor and a single throw switch for
energizing the compensator, the running side of the motor circuit being
provided with fuses or automatic circuit breakers, or the switches
provided with low voltage and overload release attachments.
~Star Delta Switches.~--These are starting switches, designed for use
with small three phase squirrel cage motors having their windings so
arranged that they may be connected in star for starting and in delta
for running.
~Ques. Describe the operation of a star delta switch.~
Ans. In starting the motor, the drum lever is thrown in the starting
direction which connects the field windings of the motor ~in star~.
When the motor has accelerated and has come partially up to speed the
starting lever is quickly thrown to the running position in which
position the field windings are connected ~in delta~. The effect of
connecting the field winding in star at starting is to reduce the
voltage applied to each phase winding, while in the running position
each phase of the field winding has full line voltage impressed upon it.
[Illustration: FIGS. 2,470 to 2,474.--General Electric ~time limit
overload relay~ _for starting compensator_. In case of overload, the
relay armature is raised and at the end of its travel, opens the small
switch at the top which in turn opens the circuit of the low voltage
release coil causing the compensator switch to return to the "off"
position. ~The oil dash pot~ provides a certain time element and can
be adjusted to operate immediately upon overload or at any interval up
to five minutes. Each relay has five calibrating points, the lowest
being the normal full load current of the motor, the highest 300 per
cent. full load current. The scale on the calibration tube reads direct
and shows various values of current at which the relay may be set to
operate. ~To change overload setting:~ 1, loosen set screw; 2, turn
relay plunger on piston rod until white mark comes opposite required
value of current; 3, tighten set screw. ~Time element adjustment:~
Removing oil dash pot by turning to the left will expose the cup shaped
piston, made of which are two concentric discs (B and C) held together
by a milled lock nut, A. There is a hole in each disc through which
the oil must pass when the plunger of the relay is raised. The time
element may be varied by changing the size of the opening between these
discs, that is, to have the relays operate in a shorter period of time,
increase the size of the opening and vice-versa. ~To change the time
setting:~ 1, remove the oil dash pot; 2, raise the discs B and C on the
piston rod; 3, loosen the lock nut A; 4, change the opening between
B and C, giving a larger opening for shorter time of operation, and
a smaller opening for longer time; 5, tighten lock nut A; 6, replace
discs in piston D; 7, replace oil dash pot.]
[Illustration: FIGS. 2,475, and 2,476.--Front and side views (oil tank
removed) of Cutler-Hammer ~star delta switch~ _for starting small three
phase squirrel cage motors_. ~In construction~, the switch consists
of one set of stationary fingers and a rotating wooden cylinder,
carrying two sets of contacts. These parts are supported from the
switch frame casting and are enclosed in a steel tank which contains
an insulating oil. Flexible oil proof cable leads are brought out
through insulated bushings in the top of the switch and tagged for
convenience in connecting to the lines and motor. To prevent seepage
of oil, the leads are sealed into the top of the cover with an oil
proof sealing wax. The lever of the star delta switch is arranged
with an interlock which prevents its being thrown directly into the
running position from the off position. It is necessary to throw the
lever first into the starting position and then with an uninterrupted
movement to the running position. The circuit of the motor is broken
only for an instant in changing from star to delta and no heavy
inrush current occurs. ~No voltage release~ protection is provided
by a latching solenoid which holds the spring centered drum cylinder
in the running position. The no voltage release coil is mounted in
the lower part of the starting switch, immersed in the oil tank, and
is protected against mechanical injury and grounding. The coil is in
circuit during the running period only and requires not more than 8
to 15 watts to hold the switch in the running position. The operation
of this protective device is such that on failure of voltage the
star delta switch will immediately be returned to the off position.
~Overload release~ protection consists of two relays on a small slate
panel, which is mounted directly on the side of the star delta switch.
The switch contacts of the overload release are connected in series
with the connections to the no voltage release coil so that when an
overload occurs the overload relay operates to open the circuit to the
no voltage release coil, thus permitting the switch lever to return to
the off position. The overload relays do not afford overload protection
during the starting period, and when such protection is desired
starting fuses should be installed. These fuses, if used, should have a
capacity of 250 to 300 per cent. of the normal full load current of the
motor.]
[Illustration: FIG. 2,477.--Diagram of connections of General Electric
three phase starting compensator with low voltage release and overload
relays for 1,040-2,500 volt circuits.]