OPERATION OF MOTORS
In operating motors of any considerable size, whether connected to the
public supply mains of a central generating station for combined lighting
and power service, or to power service mains only, there are certain
precautions to be observed in starting, stopping, and regulating the
motor, in order that the efficiency of the supply, and indirectly the
working of other motors and lamps connected to the mains in the immediate
neighborhood, may not be affected by abnormal variations of pressure.
These precautions should be observed also to prevent any danger of
the motor itself being subjected to detrimental mechanical shocks and
excessive temperatures in the working parts.
Before Starting a Motor.--The general instructions relating to inspection
and adjustment, lubrication, etc., which have already been given, should
be carefully followed preparatory to starting[E].
[E] NOTE.--In starting a motor, first see that the bearings contain
sufficient oil and that the brushes bear evenly on the commutator. If a
circuit breaker be used, close it; then close the main switch. Rotate
slowly the handle of the starting rheostat as far as it will go. Care
should be taken, in starting the motor, that the handle of the rheostat be
not rotated too fast. To stop a motor, open the circuit breaker or switch,
which will cut in the resistance of the starting box. Never attempt to
stop a motor by forcibly pulling open the starting box, _Disregard of
these instructions may cause burning out of the field coils._
Starting a Motor.--In starting a motor, resistance must be put in series
with the armature because, since there is no reverse electromotive force
to counteract the applied voltage when the motor is at rest, the switching
of the latter direct to the motor would result in an abnormal rush of
current. This, in addition to being uneconomical and productive of a drop
of voltage in the mains, would injure all except the smallest motors.
Hence motors above two horse power usually require a rheostat.
Ques. Describe a rheostat or "starting box."
Ans. It consists essentially of a suitable resistance to be inserted at
starting to reduce the initial rush of current, and which can be cut out
in sections by successive movements of a lever as the speed increases.
[Illustration: Fig. 731.--Press for forcing on and removing a commutator.
Small commutators are pressed on to the shaft by a hand press. All of
the larger commutators are pressed on by means of a power press. In the
above figure is shown a hand press. The plate _B_ is used in removing
old commutators. It is placed back of the commutator as at _x y_ with
the slot _C_ over the shaft. Bolts _a b_ are passed through the holes in
the plate and secured by nuts. The commutator can then be forced off the
shaft. In pressing on a commutator, a sleeve is placed over the shaft at
_O_, and against the commutator. The rear end of the shaft is secured so
it will withstand the pressure, and the commutator is forced on. The power
presses are built on the principle of a hydraulic press. In pressing on a
commutator a piece of babbitt metal or soft brass should be used against
the end of the shaft. The shaft should be painted with white lead before
having the commutator pressed on, in order to lubricate the shaft so that
the commutator will press on easily. The wiper rings are pressed on after
the commutator and then the armature is ready to be connected.]
Ques. Describe what occurs in starting a motor.
Ans. When the lever of the starting box is moved to the first contact some
of the resistance is cut out of the circuit and current flows through
the motor. This produces a torque and starts the armature rotating. The
movement of the armature induces a reverse voltage, which, as the speed
increases, gradually reduces the applied current. With this reduction of
current, the torque is reduced and the speed not accelerated as quickly as
at first. When the applied current has been reduced to a certain value by
the increasing reverse current, the handle of the starting box is moved
to the next contact, and so on till all the resistance in the starting
box has been cut out, the motor then attaining its normal speed.
[Illustration: Figs. 732 to 735.--Various starting resistances. The type
of resistance used in motor starting rheostats of small size consists
usually of tinned iron wire wound on asbestos tubes, as shown in fig. 732,
the tubes being firmly supported by porcelain nipples, the ends of which
fit into holes in the top and bottom of the enclosing case. In starters of
larger size, cast metal grids, as shown in fig. 733, are used. In addition
to these types of resistance, some forms of starter are equipped with
what is known as "unit" type resistance. In this form, the resistance is
built up of a number of separate sections, or units, which are connected
to form the complete starting or regulating resistance as the case may be.
A single unit consists of a moulded core of vitreous material upon which
is wound the resistance wire, as shown in fig. 734. The surface of the
unit is then coated with a special cement and baked. By this method the
resistance material is protected from mechanical injury and is also made
proof against moisture and other conditions which sometimes affect the
ordinary type of resistance. In addition to units coated with cement only,
there are still other types of units, as in fig. 735, which are provided
with a sheet metal covering around the cement, as a further precaution
against injury. Each of the various types of resistance described
possesses certain characteristics not shared by the others, the use of
any particular type being largely governed by conditions of service.]
Ques. What is the difference between a starting box and a speed regulator?
Ans. Motor starting rheostats or "starting boxes," are designed to start
a motor and bring it gradually from rest to full speed. They are _not_
intended to regulate the speed and must not be used for such purpose.
_Failure to observe this caution will result in burning out the
resistance which, in a motor starter, is sufficient to carry the
current for a limited time only_, whereas in the case of speed
regulators sufficient resistance is provided to carry the full load
current continuously.
[Illustration: Fig. 736.--View of Cutler-Hammer starter with slate front
removed showing open wire coil resistance. The type of resistance here
used consists of tinned iron wire wound on asbestos tubes. The bottom of
the casing is perforated to secure ventilation.]
Ques. For what kinds of service are speed regulators used?
Ans. In cases when the speed must be varied, as in traction motors, organ
blowers, machine tool drive, etc.
Ques. How long does it take to start a motor?
Ans. Usually from five to ten seconds.
Ques. How is the starting lever operated?
Ans. It is moved progressively from contact to contact, pausing long
enough on each contact for the motor to accelerate its speed before
passing to the next.
Ques. What are the conditions at starting in a series motor?
Ans. There is a rush of current, the magnitude of which depends on the
amount of resistance cut out at each movement of the starting lever.
[Illustration: Figs. 737 and 738.--Sliding contact starters. Fig. 737,
starter with button contacts; fig. 738, starter with renewable contacts.
Motor starters in which the successive steps of resistance are cut out by
a pivoted lever carrying a contact shoe which slides over button contacts
or over contact segments, are known as sliding contact starters. Button
contacts are usually furnished with motor starting rheostats of small size
while contact segments are used on those of greater capacity. The contact
segment being held in position by two screws, is readily renewable when
worn by long service or damaged by arcing. The fixed button contact is
not so easily renewed but being used only on small size starters is never
likely to be subjected to severe service. Some starters, however, have
renewable button contacts.]
Ques. How are small series motors started on battery circuits?
Ans. By simply closing a switch to complete the circuit, the resistance
of the battery being sufficient to prevent a great rush of current while
starting.
Ques. How is a shunt motor started?
Ans. In starting a shunt motor, no trouble is likely to occur in
connecting the field coils to the circuit. Since the resistance of the
armature is very low, it is necessary on constant voltage circuits to use
a starting rheostat in series with the armature.
The necessary connections are shown in fig. 756. The switch is first
closed thus sending current through the field coils, before any
passes through the armature. The rheostat lever P is then moved to the
first contact to allow a moderate amount of current to pass through
the armature. The resistance of the rheostat is gradually cut out by
further movement of the lever P, thus bringing the motor up to speed.
[Illustration: Figs. 739 and 740.--Multiple switch starters. Fig. 739,
starter with no voltage release; fig. 740, starter with no voltage release
and circuit breaker. The multiple switch type of starter is designed
to overcome the arcing on sliding contacts which, in the case of large
motors would be very severe. The cutting out of each step of resistance is
accomplished in the multiple switch starter by a separate carbon contact
switch which breaks the circuit with a quick snappy action.]
Ques. How does the reverse voltage affect the starting of a motor?
Ans. When a motor is standing still, there is no reverse voltage, and
the current taken at first is governed principally by the resistance of
the circuit. If the motor be series wound, there is a momentary reverse
voltage, due to self-induction while the field is building up. If the
motor be shunt wound, self-induction delays the current through the
field coils, but that through the armature is not impeded by such cause.
When the armature begins to revolve, reverse voltage is developed which
increases with the speed. The resistance of the starting box may be
gradually cut out as the armature comes to speed. Thus the reverse voltage
gradually replaces ohmic drop in limiting the current as the motor comes
to speed.
[Illustration: Fig. 741.--Starting rheostat with no voltage and overload
release. The no voltage release permits the starting lever to fly to
the "off position" should the voltage fail momentarily, thus protecting
the motor against damage should the voltage suddenly return to the
line. The movement of the lever is due to a spring. The overload device
causes the lever to back to the off position should the current exceed a
predetermined maximum for which the release is adjusted.
Fig. 742.--Compound starter. Rheostats designed for the double duty of
starting a motor and regulating its speed are commonly known as compound
starters, the resistance provided being a combination of armature
resistance for starting duty and shunt field resistance for speed
regulation.]
Failure to Start.--This fault, which is liable to occur in a motor of any
description, is similar to failure to excite in a dynamo, and is liable to
be produced by any of the causes mentioned in connection with the latter
fault, excluding insufficient speed, and insufficient residual magnetism.
When a motor fails to start, it should first be ascertained if a supply
of electrical energy be available in the mains. This may readily be
discovered by means of a voltmeter, or if low tension service, by means of
the fingers bridging across the main terminals. If the supply of energy
be present, the contact arm of the starter should be moved into such
position that all resistance is inserted into circuit with the motor. This
is important, as the motor may start suddenly while trying to ascertain
the cause of the stoppage.
[Illustration: Fig. 743.--Starting panel. In installing any kind of motor
starting rheostat, it is necessary to provide main line knife switch and
fuses in addition to the starting box. The appearance of the installation
can be much improved by mounting all of these upon one panel.]
Having closed the switch, if the motor fail to start, it will be advisable
to remove the load if possible, as the failure may arise from an overload
of the machine. This being effected and the motor not starting, the
terminals of the latter should be tested by the means already described
for voltage. If no voltage be generated, a broken circuit or a defective
contact may be looked for in the main fuse, switch, or starting box. The
resistance coils of the latter, through the heat developed, frequently
break in positions out of sight. If a defective contact of this nature
cannot readily be seen, the contact arm should be moved slowly over the
contacts, as it is possible the broken coil may be cut out of circuit by
this means.
[Illustration: Figs. 744 to 746.--Cutler-Hammer motor starting rheostats
with no voltage and overload release. Fig. 744, starter with fixed button
contact, fig. 745, with renewable button contact, and fig. 746, with
contact segments. In construction the resistance is enclosed in a pressed
steel box on which is mounted a marbleized slate panel carrying the
starting lever, contacts and protective devices. By means of a calibrated
scale, the overload release (shown in the lower left hand corner, figs.
744 and 745, and in the lower right hand corner fig. 746) can be set
to break the circuit on any overload not exceeding 50 per cent. of the
rated capacity of the motor. This calibrated scale can also be used for
determining, with a fair degree of accuracy, the amount of current being
consumed by the motor.]
If a difference of pressure exist between the motor terminals, the
field magnets will, if shunt or compound wound and in good order,
be excited, which may be ascertained by means of a bar of iron. If
no magnetism be present, it will of course, indicate a broken or
bad connection, either between the terminals of the field coils, or
one or more of the coils themselves. If the bar pull strongly, the
position of the brushes upon the commutator in regard to the neutral
points should be ascertained, and the rocker adjusted, if necessary,
to bring them into their correct positions. If this fail to start the
motor, the connecting leads from the motor terminals to the brushes
and the brushes themselves should be carefully examined for broken
or bad connections, and defective contact of the brushes with the
commutator. In the latter case, it may arise from a dirty state of
the commutator, or from the brushes not being fed properly. If due
to these causes, pressing the brushes down upon the commutator with
the fingers will probably start the motor. If the failure to start
arise from none of these causes, it is probably due to the field coils
acting in opposition, or to a short circuited armature. This latter
remark applies more especially to motors provided with drum armatures.
[Illustration: Fig. 747.--Allen-Bradley compression type resistance units.
The contact resistance between the discs composing the resistance column
is subject to variations of pressure thereby producing proportionate
resistance changes in the column as a whole. In the complete resistance
unit, the resistances column is encased in a drawn steel tube, which
is lined with a highly refractory cement, for purpose of insulation,
_affording the column both mechanical and electrical protection_ and
excluding the air which effectually prevents any combustion should the
column become red hot due to overload. The ends of the tube are closed by
means of caps through which pass electrodes for making connections between
the discs and exterior conductors. The steel tube, when necessary, is
provided with ribs or fins for the dissipation of acquired heat.]
[Illustration: Fig. 748.--Allen-Bradley type Z automatic motor starter.
The operation of this machine is as follows: When the main switch is
closed, the motor circuit is made through the fuses, resistance unit,
current relay, and the motor armature. At the same time, the solenoid
circuit is closed (this is connected directly across the line, and takes
a current which is a small fraction of an ampere), and the plunger of
the solenoid is drawn up, which produces a pressure on the resistance
unit, and increases the current in the motor circuit to the predetermined
value at which the current relay is set. When this value is reached the
current relay operates and opens the solenoid circuit, which reduces the
magnetic pull and allows the solenoid plunger to drop back slightly. This
action increases the resistance in the motor circuit, which decreases the
current sufficiently to allow the relay to close again. Similar cycles
of operation are repeated as the motor accelerates, and each time the
plunger is drawn a little farther into the solenoid, until the short
circuiting contacts on the top are pushed together, which short circuits
the current relay and resistance unit, making them inoperative, and
completing the operation of starting the motor. It will be noted that in
starting a motor with this device the current is always held down to a
certain predetermined value, and it is impossible to overload the motor
by too rapid starting. The current relay is calibrated in amperes, and
may be set to suit existing conditions. The action of the starter being
controlled by a current relay and not by an oil or air dash pot, the motor
will start rapidly when under a light load, and slowly when more heavily
loaded. The fuses or circuit breakers may be set at a value that will
furnish protection to the motor under running conditions.]
Precautions with Shunt Motors.--With motors of this type, because of the
large amount of self-induction in the shunt windings, it is important to
note: 1, that in switching on the field magnet, the current may take an
appreciable time to grow to its normal value, and 2, that in switching
off, especially with quick break switches, high voltages are induced in
the windings, which may break down the insulation.
[Illustration: Fig. 749.--Monitor starter giving automatic start with
knife switch control; designed for use with printing presses. It consists
of a set of solenoids connected in series and so interoperating as to cut
resistance out of the armature circuit of the motor as the apparatus it
is driving comes up to speed. This type is for small motors or where no
need arises for speed regulation; there is, therefore, no adjustment of
speed possible aside from an actual change in motor conditions. At full
speed the motor is directly across the main supply line.
Fig. 750.--Monitor automatic starter, equipped with relay for push button
control.]
Ques. What provision is made so that the magnetizing current will have
time to reach its normal value?
Ans. The field connections are generally separated from the actual
starter, and taken to the main switch, so that wherever the main switch
is closed, the current flows through the field coils, before the starting
lever is moved.
Ques. How are the connections arranged to avoid excessive voltage in the
windings due to self-induction?
Ans. Generally the armature and field magnet circuits are placed in a
closed circuit that is never opened.
In other cases, in order that the rise of voltage may not injure the
insulation when the shunt is opened, a special form of main switch
is sometimes used which, before breaking from the supply, puts a
non-inductive resistance across the shunt of the motor. This is known
as a _flashing resistance_.
[Illustration: Figs. 751 to 753.--Monitor control switches. Fig. 751, push
button "start" and "stop" switch; fig. 752, safety lever control switch
with "slow" and "fast" buttons for rotary printing presses. This device
will upon pressure of the "start" button, set the machine in motion and
bring it up to the predetermined speed, either as previously set by the
starter limits or by the setting of the rheostat arm. The stop button
projects some distance beyond any other portion of the device, in order
that in case of emergency the operator may stop the machine merely by
hitting the face of the switch with his open hand. The lever control
switch, fig. 753, is similar in its action to the push button switch but
combines two other features: locking point, and visual indication of the
station from whence the press has been stopped. With the lever at the
downward position, the press is locked and cannot be started from any
other station. In order to make the press ready to start the lever must
be raised to the central position. Thus a man may safely enter the press
without delay by setting his station to the locked position, knowing
that it cannot be started except by some one coming to that station and
realizing that the press has been purposely locked. Also, by looking along
the press, it is possible to tell from which station it has been locked,
and proper action can be immediately taken. The safety control station
is usually combined for use on large rotary presses with the "slow" and
"fast" push buttons as shown in fig. 752. A pressure upon the fast or
slow buttons will cause the press speed to be correspondingly accelerated
or retarded, and this action will continue so long as the button is
pressed. The press continues to run at the speed attained at the instant
of releasing the button. Any speed may, therefore, be selected or changed
to suit momentary requirements. This gives complete control excepting
reversal which is not required of such a press.]
[Illustration: Fig. 754.--Wiring diagram of the standard form of
Monitor controller. A set of solenoids are connected in series and so
interoperating as to cut resistance out of the armature circuit of the
motor as the apparatus it is driving comes up to speed. The controller
is designed to be used on all classes of work. In its simplest form,
a single copper and graphite contact, is controlled by two magnets,
so proportioned as to cut out the entire starting resistance when the
armature current falls to a predetermined value. In the larger sizes,
the number of steps controlling the resistance is increased and arranged
to produce the correct acceleration. In every case the regulation of the
starting resistance is effected entirely by the current passing to the
motor without the use of dash pot, mechanical governor or delicate cut
outs. The time element is thus directly proportioned to the load and the
motor brought up to speed in the shortest time consistent with the load,
but always with safe limitation of the armature current. The distinction
between the current controlled starter and the starter with dash pot
governor should be noted. The starter here shown limits the starting
current to a fixed value throughout the starting operation, which is an
ideal condition and prevents blowing fuses in starting.]
Ques. How can shunt motors be controlled from a distant point?
Ans. The starter and switch are placed at the desired point and the two
main wires and the field wires run from that point to the motor.
This requires additional wire which increases the cost and line loss.
Regulation of Motor Speed.--Motors are generally run on constant voltage
circuits. Under these conditions, the speed of series motors varies with
the load and at light loads becomes excessive. Shunt motors run at nearly
constant speeds.
For many purposes, particularly for traction, and for driving tools, it
is desirable to have speed regulation, so that motors running on constant
voltage circuits may be made to run at different speeds.
The following two methods are generally used for regulating the speed of
motors operated on constant voltage circuits:
1. By inserting resistance in the armature circuit of a shunt wound motor;
2. By varying the field strength of series motors by switching sections
of the field coils in or out of circuit.
Ques. Describe the first method.
Ans. This method is illustrated in fig. 756. When the main switch is
closed, the field becomes excited, then by moving the lever P of the
starting rheostat the various contacts (1, 2, 3, 4, 5), more or less of
the rheostat resistance is cut out of the armature circuit, thus varying
the speed correspondingly.
This is the same as the method of starting a motor, that is, _by
variation of resistance in armature circuit_, but it should be
noted that when this method is used for speed regulation, _a speed
regulating rheostat should be used instead of the ordinary starting
box_, because the latter, not being designed for the purpose, _will
overheat and probably burn out_.
[Illustration: Fig. 755.--Monitor printing press controller. It provides
variable speed and other control features required in the operation
of large rotary presses, such as those used for printing newspapers.
From any one of various stations similar to the one illustrated in fig.
753, located at all desirable places about the press, the latter may be
started, stopped, accelerated, slowed down or locked. It differs from
other types of printing press controller in that the solenoid has an
overall maximum pull of less than one inch and does not actuate the main
line current directly but through pilot circuits, which in turn, operate
flapper switches; there are no sliding contacts. At the control stations,
the operator can distinguish the accelerating button from the retarding
button by the sense of touch and obviously he can in the same manner
ascertain the position of the lever. The position of the lever whether
at start, stop or safety, can be readily observed at a distance. When
the lever of either control station is placed at stop, the current is
disconnected from the motor and a powerful dynamic brake brings the press
to rest without delay and without shock or harmful strain. The start will
always be made with all resistance in the armature circuit, and with full
field, and should the current supply fail, the controller will release and
open the circuit to the motor. This controller will give a speed range as
low as 10% of normal speed by armature resistance and, by field control,
any increase within the speed of the motor.]
Ques. Describe the second method.
Ans. This method of regulating the speed of a series motor is shown in
fig. 757. The current through the armature will flow through all the field
windings when the position of the switch lever S, is on contact 4, and the
strength of the field will be the maximum. By moving the arm to contact
3, 2, etc., sections of the field winding are cut out, thus reducing the
strength of field and varying the speed.
[Illustration: Fig. 756.--Speed regulation of shunt motor by variable
resistance in the armature circuit.]
Ques. How does the speed vary with respect to variation of field strength?
Ans. Decreasing the field strength of a motor increases its speed, while
increasing the field strength decreases the speed.
Under the conditions of maximum field strength, as with switch S on
point 1, the torque will be greatest for any given current strength
and the reverse voltage also greatest at any given speed. The current
through the armature of the motor, to perform any given work, will
thus be a minimum, as well as the speed at which the motor has to run,
in order to develop sufficient reverse voltage to permit this current
to flow. Regulation of speed by varying the field strength is limited
in range of action, since the field saturation point is soon reached,
moreover, with too low a field strength, armature reaction produces
excessive field distortion, sparking, etc.
[Illustration: Fig. 757.--Speed regulation of series motor by cutting out
sections of the field winding. In this method the field winding is tapped
at several points, dividing the coil into sections and the leads from
these points are connected a multi-point switch of the type that would
be used on a rheostat. By moving the lever S, to the left or right, the
current will flow through one or more sections of the field winding, thus
decreasing or increasing the ampere turns and thereby providing means of
regulation.]
[Illustration: Fig. 758.--Speed regulation of a series motor by the method
of short circuiting sections of the field winding. It will be seen that
there are seven different positions for the contact springs on the barrel
contacts. A. represents the armature and brushes, little A, B, and C,
the divided field magnet coils, L the line connection, and G the earth
connection. The diagram shows the connections for trolley car operation.]
Ques. How is the speed of shunt and compound motors varied with respect
to the normal speed in the two methods?
Ans. The first method (variable resistance in armature circuit) reduces
the speed _below_ the normal or rated speed of the machine, while the
second method increases the speed _above_ the normal.
In the first method the amount of speed reduction depends partly upon
the amount of resistance introduced into the armature circuit, and
partly upon the load.
In the second method the amount of speed increase depends entirely
upon the amount of resistance placed in the shunt winding circuit.
Eighty-five per cent. is about the maximum speed reduction obtainable by
armature resistance but so great a reduction is seldom satisfactory since
comparatively slight increases in the load will cause the motor to stall.
Shunt field regulation may be obtained up to any point for which the
motor is suited, the only limitation in this case being the maximum
speed at which the motor may be safely operated.
It should be remembered, however, that speed increase by shunt field
weakening increases the current in proportion to the increase in
speed, and care should be taken not to overload the armature.
NOTE.--A compound motor may be made to run at constant speed, if
the current in the series winding of the field be arranged to act in
opposition to that of the shunt winding. In such case, an increase
of load will weaken the fields and allow more current to flow through
the armature without decreasing the speed of the armature, as would be
necessary in a shunt motor. Such motors, however, are not very often
used, since an overload would weaken the fields too much and cause
trouble. If the current in the series field act in the same direction
as that in the shunt fields, the motor will slow up some when a heavy
load comes on, but will take care of the load without much trouble.
NOTE.--Motors have much the same faults as dynamos, but they make
themselves manifest in a different way. An open field circuit will
prevent the motor starting, and will cause the melting of fuses or
burning out of the armature. A short circuit in the fields, if it cut
out only a part of the winding, will cause the motor to run faster and
very likely spark badly. If the brushes be not set exactly opposite
each other, there will also be bad sparking. If they be not at the
neutral point, the motor will spark badly. Brushes should always be
set at the point of least sparking. If it become necessary to open the
field circuit, it should be done slowly, letting the arc gradually
die out. A quick break of a circuit in connection with any dynamo,
or motor is not advisable, as it is very likely to break down the
insulation of the machine. The ordinary starting box for motors is
wound with comparatively fine wire and will get very hot if left in
circuit long. The movement of the arm from the first to the last point
should not occupy more than thirty seconds and if the armature do not
begin to move at the first point, the arm should be thrown back and
the trouble located.
[Illustration: Fig. 759.--Cutler-Hammer multiple switch starter with no
voltage release; for use with large motors, or with motors of medium size
where the starting conditions are severe or when more than fifteen seconds
are required to accelerate the motor. In operation, the cutting out of
each step of resistance is accomplished by a separate lever and the levers
themselves are so interlocked as to prevent closing switches except in
proper order, beginning with the lever on the left. The last switch (the
one on right hand side) is held by an electro-magnet when closed, each
of the other switches being held in the closed position by a latching
device on the switch next to it. In front of each switch is placed a metal
stop, so arranged as to prevent any switch being operated until the one
next to it on the left has been closed. These metal stops constitute the
interlocking mechanism and prevent the starting of the motor in any way
except by closing the switches in regular rotation, thus insuring proper
resistance in the circuit and protecting the motor from excessive starting
currents. When the current is interrupted, the electro-magnet releases
the last switch, which, on opening, releases the latch on the switch next
to it, allowing that switch to open, and this in turn releases the next
latch and so on, the switches opening automatically one after another.
In starting the motor, each switch should be closed quickly and firmly,
pausing a second or two before closing the next switch to give the motor
time to accelerate.]
[Illustration: Fig. 760.--Cutler-Hammer speed regulator with no voltage
release, _regulation by armature resistance only_, reducing speed of motor
_below normal_. No resistance in the armature circuit. No provision is
made in regulators of this type for increasing the speed of the motor. The
maximum speed obtainable when these regulators are used is, therefore, the
normal speed at which the motor is designed to operate with no resistance
in circuit. With all resistance in circuit and the motor taking normal
current these regulators will reduce the speed of the motor 50 per cent.
If the motor be taking less than normal current the percentage of speed
reduction obtainable will be correspondingly less. The notched fan tail
extension on the lower end of the lever engages with a magnetically
operated pawl to hold the lever squarely on any contact so long as the no
voltage release magnet is energized.]
Ques. How is a wide range of speed regulation secured?
Ans. By a combination of the two methods.
Regulation by Armature Resistance.--Speed regulators for this method
of regulation, are designed to carry the normal current on any contact
without overheating and when all the resistance is in the circuit, they
will reduce the speed of the motor about 50 per cent. provided the motor
be taking the normal current. When operating without resistance in the
armature circuit, shunt wound and compound wound motors will regulate
to approximately constant speed regardless of load. This characteristic
of inherent regulation is lost, however, when armature resistance is
employed to reduce the speed of the motor, fluctuations in load resulting
in fluctuations in speed, which become more noticeable as the amount of
resistance inserted in the armature circuit is increased. Accordingly,
it becomes necessary to move the lever of the speed regulator forward
or backward to again obtain the speed at which the machine was operating
before the load changed.
[Illustration: Fig. 761.--Cutler-Hammer compound starter with no voltage
and overload release. This is a starting rheostat and field regulator
combined. In operation, two levers are employed, both being mounted on
the same hub post and one lying directly under the other. The upper lever
only is provided with a handle, but when moving from the off position to
the starting position (that is to say, from left to right) the lower, or
starting, lever is carried along by the upper, or speed regulating, lever
until it comes in contact with the no voltage release magnet where it is
held fast by the attraction of the magnet, leaving the upper lever free
to be moved backward over the field contacts, thus weakening the shunt
field of the motor little by little until the desired speed is attained.
During the operation of starting the motor, the field resistance is
short circuited by an auxiliary contact (the slotted metal strip shown
near center of rheostat) but as soon as the starting lever touches the
no voltage release magnet or, in other words, when the motor has been
accelerated to normal speed, this short circuit is removed, and the
field resistance becomes effective for speed regulation. The motor is
accelerated from rest to normal speed by moving both levers from left to
right, while increases in speed above normal are obtained by moving the
upper lever from right to left. Only the lower, or starting lever comes
into contact with the no voltage release magnet. This lever is provided
with a strong spiral spring which tends always to throw the lever back
to the off position. Hence should the voltage fail, the no voltage
release magnet releases the starting lever and this, in flying back to
the off position, opens the armature circuit of the motor and carries the
speed regulating lever with it to the off position. The upper, or speed
regulating lever, not being influenced by the spring, though mounted on
the same hub post as the starting lever, may be moved back and forth at
will or left indefinitely in the position which gives the speed desired.]
When the speed of a motor driving a constant torque machine is
reduced by inserting resistance in the armature circuit there is
no corresponding reduction in current consumed. The motor runs more
slowly simply because a part of the energy impelling it is shunted
into the resistance and there dissipated in the form of heat. Hence,
whether the motor be operating at full speed or half speed, the amount
of current consumed is the same; the only difference being that in the
one case all the energy taken from the line is expended in driving the
motor while in the other case only one half is utilized for power,
the other half being dissipated in the resistance. Speed regulation
by armature resistance only is therefore open to two objections:
1, the difficulty of maintaining constant speed under varying load
conditions, and 2, the necessity of wasting energy to secure speed
reduction. These objections are, in part, offset by the fact that
speed reduction by armature resistance may be applied to any motor of
standard design and requires nothing more than the simplest and least
expensive speed regulating rheostat.
In cases where the motor will be operated nearly always at full speed,
the difference in first cost of the installation may justify the
use of the armature resistance method of control. As a rule, speed
regulation by shunt field resistance is preferable.
[Illustration: Fig. 762.--Cutler-Hammer compound speed regulator with no
voltage and overload release; _regulation by combined armature and shunt
field resistance_, designed to both decrease and increase the speed of
a motor. Speed reduction is accomplished by inserting resistance in the
armature circuit, the maximum amount of speed reduction obtainable with
these controllers being 50 per cent. below normal. Speed increase is
obtained by inserting resistance in the shunt field circuit, the maximum
amount of speed increase obtainable with these controllers being 25 per
cent. above normal.]
Regulation by Shunt Field Resistance.--Since regulation by this method is
for speeds above normal, a starter must be used to bring the motor up to
its rated speed. Usually the starter is combined with the regulator, as
shown in fig. 761, the device being called a _compound starter_.
[Illustration: Figs. 763 to 765.--Holzer-Cabot shunt wound motor; diagrams
showing connections and positions of index point for forward and reverse
rotation.
LOCATION AND SETTING.--The motor should be placed in as cool, clean and
well ventilated a location as possible, away from acid or other fumes
which would attack the metal parts or insulation, and should be easily
accessible for cleaning and oiling. Do not put it in some corner where
care of motor will be neglected because of the trouble of getting at
it. The motor should be set so that the shaft is level and parallel with
the shaft it is to drive so that the belt will run in the middle of the
pulleys. Do not use a belt which is much too heavy or too light for the
work it has to do, as it will materially reduce the output of the motor.
The belt should be about one-half inch narrower than the pulley.
ROTATION.--In order to reverse the direction of rotation, interchange
leads A and B, and shift brush ring as shown in the diagram above.
SUSPENDED MOTORS.--Motors with ring oil bearings may be used on the wall
or ceiling by taking off end caps and revolving 90 or 180 degrees until
the oil wells come directly below the bearings.
STARTING.--Before starting the motor see that the armature revolves
freely, that the bearings are full of oil, and the oil rings are in place
and free to turn.
Examine connections carefully to see that they are according to above
diagram, after which proceed as follows:
1. Close the main knife switch. This action should not allow any current
to pass through the motor (see Note 2);
2. Move the lever of the starting rheostat quickly and squarely onto the
first segment, and hold it there for about a second;
3. Move the lever to the second segment and hold it there for about a
second;
4. Move the lever to the third segment and hold it there for about a
second, and so on from one segment to the next until the lever has
been moved over all the segments to the short circuit position,
where it should be held firmly by the retaining magnet.
If the motor do not start when the lever of the starting rheostat is on
the third segment, open the main knife switch and look for the trouble.
This may consist of any of the following:
a. Wrong connections;
b. Too great a load on the motor;
c. The motor brushes not in proper position;
d. An open circuit of some kind;
e. A short circuit of some kind.
NOTE 1.--It is always advisable, in case of trouble, to make sure that
the fields of the motor are magnetized. This test is easily made by first
closing the main knife switch, then moving the lever of the starting
rheostat to the first segment, and finally having an assistant place a
screw driver or other piece of iron against the pole pieces of the motor.
If the fields be magnetized, a heavy pull on the iron should result.
NOTE 2.--Any possibility of arcing on the first contact of the starting
rheostat when starting can be obviated by _first_ moving the lever onto
the initial contact, holding it there, and then closing the main line
switch, after which proceed as per paragraphs 3 and 4.
TO STOP THE MOTOR.--Open the main knife switch and let the starting
rheostat take care of itself. The lever will not fly back immediately,
but will hold until the motor has slowed down considerably.
NOTE.--The above directions apply only to starters of the sliding contact
type.
TEMPERATURES.--If located as instructed above, these motors will carry
full load as indicated on the name plate on the motor with a temperature
rise of not over 40 degrees Centigrade, or 75 degrees Fahrenheit above
the surrounding air. This will feel hot to the hand but is far below the
danger point. If the motor feel too hot, get a thermometer and measure
the temperature. To do this, place the bulb of the thermometer for 10
minutes against the frame, cover with a cloth or piece of waste, and note
temperature as compared with that of room. If the motor run in a small,
enclosed space with no ventilation, the temperatures will be somewhat
higher than those given above.
OILING.--Fill the oil wells to the overflow before starting and keep them
full. Use good "dynamo oil." Be sure that the oil rings turn freely while
the motor is running. If in a dirty place, draw off the old fluid and fill
with new every two or three months.
CARE OF MOTOR.--The motor must be kept clean. If the commutator become
rough, smooth it up with No. 00 sandpaper moistened with oil. When fitting
new brushes or changing them, always sandpaper them down until they fit
the commutator perfectly, by passing to and fro beneath the brush a strip
of sandpaper, having the rough side toward the brush.
Brushes must _always_ be renewed before the metal of the holder comes in
contact with the commutator.
Don't use anything on commutator except good mineral machine oil, or
kerosene, and this only in very small quantities applied with a cloth
having no lint or threads.]
[Illustration: Fig. 766.--Sectional view showing principal parts of
Reliance adjustable speed motor: 1, lever fulcrum pin; 2, lever; 3,
sliding thrust bearing box; 4, ball bearing; 5, armature shaft end nut;
6, cap; 7, commutator end yoke; 8, lever rod; 9, compression spring; 10,
steel frame; 11, speed adjustment nut; 12, thrust collars and pins; 13,
hand wheel rod; 14, hand wheel; 15, sleeve nut; 16, oil well cover; 17,
bearing bushing; 18, sleeve; 19, oil ring; 20, pinion end yoke; 21, rocker
arm; 22, brush holder stud; 23, brush; 24, commutator; 25, armature; 26,
armature laminations; 27, armature coils; 28, armature end plate; 29,
armature shaft; 30, leads; 31, axial position of commutating pole; 32,
axial position of main field pole; 33, slide rail screws; 34, end yoke
cap screws; 35, slide rails; 36, commutating coil; 37, commutating pole;
38, main field pole; 39, main field coil.]
[Illustration: Fig. 767.--Cutler-Hammer reversible starter with no
voltage release, adapted to start and operate motor at full speed in
either direction, such for instance as motors driving auxiliary motions
on lathes, planers and other machine tools which may rotate in either
direction but always at constant speed. They are not designed to reduce
the speed of the motor, but merely to start it and bring it smoothly up to
full speed in either direction. Two no voltage release latching devices
are provided so that the lever will be held in the full speed position
in either direction so long as the voltage of the line remains constant.
On failure of voltage a strong centering spring attached to the hub-post
of the lever throws the latter to the central, or off position. The shunt
field circuit is not opened by starters of this type.]
The weakening of the shunt field of a motor by the insertion of resistance
in the shunt field circuit causes the armature to revolve more rapidly.
One advantage of this method of control is that the motor will inherently
regulate to approximately constant speed under widely varying load
conditions. Another advantage is found in the fact that all of the
current taken from the line is utilized for power, the changes in speed
being obtained not by dissipating a portion of the effective energy
in the resistance (as in the case of the armature resistance method of
control) but by weakening the reverse voltage by inserting resistance
in the shunt field circuit. Speed increase by shunt field weakening is
limited, however, to about 10 to 15 per cent. above the normal speed in
motors of standard construction. Greater ranges of speed can be obtained
from motors especially designed for shunt field control but should not be
attempted with motors of standard design without first ascertaining from
the manufacturer the maximum safe speed.
Combined Armature and Shunt Field Control.--Regulation by combined
armature and shunt field resistance is by far the easiest way of obtaining
a wide range of speeds. Rheostats embodying these methods are known as
_compound speed regulators_, one form being shown in fig. 762. Standard
regulators can be obtained giving a wide range of speed variation, and
special regulators may be constructed giving practically any desired
range.
Selection of Starters and Regulators.--Unsatisfactory operation of these
devices is, in nearly all cases, due to lack of precaution in selecting
the proper piece of apparatus for the work to be done. One of the
commonest errors is to select a rheostat of insufficient capacity. If the
current required to operate the motor at full speed with no resistance in
circuit be greater than the rated capacity of the rheostat, overheating
of the resistance will result. An increase in temperature even to a
point where the hand cannot be held on the enclosing case need cause no
apprehension, but should the resistance become red hot it indicates that
the apparatus is being worked far beyond its capacity, and the load on the
motor should be reduced or a regulator of greater capacity substituted.
If the current required to operate the motor at full speed with no
resistance in circuit be less than the rated capacity of the rheostat no
overheating will occur, but it will not be possible to secure the full
50 per cent. speed reduction the rheostat is designed to give with all
resistance in circuit.
[Illustration: Fig. 768.--Various sizes of Watson commutator. The segments
are punched from hard drawn copper strip and are insulated from each
other and the core by amber mica, of hardness corresponding to that of
the copper in order that the wear of mica and copper may be uniform. The
segments are assembled in a ring under great pressure and are repeatedly
heated and tightened, being finally secured and rigidly locked together.]
In ordering a starter or regulator, the manufacturer should be furnished
with the following information:
1. Horse power of motor with which speed regulator will be used;
2. Voltage of motor;
3. Winding of motor, whether series, shunt, or compound wound;
4. Nature of the machine which motor is to operate;
5. Normal rated speed of motor to be used;
6. Maximum speed at which it is desired to operate the motor;
7. Minimum speed at which it is desired to operate the motor;
8. Whether controller will ever be required to reverse direction of
motor or to operate it in one direction only;
9. If reversible controller be desired, whether or not full range of
speed control is required in both directions;
10. Whether the regulator shall be equipped with any of the following
devices: no voltage release, overload release, knife switch, fuses;
11. Whether button contacts or renewable contact segments are preferred;
12. Giving, also, if possible, the resistance of the shunt field
cold, and the shunt field current at the maximum speed required.
If this cannot be ascertained, give horse power, voltage, normal
speed, maximum speed required, serial number of motor and name of
manufacturer.
[Illustration: Fig. 769.--Organ blower speed regulator; diagram showing
operation and method of installing. A cord running from the top of the
organ bellows passes over two pulleys and is then made fast to the chain
furnished with the regulator. This chain passes around a sheave which
turns on a post projecting from the center of the slate panel. Attached to
the lower end of the chain is a weight, also furnished with the regulator.
As the air is exhausted from the bellows the latter slowly collapses,
drawing the rope down with it, and in so doing turns the sheave from left
to right, thus cutting resistance out of circuit and increasing the speed
of the motor which pumps air into the bellows. Responding to the inrush of
air, the bellows expands, relaxing the tension on the rope which is now
pulled in the opposite direction by the weight, thus turning the sheave
from right to left, cutting resistance into circuit once more and slowing
down the motor. The speed of the motor is thus automatically regulated
by the bellows, with the result that a practically uniform pressure is
maintained at all times. In connection with an organ blower regulator it
is necessary to install a separate starting rheostat. This is required
for the reason that all organ bellows leak. During the intermissions in
the musical part of the service, or at other times when the blower is not
operating, the air gradually escapes and the bellows settles down, moving
the rheostat arm to the right and cutting out resistance. With the motor
at rest and the bellows empty all the blower regulator resistance would
be short circuited and it is therefore necessary to avoid throwing the
motor directly across the line when starting again. A starting rheostat
with no voltage release is suitable for this purpose, and should be
installed within easy reach of the organist, so that a moment or two
before beginning to play he can move the lever of the starting box and get
the motor into operation. Where remote control is desirable a self starter
can be substituted for the manually operated starting box, in which case
the entire installation can be controlled by a push button, or single
throw knife switch.]
[Illustration: Fig. 770.--General Electric type K7 controller with cover
open showing construction. The mechanism consists of a long spindle,
carrying a number of heavy brass or gun metal segments, making contact for
a longer or shorter time with a corresponding number of spring contacts.
The spindle is provided at its upper end with a handle, and the various
contacts are made by turning it through an arc of about 150°. For this
method a moderate amount of resistance is employed. The first contact
joins both motors and the full amount of resistance in series across
the line, and as the motors are standing still, maximum current flows so
that they exert their full torque. The moment they start to revolve, the
current tends to fall, due to the generation of a reverse voltage; to
prevent this and maintain a heavy current for some time, thus obtaining
rapid acceleration, the resistance is arranged so that it can be gradually
reduced, until at about the fourth notch the two motors are in series
without resistance across the line. To increase still further the speed in
the above type of controller, the series fields may be shunted, and then
the next steps place the motors in parallel with the resistance.]
Speed Regulation of Traction Motors.--The speed regulator for motors of
this class is called a _controller_, and being located in an exposed place
is enclosed in a metal casing. Controllers are designed to be used for
starting, stopping, reversing, and regulating the speed of motors where
one or more of these operations have to be frequently repeated.
The controller used with a single motor equipment is practically
the same as any other single motor starting box, excepting that
the resistance has sufficient carrying capacity to be left in the
circuit some time. When the motor is to operate at full speed all the
resistance is cut out. To reverse, a reversing notch is placed in the
armature or field circuit, but not in both.
Ques. What provision is made to overcome the arc when the circuit is
opened?
Ans. A magnetic field is used with such polarity that it blows out the arc.
[Illustration: Fig. 771.--Controller of the Rauch and Lang electric
vehicles. It is of the flat radial type. Two movable copper leaf contacts
of ample size make all commutations necessary to obtain the various
speeds. Five speeds forward and reverse are provided.]
Magnetic blow out coils are used on all controllers designed for 500
volt circuits, and on types designed for lower voltages requiring more
than 60 amperes normal capacity.
The coils are wound with either copper wire or flat strips of
sufficient capacity to carry full load current continuously without
undue heating, and after being wound they are treated with an
insulating compound making them moisture proof.
Ques. What provision is made to prevent reversal before bringing the
controller lever to the "off" position?
Ans. Controllers having separate reversing cylinders are fitted with
mechanical interlocks making it necessary to place lever in off position
before reversing.
[Illustration: Figs. 772 to 782.--Diagram of controller connections,
illustrating the series parallel method of two motor control.]
Two Motor Regulation.--With a two motor equipment, the controller becomes
more complicated because it must be arranged to switch the motors in
series or in parallel, so as to secure economy at half and full speed.
The various connections of series-parallel regulation are shown in figs.
772 to 782.
From these diagrams it is seen that the motors are first operated in
series until all the resistance is cut out by the controller (figs.
772 to 777).
The next point on the controller puts the two motors in parallel
with some resistance in the circuit (fig. 778), which resistance is
gradually short circuited on the remaining controller points, until
at full speed all the resistance is cut out, the two motors remaining
in parallel (figs. 778 to 782).
Stopping a Motor.--If it be desired to stop a motor, the main switch
is opened. As the armature of the motor continues to operate, due to
its inertia, it generates an electromotive force which sends a current
through the shunt connected field circuit and helps to maintain the field
excitation. When the speed of the motor has decreased sufficiently so as
not to endanger the motor should the main switch be thrown, the current
in the series magnet becomes weakened, and the spring throws back the
starting box arm.
It should be noted that in stopping a motor having a starting box provided
with a no voltage release simply open the main switch and do not touch the
lever because otherwise, the self induced voltage of the field circuit
may puncture the field winding or the insulation of the adjoining wires
in the starting box.
HAWKINS PRACTICAL LIBRARY OF
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ELECTRICAL GUIDE, NO. 1
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
ELECTRICAL GUIDE, NO. 2
The construction of Dynamos, Motors, Armatures, Armature Windings,
Installing of Dynamos.
ELECTRICAL GUIDE, NO. 3
Electrical Instruments, Testing, Practical Management of Dynamos
and Motors.
ELECTRICAL GUIDE, NO. 4
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
ELECTRICAL GUIDE, NO. 5
Principles of Alternating Currents and Alternators.
ELECTRICAL GUIDE, NO. 6
Alternating Current Motors, Transformers, Converters, Rectifiers.
ELECTRICAL GUIDE, NO. 7
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
ELECTRICAL GUIDE, NO. 8
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
ELECTRICAL GUIDE, NO. 9
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
ELECTRICAL GUIDE, NO. 10
Modern Practical Applications of Electricity and Ready Reference
Index of the 10 Numbers.
Theo. Audel & Co. Publishers 72 FIFTH AVENUE, NEW YORK