MOTORS
An electric motor is just the reverse of a dynamo; it is _a machine for
converting electrical energy into mechanical energy_.
The electrical energy delivered by the dynamo must be obtained from
a steam engine, gas engine, or other power; the mechanical energy
obtained from the motor comes from the energy of the current flowing
through its armature.
=Ques. What is the construction of a motor?=
Ans. It is constructed in the same manner as a dynamo.
Any machine that can be used as a dynamo will, when supplied
with electrical power, run as a motor, and conversely, a motor
when driven by mechanical power, will supply electrical energy
to the circuit connected to it. Dynamos and motors, therefore,
are convertible machines, and the differences that are found
in practice are largely mechanical; they arise chiefly from
the conditions under which the motor must work. Hence, the
study of the motor begins with a knowledge of the dynamo, and
accordingly the student should understand thoroughly all the
fundamental principles of the dynamo, as already given, before
proceeding further with the study of the motor.
=Principles of the Motor.=--All the early attempts to introduce
motors failed, chiefly because the law of the conservation of energy
was not fully recognized. This law states that _energy can neither be
created nor destroyed_.
Early experimenters discovered, by placing a galvanometer in a circuit
with a motor and battery, that, _when the motor was running, the
battery was unable to force through the wires so strong a current as
that which flowed when the motor was standing still_. Moreover, the
faster the motor ran, the weaker did the current become.
[Illustration: FIG. 389.--Conductor, lying in a magnetic
field and carrying no current; the field is not distorted whether the
conductor be at rest or in motion.]
=Ques. Why does less current flow when the motor is running than when
standing still?=
Ans. Because the motor, on account of its rotation acts as a dynamo and
thus tends to set up in the circuit a _reverse electromotive force_,
that is, an electromotive force in opposite direction to the current
which is driving the motor.
=Ques. What is the real driving force which causes the armature of a
motor to rotate?=
Ans. _The propelling drag_, that is, the drag which the magnetic field
exerts upon the armature wires through which the current is flowing, or
in the case of deeply toothed cores, upon the protruding teeth.
=The Propelling Drag.=--In fig. 389 is shown the condition which
prevails when a conductor carrying no current is placed in a uniform
magnetic field. The magnetic lines pass straight from one pole to the
other. The field is not distorted whether the conductor be at rest or
in motion, so long as there is no flow of current. This represents
the condition in the air gap of a motor or dynamo, when no current is
flowing in the armature.
=Ques. What happens when a current flows in the conductor of fig.
389.=
Ans. It sets up a magnetic field of its own as shown in fig. 390.
=Ques. What is the effect of this magnetic field?=
Ans. It distorts the original field (fig. 389) in which the conductor
lies, making the magnetic lines denser on one side and less dense on
the other as in fig. 390.
=Ques. What is the nature of these distorted magnetic lines?=
Ans. They tend to shorten themselves to their original form of straight
lines.
=Ques. What effect has this on the conductor?=
Ans. It produces a force on the conductor tending to push it in the
direction indicated by the arrow, fig. 390.
[Illustration: FIG. 390.--Conductor carrying a current in a
magnetic field. The current flowing in the conductor sets up a magnetic
field which distorts the original field as shown, making the magnetic
lines denser on one side and less dense on the other. This results in a
force upon the wire, which, in the case of a dynamo (fig. 391) opposes
its movement, and which forms the _propelling drag_ in the case of a
motor (fig. 392).]
The distorted magnetic lines may be regarded as so many rubber
bands tending to straighten themselves; The result then is
clearly to force the conductor in the direction indicated.
According to Lenz' law, the direction of the current in the
armature of a dynamo is such as to oppose the motion producing
it. When the armature of a dynamo is rotated, the bending of
the lines of force of the main magnetic field due to armature
reaction acts as a drag against the motion of the armature.
Armature reaction increases with the increase of the armature
current. Therefore, the effect of the drag increases with the
increase of load and requires an additional expenditure of
power to drive the armature.
In a motor, the direction of the actuating current is
the reverse of that of the armature current of a dynamo,
consequently, the armature reaction which constitutes a drag,
acting against rotation of the armature of a dynamo, becomes a
pull in the direction of rotation of the armature of a motor
and constitutes its real turning effect or _torque_ which is
used at the pulley to do mechanical work. The greater the load
applied to the motor, the greater will be the amount of current
taken from the supply mains, and consequently, the greater the
_torque_.
[Illustration: FIGS. 391 and 392.--Action of the magnetic
force in a dynamo and motor. In the first instance, according to
Lenz' law, the direction of the current induced in the wire is such
as to oppose the motion producing it. In the operation of a motor,
the current supplied in flowing through the armature winding distorts
the field and thus produces rotation. In the figures, the direction
of the force is clearly indicated by remembering that the distorted
lines of force act like rubber bands tending to straighten and shorten
themselves.]
=Ques. What are the essential requirements of construction in a
motor?=
Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular
to the field, and 3, provision for motion, of the conductors across the
field in a direction perpendicular to both themselves and the field.
=The Reverse Electromotive Force.=--When an electric current flows
through some portion of a circuit in which there is an electromotive
force, the current will there either receive or give up energy,
according to whether the electromotive force acts _with_ or _against_
the current.
[Illustration: FIG. 393.--Force exerted on a current carrying
conductor placed across a magnetic field. Let N, S, be the pole pieces
of an ordinary electromagnet, having their faces flat and with only a
narrow air gap between. In this gap is stretched the vertical copper
wire A B, kept taut by a strong spring at A; current can be passed into
the wire from the leads C and D. Attached to the wire in the middle of
the gap is a horizontal cord passing over a pulley P and kept taut by
a weight W; the pulley carries a pointer F which moves in front of a
scale _s s_. If the electromagnet be now excited and have the polarity
indicated, it will be found that on passing a strong current _down_
the wire, the index F moves toward the _right_, showing a similar
movement in the wire. The index returns to zero when the current in
the wire ceases, and moves in the opposite direction if the current
in the wire be reversed and sent _up_ instead of down. The experiment
can be further varied by reversing the magnetizing current of the
electromagnet.]
This is illustrated in fig. 395, which represents a circuit
in which there is a dynamo and a motor. Each is rotating
clockwise, and accordingly, each generates an electromotive
force tending upward from the lower to the upper brush.
In both cases the upper brush is positive. In the dynamo,
however, where energy is being supplied to the circuit, the
electromotive force is in the same direction as the current,
and in the motor, where work is being done, the electromotive
force is in the reverse direction to that of the dynamo.
[Illustration: FIG. 394.--Showing relative directions of
armature current and reverse electromotive force of a motor. When a
motor is in operation, the wires around the periphery of its armature
"cut" the magnetic lines of force produced by the field magnet exactly
as in the case of the dynamo. Consequently, an electromotive force
is induced in each wire, as in the dynamo armature. This induced
electromotive force is in opposition to the flow of current due to
the electromotive force of the supply circuit, and tends, therefore,
to keep down the flow of current. The figure shows a single loop
of wire, on the armature core connected directly to the source of
electricity. With current flowing in the loop in the direction
indicated by the arrows marked _c_, a magnetic field is set up in the
direction indicated by the large arrow marked "direction of armature
flux." With the field magnet energized so as to produce a field in the
direction indicated by the large arrow F, the reaction between the
two fields will turn the armature core in the direction indicated by
the arrow R. As the core turns, the upper wire of the loop will cut
the flux under the south pole of the field magnet, and the other side
of the loop will cut the flux under the north pole. The result will
be the induction of a reverse electromotive force in the loop, the
direction being indicated by the small arrows marked _e_. The actual
flow of current in the armature is that due to the difference between
the impressed and reverse voltage; the latter is proportional to the
speed of the armature, the number of armature wires and the strength
of the magnetic field in the air gaps between the armature and the
pole faces. The speed of a motor supplied with current at constant
voltage varies directly with the reverse electromotive force, also with
other conditions fixed, the stronger the field, the slower the speed.
Weakening the field will increase the speed up to the point where the
increase in reverse electromotive force due to the increased speed
cuts down the armature current below the value necessary to give the
requisite pull at the armature periphery. When this point is reached,
any weakening of the field will reduce the speed of the armature. The
pull or _torque_ of a motor armature is directly proportional to the
strength of the magnetic field, and to the strength of the armature
current, the number of armature inductors being fixed. In a field of
constant strength, therefore, the pull of the armature depends on the
amount of current passing through the winding. The torque must be just
sufficient to overcome the load; if in excess, the speed will increase
until the increase of the reverse electromotive force reduces the
current and the increase of speed increases the load to the point of
equilibrium between load and torque. If the torque be insufficient for
the load, the speed will diminish until equilibrium is established,
assuming the motor is running on constant voltage circuit.]
=Ques. Describe similar conditions which prevail in the operation of
a dynamo.=
Ans. When no current is being generated by the dynamo, little power
is required to drive it, but when the external circuit is closed and
current is forced through it against more or less resistance, work is
being done, hence more power is required. In other words, there is
an opposition to the mechanical force applied at the pulley which is
proportional to the electric power delivered by the dynamo. An opposing
reaction or _reverse force_ then is set up in a dynamo when it does
work.
[Illustration: FIG. 395.--Circuit with generator and motor.
Whenever current flows through some portion of a circuit in which
there is an electromotive force, the current will there either receive
or give up energy according to whether the electromotive force acts
_with_ the current or _against_ it. In the figure, the generator and
motor are rotating clockwise, and hence each generates an electromotive
force tending upwards from the lower brush to the higher. In each case
the upper brush is the positive one. In the dynamo, where energy is
being supplied to the circuit, the electromotive force is in the same
direction as the current, while in the motor where work is being done
and energy is leaving the circuit, the electromotive force is in a
direction which opposes the current.]
=Ques. In the operation of a motor what is the nature of the reverse
electromotive force?=
Ans. It is proportional to the velocity of rotation, the strength of
the magnets, and to the number and arrangement of the wires on the
armature, that is, the reverse voltage depends on the _rate_ at which
the lines of force are cut.
[Illustration: FIGS. 396 and 397.--Water and electric
circuits. Diagrams showing comparison between water motor and electric
motor.]
In the diagrams:
The pump corresponds to the dynamo.
The high level pipe " " " positive conductor.
The low level pipe " " " negative conductor.
The valve " " " switch.
The water motor " " " electric motor.
The water pressure (called head) " " " electric pressure
(called voltage).
The flow in gallons per minute " " " amperes.
The size of pipe " " " size of conductor.
The foot pounds " " " watts.
The greater the difference between the height of the two pipes
the higher the pressure, and the greater the difference between
the pressures of the two conductors the higher the voltage.
The larger the diameter of the pipes the less resistance is
offered to the flow of water, and the larger the diameter of
the conductors the less resistance is offered to the flow of
electricity. The more water required by the water wheel, the
more power is required to drive the pump. The more electricity
required by the motor the more power is required to drive the
generator.
[Illustration: FIG. 398.--Fairbanks-Morse standard TR type
motor. This type is built in the smaller sizes and the design is such
that the motor can be installed upon the floor, wall or ceiling, the
bearing yokes being attached to the frame by four equally spaced bolts
so that they can be turned to provide for proper operation of the
oiling devices in either position. A substantial base is provided with
a thrust screw for adjusting the belt tension. This base has clamping
bolts which permit adjusting the position of the motor while suspended.
There is a cast ring type frame having steel side pieces which press
firmly together, the steel laminations making up the pole pieces. The
field coils armature, and armature coils are illustrated in detail in
figs. 399 to 401. The commutator bars are of drawn copper, insulated
with mica. The lugs which extend outward from the bars to receive the
lead wires from the armature windings are formed in one piece with
the bars, and are of the full width of the bars with the insulator
extending outward between them, so that when assembled a solid flange
is formed to receive the armature connections. Self-oiling bearings
are provided and the location of the bearing sleeves in the housing
is adjustable so that the armature may be centered in the magnetic
field. The brush rigging is carried on a skeleton rocker supported in
a groove, turned in the edge of the frame. The brush holders are of
the box type with independently adjustable tension spring for each.
Standard shunt windings are for 115, 230 and 550 volts. The compound
wound motors operate at approximately the same full load speeds as the
shunt wound, but the no load speeds will be about 20 per cent. higher
than the full load speeds. They have, however, the ability to exert a
more powerful starting effort than shunt motors without drawing such a
heavy current from the line, and are, therefore, especially adapted for
driving apparatus that has to be frequently started and stopped under
load and where close speed regulation is not required.]
=Ques. Describe an experiment which shows the existence of a reverse
electromotive force in a motor.=
Ans. The apparatus required consists of a small motor, battery, and
ammeter. They should be connected in one circuit and the deflection of
the ammeter observed when the armature is held stationary, and when it
rotates with various loads.
In an experiment of this kind made on a motor with separately
excited magnets, the following figures were obtained:
Revolutions per minute 0 50 100 160 180 195
Amperes 20 16.2 12.2 7.8 6.1 5.1
Apparently, if the motor had been helped on to run at 261½
revolutions per minute, the current would have been reduced to
zero. In the last result obtained, the current of 5.1 amperes
was absorbed in driving the armature against its own friction
at the speed of 195 revolutions per minute.
[Illustration: FIG. 399.--Fairbanks-Morse field coil and pole
piece. The field coils are wound upon iron forms, each layer treated
with insulating compound. Afterward they are removed from the forms and
baked hard and dry and finally wrapped with insulating materials; all
but the three smaller sizes are wrapped with a protecting cord. The
series and shunt coils of the compound winding here shown are wound
separately, the smaller one being the series coil and the larger the
shunt coil.]
=Ques. Explain the action of the current supplied to a motor for its
operation.=
Ans. The motor current passing through the field magnets polarizes them
and establishes a magnetic field, and entering the armature, polarizes
its core in such a way that the positive pole of the core is away from
the negative pole of the magnetic field, and the negative pole is away
from the positive pole of the magnetic field. The magnetic repulsions
and attractions thus created cause the armature to rotate in a position
of magnetic equilibrium or so as to bring its positive and negative
poles opposite the negative and positive poles respectively of the
magnetic field. It is evident that unless suitable means were provided
to reverse the polarity of the armature core at the instant it reached
the position of the magnetic equilibrium, the armature would not rotate
any further. The construction is such that the polarity of the armature
core, or the direction of the current in the armature coils is reversed
at the proper instant automatically by the commutator, thus giving
continuous rotation.
[Illustration: FIG. 400.--Fairbanks-Morse armature for 7½
H.P., 1300 R.P.M., TR type motor. The armature core is built up of thin
sheet steel laminations with notches in the circumference, which, when
the discs are placed together, form grooves or slots to receive the
armature coils. The armature cores for the larger machines are mounted
on a cast iron spider, which also carries the commutator, making
the two parts entirely self-contained, and with this construction,
it is possible to remove the armature shaft, without disturbing the
core, commutator or windings. Cores of all sizes are provided with
ventilating spaces, running from the surface to the central opening of
the core, so that air is drawn through the core and blown out over the
windings by the revolution of the armature.]
=Direction of Rotation of Motors.=--In the case of either a motor,
or a dynamo used as a motor, the direction in which the armature will
rotate is easily found by the left hand rule, as illustrated in fig.
411, when the polarity of the field magnets and the direction of
currents through the armature are known.
=Ques. How may the rotation of a motor be reversed?=
Ans. By reversing either the current through the fields, or the current
through the armature.
=Ques. What will happen if both currents be reversed?=
Ans. The motor will run in the same direction as before.
[Illustration: FIG. 401.--Fairbanks-Morse wire wound armature
coils. These coils are form wound and are thoroughly insulated and
baked before assembling in the slots. Material of great mechanical
strength as well as high insulating value is used, and the coils
are subjected to dippings in insulating compound and to bakings,
thus driving out all moisture and making a coil which is practically
waterproof and which will withstand rough handling. These coils, when
completed, are placed in the slots, where they are retained by bands on
the three smaller sizes and by hardwood wedges on the larger sizes.]
=Ques. What is the effect of supplying current to a series dynamo?=
Ans. It will run in a direction opposite to its motion as a dynamo.
=Ques. What is the result of reversing the direction of current at
the terminals of a series motor?=
Ans. It will not change its direction of rotation, since the current
still flows through the armature in the same direction as through the
field.
[Illustration: FIGS. 402 to 410.--Diagrams showing relative
direction of rotation of motors and dynamos. From figs. 391 and 392, it
is seen that the direction of the current in a motor armature must be
such as will increase, by the flux it produces, the intensity at the
_leading polar edge_ and decrease the intensity at the _trailing polar
edge_. In a dynamo, the armature has to be moved by mechanical force,
against a magnetic force, hence the leading polar edge is weakened,
while the trailing edge is strengthened. The magnetomotive force in a
motor armature is, therefore, opposed to the direction of that in a
generator armature, when the direction of rotation and the direction of
the field magnetomotive force are the same. Upon this depends all the
relations existing between the direction of rotation of a machine when
acting as a motor or as a dynamo.]
=Ques. What is the behavior of a shunt dynamo when used as a
motor?=
Ans. Its direction of rotation remains unchanged.
=Ques. Why is this?=
Ans. Because if the connections be such that the current supplied will
flow through the armature in the same direction as when the machine is
used as a dynamo, the current through the field will be reversed, since
the field windings are in parallel with the brushes.
[Illustration: FIG. 411.--The "left hand rule" for direction
of motion in motors. _Place the left hand_, as shown, _so that the
thumb points in the direction of the current, the 3rd, 4th and 5th
fingers in the direction of the lines of force, then will the 2nd or
forefinger, at right angles to the others, point in the direction in
which the conductor is urged_.]
=Armature Reaction in Motors.=--In the operation of a motor the
reaction between the armature and field magnets distorts the field in a
similar manner as in the operation of a dynamo. A current supplied from
an outside source magnetizes the armature of a motor and transforms it
into an electromagnet, whose poles would lie nearly at right angles
to the line joining the pole pieces, were it not for the fact that
_negative_ lead must be given to the brushes.
[Illustration: FIG. 412.--Principle of the electric motor
as illustrated by experiment showing effect of a magnetic field on a
wire carrying an electric current. Let a vertical wire _ab_ be rigidly
attached to a horizontal wire _gh_, and let the latter be supported
by a ring or other metallic support as shown, so that _ab_ is free
to oscillate about _gh_ as an axis. Let the lower end of _ab_ dip
into a trough of mercury. When a magnet is held in the position shown
and a current from a cell is sent through the wire as indicated, the
wire will move in the direction shown by the arrow _f_, that is, at
right angles to the direction of the lines of magnetic force. Let the
direction of the current in the wire be reversed, then the direction
of the force acting on the wire will be found to be reversed also. The
conclusion is that _a wire carrying a current in a magnetic field tends
to move in a direction at right angles both to the direction of the
field and to the direction of the current_. The relation between the
direction of the magnetic lines, the direction of the current, and the
direction of the force, is often remembered by means of the following
rule, known as the _motor rule_, and which differs from the dynamo rule
only in that it is applied to the fingers of the _left hand_ instead
of to those of the right. _Let the forefinger of the left hand point
in the direction of the magnetic lines of force and the middle finger
in the direction of the current sent through the wire, then will the
thumb, at right angles to the other two fingers, point in the direction
in which the wire is urged._]
Negative lead is the amount of backward advance of the brushes
against the direction of the rotation of the armature, measured
in degrees from the neutral plane.
If the brushes be given positive lead, that is, placed in
advance of the neutral plane _in the direction of rotation_,
the cross magnetizing force is converted into one that tends
to increase that of the field magnet, while if they be given
negative lead, it tends to demagnetize the field magnet.
Since with positive lead the armature polarity strengthens that
of the field magnet, it is possible, disregarding sparking,
to operate a motor without any other means being taken to
magnetize the field magnets, because the armature will induce a
pole in the field magnet and then attract itself towards this
induced pole.
=Ques. What effect has the cross magnetizing force on the field?=
Ans. It tends to shift the field around in a direction opposite to that
of the rotation.
[Illustration: FIG. 413.--Current commutation in a motor.
Considering the coil W which is ascending, current is flowing through
it from the top brush, while it is itself the seat of an electromotive
force that tends to stop or reverse its current. The condition for
sparkless commutation requires that during the interval the coil is
short circuited by the brush, the coil should be passing through a
field that is not only sufficiently strong but one that tends to
reverse the direction of its current. The coil is already in such a
field, hence, commutation must take place _before_ it passes put of
this field. To accomplish this the brushes must be shifted backward,
that is, given _negative lead_, to overcome sparking. In other words,
the _commutating plane_ must be shifted _back_ of the neutral plane in
a motor instead of being placed _in advance_ as in a dynamo.]
=Ques. What are the conditions of minimum sparking?=
Ans. The same conditions must obtain as in a dynamo, that is, the
current in the coil undergoing commutation must be brought to rest and
started again in the opposite direction. This involves that while the
coil is short circuited by the brush, it should be passing through a
field that tends to reverse the direction of the current. Since the
coil is already in such a field, the act of commutation must take
place before it passes out of this field. Accordingly, a negative lead
must be given the brushes.
[Illustration: FIG. 414.--Railway motor. This type of motor,
since it must operate under cars, has taken on the peculiar form under
which it is most familiar. As illustrated, the case is of such shape
that compactness and water proofing are secured, and the means of
attachment to the car axle and support from the axle and truck frame
are provided.]
=Method of Starting a Motor.=--Although motors and dynamos are
practically similar in general construction and either one of them
will act as the other when suitably traversed by an electric current,
there are certain differences between the connections and accessories
of a machine operated as generator and one employed as a motor. For
instance, when a machine is operated as a dynamo, it is first driven up
to speed until it has excited itself to the right pressure, and then
it is connected to the circuit; but when a machine is used as a motor
it will not start until it has been connected to the circuit, and this
must not be done until the proper precautions have been taken to ensure
that the current, which will pass through it when so connected, will
not be excessive and thereby result in serious injury to the motor.
For this reason a rheostat or variable resistance, commonly called a
starting box is usually inserted in the armature circuit of a motor to
prevent an undue rush of current before the motor attains its speed,
and subsequently the speed is regulated by the cutting in or out of the
circuit of certain extra resistances which constitute the controller
used on a series motor requiring variable torque at variable speed, as
in the case of elevator or electric traction service.
[Illustration: FIG. 415.--View of railway motor, open. The
frame is of cast steel for lightness, and which serves as magnetic
circuit and protecting case. It is circular or octagonal in form except
in very large motors. Four short magnets project from the case. The
armature is large in order to secure the required torque. It is always
series wound, requiring two brushes. The brush holders are mounted upon
a frame of insulating material which is attached to the upper half of
the case. The brushes are adjustable radially, but usually it is not
necessary to provide for shifting as they remain in the neutral plane.
In motors which receive so little attention as these, special attention
must be given to the design of devices for keeping oil and grease
out of the case. These would injure the insulation of the coils and
produce sparking at the commutator. Oil rings are, therefore, placed
on the shaft, and these discharge into chambers connected to the oil
wells or allow the oil to overflow on the track. The bearings are made
self-oiling or self-greasing by means of rings or wicks and will run
for weeks without attention.]
=Classes of Motor.=--Motors are classified in the same manner as
dynamos. The fields may be either bipolar or multi-polar, and with
respect to the type of armature winding employed, motors are classed as:
1. Series wound;
2. Shunt wound;
3. Compound wound.
[Illustration: FIG. 416.--Series motor connections. A series
motor on a constant voltage circuit does not have a constant field
strength, and does not run at uniform speed. If the load be taken
off it will run at excessive speed. To start the motor, the circuit
is completed through a variable resistance or rheostat by moving the
switch S so that the resistances R, R_{1}, R_{2}, R_{3}, are gradually
cut out of the circuit. To stop, the switch S is moved back to its
"off" position.]
=Series Motors.=--A series motor is one in which the field magnet
coils, consisting of a few turns of thick wire, are connected in series
with the armature so that the whole current supplied to the motor
passes through the field coils as well as the armature. Fig. 416 is a
diagram of a series motor showing the connections and rheostat.
=Ques. What are the characteristics of a series motor?=
Ans. The field strength increases with the current, since the latter
flows through the magnet coils. If the motor be run on a constant
voltage circuit, with light load, it will run at a very high speed;
again, if the motor be loaded heavily, the speed will be much less than
before.
[Illustration: FIG. 417.--General Electric type CL-B motor
for slow and moderate speeds. It is of multipolar construction, having
six pole pieces. The advantages of slow speed machinery are generally
understood, and in motors the additional outlay to secure slow speeds
is warranted, inasmuch as it results in diminished wear and friction
losses in gearing, belting, bearings, and commutators, and decreased
brush renewals. The comparatively slow speeds of these motors are of
importance in that they permit belting or gearing the motors directly
to ordinary slow speed line shafting without employing intermediate
counter shafting. When motors are geared to heavy duty machines, it is
considered better practice to supply an outboard bearing to take up
the additional strain that would otherwise be put on the gearing and
bearing.]
=Ques. For what kinds of service are series motors unsuited?=
Ans. Series motors should not be employed where the load may be
entirely removed because they would attain a dangerous speed. They
should not be used for driving by means of belts, because a sudden
release of the load due to a mishap to the belt would cause the motor
to "run away."
Very small series motors may be used with belts since their
comparatively large frictional resistance represents an
appreciable load, restraining the motor from reaching a
dangerous speed.
=Ques. For what service are series motors adapted?=
Ans. For gear drive.
In the case of a sudden release of the load the gears provide
some load on account of the frictional resistance of the gear
teeth.
[Illustration: FIG. 418.--Shunt motor connections. A shunt
motor runs at constant speed on a constant voltage circuit. In
connecting the motor in circuit, the field coils must be placed in
circuit first, so that there is a certain amount of field strength to
produce rotation of the armature and thus prevent excessive current
through the armature. If the field magnets were not put in the circuit
first, the armature, at rest on receiving current, would probably burn
out, because it is of low resistance, and would take practically all
the current supplied, especially since no reverse voltage is generated
in the armature at rest. The method of starting is shown in the
illustration. To start, the switch is closed, and the rheostat lever
pushed over so as to make contact with A and B, thus _first_ exciting
the magnets. On further movement of the lever, the rheostat resistances
R, R_{1}, R_{2}, R_{3}, etc., are gradually cut out as the speed
increases, until finally all the resistance coils are cut out. To stop,
the lever is brought back to its original position.]
=Ques. What advantage is obtained with series motors with respect to
the connections?=
Ans. A single wire only proceeds from the rheostat to the motor, so
that, with the return wire, only two wires are required.
=Ques. For what service are series motors specially adapted?=
Ans. Series motors are used principally for electric railways,
trolleys, and electric vehicles, and similar purposes where an
attendant is always at hand to regulate or control the speed. They
are also used on series arc light circuits in which the current is of
constant strength. Very small motors are generally provided with series
windings.
[Illustration: FIG. 419.--Speed regulation of a shunt motor.
The speed of a motor depends on the voltage of the current supplied and
the field strength. The motor tends to rotate so fast as to produce
a _reverse voltage_ nearly equal to that supplied to the brushes;
hence, the speed varies with the voltage supplied. By decreasing this
voltage then, the speed is decreased. Accordingly, the speed may be
reduced by inserting, by means of a rheostat, a resistance in series
with the motor. By inserting this resistance in the field circuit,
the voltage at the terminals of the motor is lowered, thus giving the
condition necessary to reduce the speed. The arrangement for speed
regulation shown in the figure includes a starting regulator and a
shunt regulator.]
=Shunt Motors.=--A shunt motor may be defined as one in which
the field coils are wound with many turns of comparatively fine wire,
connected in parallel with the brushes. The current then is offered
two paths: one through the armature, and one through the field coils.
[Illustration: FIGS. 420 to 422.--Reversing the direction of
rotation of a series motor. Fig. 420 shows the connections for counter
clockwise rotation. The motor may be reversed: 1, by allowing the
current to flow in its original direction (from D to C) in the field
magnet coils, and altering the direction of the armature current by
changing the two connections on the brushes A and B, thus connecting
C to A and B to the return wire as in fig. 421, or 2, by leaving the
direction of the current in the armature in its original direction,
and reversing that of the field current, as in fig. 422. If the wires
leading to the rheostat and motor directly, were reversed there would
be no reversal of the motor, because by so doing, both the armature and
field magnet currents would be reversed.]
[Illustration: FIGS. 423 to 425.--Reversing the direction of
rotation of a shunt motor. Fig. 423 shows the connections for counter
clockwise rotation. The motor may be reversed: 1, by allowing the
current to flow in its original direction through the field magnet
coils (from D to C), and reversing its direction through the armature
(from A to B) as in fig. 424, or 2, by allowing the armature current to
flow in its original direction (from B to A) and reversing the current
through the field coils (from C to D) as in fig. 425.]
=Ques. What may be said with respect to the speed of a shunt
motor?=
Ans. It is practically constant with varying loads.
The variation of speed ranges from 1/10 to 5 per cent., except
in the case of small motors, in which the variation may be much
greater.
=Ques. How should a shunt motor be started?=
Ans. To properly start the machine, the field coils must be fully
excited.
It is, therefore, necessary to switch the magnet coils
immediately on to the voltage of supply, while a variable
resistance must be provided for the armature circuit. To get
both connections at the same time, rheostats for shunt motors
are arranged as shown in fig. 418.
=Influence of Brush Position on Speed.=--In the case of a shunt
motor supplied with current at constant pressure, the speed is a
minimum when the brushes are in the neutral plane, and the effect of
giving the brushes either positive or negative lead is to increase the
speed, especially with little or no load.
=Ques. Why does the speed increase?=
Ans. When the brushes are shifted from the neutral plane, the reverse
voltage between the brushes is decreased, speed remaining unchanged.
Accordingly, the pressure in the supply mains forces an increased
current through the armature thus producing an increased armature pull
which causes the speed to increase until the reverse voltage reaches a
value sufficiently large to reduce the current to the value required to
supply the necessary driving torque.
=Compound Motors.=--This type of motor has to a certain extent,
the merits of the series motor without its disadvantages, and is
adapted to a variety of service. If the current flow in the same
direction through both of the field windings, then the effect of the
series coil strengthens that of the shunt coil; this strengthening is
greater, the larger the armature current.
[Illustration: FIG. 426.--Compound motor connections for
starting from a distant point. A compound winding may be used on motors
for many different purposes. If the current flow in the same direction
through both windings, then the effect of the series coil strengthens
that of the shunt coil. This strengthening increases with the load.
Thus the motor gets, at increasing load, a stronger magnetic field, and
will therefore, if the voltage remain constant run slower than before.
Accordingly, for a given current, the starting power will be greater
than that of a shunt motor. With a decreasing load the motor will run
faster. The compound motor has, to a certain extent, the merits of the
series motor without its disadvantages. By means of compound motors
the starting at a distance with only two mains may be effected, just
as in the case of the series motor. The connections are shown in the
diagram. If the motor be regarded as being without the shunt coil, then
it is connected up exactly as the series motor in fig. 416. The current
coming from the starter enters the series coil at F, flows through the
series coil and leaves it at E, flowing from there to the armature
brush B, through the armature to brush A, and from there through the
second main back to the generator. The shunt winding is connected
directly with the armature brushes A and B, and gets at starting,
therefore, only a very small voltage, hence its field is nearly
ineffective. But on account of the series winding, the motor starts
as a series motor. Obviously such a motor will not develop a very
large starting power like a real series motor, for, on account of the
large space occupied by the shunt coils, there is less space available
for the series coils than with a series motor. A compound motor may,
however, even with this arrangement, be easily started, provided the
load on starting be not too heavy. When once running the armature will
produce a reverse voltage and the shunt coil will be supplied with
nearly the full terminal voltage.]
=Ques. Mention some characteristics of the compound motor.=
Ans. Since it is a combination of the shunt and series types, it
partakes of the properties of both. The series winding gives it strong
torque at starting (though not as strong as in the series motor), while
the presence of the shunt winding prevents excessive speed. The speed
is practically constant under all loads within the capacity of the
machine.
=Ques. Describe the connections for starting a compound motor at a
distance.=
Ans. Control at a distance can be effected with only two wires, just
as in the case of a series motor. In the diagram fig. 426, the current
coming from the rheostat enters the series coil at F, and leaves it
at E, thence it flows to the armature brush B, through armature to
brush A, and from here back to the dynamo. The shunt winding, which is
connected across the brushes, gets a very small voltage at starting
and is accordingly very ineffective. The motor then starts as a series
motor. The starting effect is smaller than in a series motor because
of the fewer turns in the series winding, most of the available space
being occupied by the shunt coils.
=Power of a Motor.=--The word "power" is defined as _the rate
at which work is done_, and is expressed as the quotient of the work
divided by the time in which it is done, thus:
work
power = ------
time
The difference between power and work should be clearly understood.
_Work is the overcoming of resistance through a certain distance._ It
is measured by the product of the resistance into the space through
which it is overcome, thus:
work = distance × space
For instance, in lifting a body from the earth against the
attraction of gravity, the resistance is the weight of the
body, and the space, the height to which the body is raised,
the product of the two being the work done.
The unit of work is the _foot pound_, which is _the amount of work done
in overcoming a pressure or weight equal to one pound through one foot
of space_.
The unit of power is the _horse power_ which is equal to 33,000 _foot
pounds of work per minute, that is_:
foot pounds per minute
horse power = ------------------------
33,000
The unit of power was established by James Watt as the power
of a strong London draught horse to do work during a short
interval, and used by him to measure the power of his steam
engines.
In order to measure the mechanical power of a motor, it is necessary
to first determine the following three factors upon which the power
developed depends:
1. Pull of the armature, in pounds;
2. Distance in feet at which the pull acts from the center of the shaft;
3. Revolutions per minute.
EXAMPLE.--If the armature pull of a motor having a two
foot pulley, be such that a weight of 500 lbs. attached to the
rim, is just balanced, and the speed be 1,000 revolutions per
minute, what is the horse power?
Here, the distance that the pull acts from the center of the
shaft is one foot, hence for each revolution the resistance
of 500 pounds is overcome through a distance equal to the
circumference of the pulley or
π × diameter = 3.1416 × 2 = 6.2832 feet.
[Illustration: FIG. 427.--General Electric type CQ Motor.
These motors range in capacity from 1/6 to 20 horse power. The
small sizes are bipolar, and the larger sizes have four poles. For
installations where the motor is exposed to dust, mechanical injury or
moisture, it may be partially or entirely enclosed by means of hand
hole covers. The standard voltages are 115, 230 and 550.]
The _work done_ in one minute is expressed by the following equation:
{ work } {weight} {circumference} {revolutions}
{ per } = { in } × { of pulley } × { per } = foot pounds
{minute} { lbs. } { in feet } { minute }
= 500 × 6.2832 × 1,000 = 3,141,600.
Hence, the power developed is
3,141,600 ÷ 33,000 = 95.2 horse power.
=Ques. What is "brake" horse power?=
Ans. The net horse power developed by a machine at its shaft or pulley;
so called because a form of brake is applied to the pulley to determine
the power.
=Ques. Describe the apparatus used in making a brake test.=
[Illustration: Fig. 428.--Prony brake for determining brake horse
power. It consists of a friction band ring which may be placed around a
pulley or fly wheel, and attached to a lever bearing upon the platform
of a weighing scale in such a manner that the friction between the
surfaces in contact will tend to rotate the arm in the direction in
which the shaft revolves. This thrust is resisted and measured _in
pounds_ by the scale. In setting up the brake the distance between the
center of the shaft and point of contact (knife edge) with the scales
must be accurately measured, _the knife edge being placed at the same
elevation as the center of the shaft_. An internal channel permits
the circulation of water around the interior of the rim as shown, to
prevent overheating.]
Ans. Tests of this kind are usually made with a Prony brake as shown in
fig. 428. It consists of a band of rope or strip iron--the latter is
the arrangement shown--to which are fastened a number of wooden blocks,
several carrying shoulders to prevent the contrivance from slipping
off the wheel rim. The brake band is drawn tight, as shown, so that
the blocks press against the surface all around. The brake thus formed
is restrained from revolving with the pulley by two arms attached near
the top and bottom centers of the wheels, and joined at the opposite
ends to form a lever which bears upon an ordinary platform scale, a
suitable leg or block being arranged to keep its end level with the
center of the shaft. By this arrangement the amount of friction between
the brake band and the revolving wheel is weighed upon the scales.
Since the brake fits tightly enough to be carried around by the wheel,
but for the arms bearing upon the scale, the amount of frictional
power exerted by the wheel in turning free within the blocks may be
transmitted and measured, just as would be the case were a machinery
load attached, instead of a friction brake.
=Ques. Why must the point of contact of the brake with the scales be
level with the center of the shaft?=
Ans. In order to determine the force acting at right angles to the line
joining the point of contact and center of the shaft.
=Ques. What is the distance between the center of the shaft and point
of contact with the scales called?=
Ans. The lever arm.
=Ques. What three quantities must be determined in a test in order to
calculate the brake horse power?=
Ans. The lever arm, the force exerted on the scales, and the
revolutions per minute.
=Ques. How is brake horse power calculated?=
Ans. From the following formula:
B. H. P. = 2 π L N W
-----------
33,000
in which
B. H. P. = brake horse power;
L = lever arm, _in feet_;
N = number of revolutions per minute;
W = force _in pounds_ at end of lever arm as measured by
scales.
EXAMPLE--In making a brake test on a motor, the lever
arm of the brake is 3 ft., and the reading of the scales is 30
lbs. When the motor is running 1,000 revolutions per minute,
what is the brake horse power?
Substituting the given values in the formula,
2 π × 3 × 1,000 × 30
B. H. P. = --------------------- = 17.1
33,000
Now, if the voltmeter and ammeter readings be 220 and 65 respectively,
what is the efficiency of the motor at this load?
The amount of power absorbed by the motor, or in other words, the
_input_ is
220 × 65
E. H. P. = -------- = 19.16
746
and since the output is 17.1 horse power,
output brake horse power 17.1
efficiency = ------ = ---------------------- = ----- = 89%.
input electrical horse power 19.16
=Speed of a Motor.=--The normal speed at which any motor will run
is such that the sum of the reverse electromotive force and the drop in
the armature will be exactly equal to the electromotive force applied
at the brushes. The drop in the armature is the difference between the
applied voltage and the reverse voltage.
=Mutual Relations of Motor Torque and Speed.=--The character of
the work to be done not only determines the condition of the motor
torque and speed required, but also the suitability of a particular
type of motor for a given service. There are three general classes of
work performed by motors, and these require the following conditions of
torque and speed:
1. Constant torque at variable speed;
Suitable for driving cranes, hoists, and elevators, etc., where
the load is constant and has to be moved at varying rates of
speed.
[Illustration: FIG. 429.--Two path method of speed regulation
of series motor. A rheostat is connected in shunt to the field coils as
shown. The current passing from _a_ to _b_ divides between the magnet
coils and the rheostat coils; the higher the resistance of the rheostat
the less current passes through it, and the more through the magnet
coils, hence the stronger the field magnet.]
2. Variable torque at constant speed;
Suitable for driving line shafting in machine shops, which must
run at constant speed regardless of variations of torque due to
variations in the number of machines in operation at a time, or
the character of work being performed.
3. Variable torque at variable speed.
Suitable for electric railway work. For example: when a car is
started, the torque is at its maximum value and the speed zero,
but as the car gains headway, the torque decreases and the
speed increases.
=Speed Regulation of Motors.=--The speed of motors connected to
constant voltage circuits is usually regulated by the two following
methods:
[Illustration: FIG. 430.--Variable field method of speed
regulation of series motor. The field winding is divided into a number
of sections with leads connecting with switch contact points as
illustrated. The speed then is regulated by cutting in or out of the
circuit sections of the field winding thus varying the strength of the
field.]
1. By inserting resistances in the armature circuit of a shunt wound
motor;
2. By varying the strength of the field of a series motor.
The first method is sufficiently explained under fig. 418 and the
second method is illustrated in fig. 430. The controller switch S is so
arranged that a greater or lesser number of field coils can be inserted
in the field circuit. When the switch arm is on point 1, the motor
current will flow through all the field windings, and the strength
of the field will be at its maximum. When the switch arm is moved so
as to successively occupy positions 2, 3, and 4, thus cutting out of
circuit a greater and greater number of field coils the strength of the
field will be gradually decreased until practically all of the motor
current is led or wired through the armature. Under these conditions,
when the field of a motor is at its maximum strength, the motor torque
will be at a maximum for any given strength of current, and the reverse
electromotive force will also be at a maximum for any given speed,
therefore, when the field strength is increased the speed will decrease
and _vice versa_.
=Ques. What results are obtained by this method of regulation?=
Ans. The speed of a series motor may be nearly doubled, that is, if the
lowest permissible speed of the motor be 250 revolutions per minute it
can be readily increased to 500 revolutions per minute by changing the
field coil connections from series to parallel. It is on this account,
as much as on their powerful starting torque, that series motors have
been until recently almost exclusively employed for electric traction
purposes.
=Series Parallel Controller.=--When two motors are used in
electric railway work, their armatures are connected in series with
each other and an extra resistance which prevents the passage of an
excessive current through the armature before the motor starts. As
the speed of the car increases, the extra resistance is gradually cut
out of circuit and the field winding connections changed from series
to parallel by means of a series parallel controller, which finally
connects each motor directly across the supply mains, or between the
trolley line and the track or ground return.
=Efficiency of a Motor.=--The commercial efficiency of a motor is
the ratio of the output to the input. As a rule, the power developed
by a motor increases as the reverse voltage generated by it decreases,
until this voltage equals one half of the voltage applied at the
brushes. After this point is reached, the power developed by the motor
decreases with the decrease of the reverse voltage. Therefore, a motor
performs the largest amount of work when its reverse voltage is equal
to one half the impressed voltage.
[Illustration: FIG. 431.--Double throw, double pole switch for
reversing direction of rotation of a motor. The direction of rotation
can be reversed by changing the direction of current in either the
armature or the field coils. It is preferable, however, to reverse the
direction of rotation by changing the direction of current through
the armature. The switch is wired as shown, means of reversal being
provided by running the wires as indicated by the dotted lines.]
The efficiency of a motor as just stated is the ratio of the output to
the input; this is equivalent to saying that the efficiency of a motor
is equal to the brake horse power divided by the electrical horse power.
The electrical horse power is easily obtained by multiplying the
readings taken from volt meter and ammeter, which gives the watts, and
dividing the product by 746, the number of watts per horse power. That
is:
volts × amperes watts
Electrical horse power = --------------- = -----
746 746
[Illustration: FIG. 432.--Wiring diagram, showing electrical
connections between the armature, field, and interpoles of an interpole
motor. As the name implies, an interpole motor has in addition to the
main poles, a series of interpoles which are placed between the main
poles, and whose function is to assist in the reversal of the current
under the brushes. They provide a separate commutating field of a
correct value at all loads and speeds, and their windings are for this
purpose connected in series with the armature. The proper functioning
of the interpoles is independent of the direction of rotation of the
armature, also of the load carried over the whole speed range. In
an ordinary motor without interpoles, commutation is assisted by a
magnetic fringe emanating from the main poles, but as the value of this
fringe is altered by the load of the motor and by rheostatic field
weakening, if higher speeds be desired from such a machine, commutation
becomes imperfect and sparking results, making a readjustment of the
brushes necessary.]
=Interpole Motors.=--An interpole motor has in addition to the
main poles, a series of interpoles, placed between the main poles. The
object of these poles is to provide an auxiliary flux or "commutating"
field at the point where the armature coils are short circuited by the
brush.
[Illustration: FIGS. 433 to 437.--Parts of the type S
interpole motor built by Electro-Dynamic Co. They are as follows: 1.
yoke--commutator view; 2. interpole coil; 3. top R.H. main coil; 4.
bottom R.H. main coil; 5. main pole; 6. interpole; 7. armature shaft,
R.O. bearing; 8. commutator; 9. armature wedge; 10. armature coil;
11. brush ring; 12. brush carrier insulation; 13. brush carrier; 14.
brush guard; 15. carbon brush; 16. brush holder; 17. cross connecting
cable; 18. oil ring; 19. commutator end bearing bushing; 20. pulley end
bearing bushing.]
=Ques. What is the object of the commutating field produced by the
interpoles?=
Ans. Its object is to assist commutation, that is, to help reverse
the current in each coil while short circuited by the brush, and thus
reduce sparking.
[Illustration: FIG. 438.--Interpole motor as built by the
Electro Dynamic Co. This type of motor is devised to prevent sparking
at all loads by the use of interpole magnets, that is, small magnets
placed between the field magnets. The interpoles set up a field in a
direction to stop and reverse the current in the armature coils while
they are short circuited by the brushes.]
=Ques. What is the nature of the commutating field?=
Ans. The excitation of the interpoles being produced by series turns,
the field will vary with the load, and will, if once adjusted to give
good commutation at any one load, keep the same proportion for any
other load, provided the iron parts of the circuit be not too highly
saturated.
=Ques. State briefly how sparking is reduced or prevented by the
action of the interpoles.=
Ans. Sparking is due to self induction in the coil undergoing
commutation, which impedes the proper reversal of the current. The
action of the interpoles corrects this in that they set up a field in
a direction that causes a reversal of the current in the coil while it
is short circuited. Thus, the coil at the instant it leaves the brush,
is not an idle coil, but has a current flowing in it in the right
direction to prevent sparking.
=Ques. Mention some of the claims made for interpole motors.=
Ans. Constant or adjustable speed, and momentary overloads without
sparking; constant brush position; operation at adjustable speeds on
standard supply circuits of 110, 220, and 500 volts; constant speed
with variable load; reversal without changing the position of the
brushes.