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FUNDAMENTALS OF POLYPHASE ELECTRIC MOTORS
Manufacturer of fractional and integral horesepower, AC, Squirrel Cage Motors
Totally Enclosed - Drip Proof - Encapsulated
FUNDAMENTALS
OF
POLYPHASE ELECTRIC MOTORS
D
By: 2
This bulletin provides basic information on the nature and
design of polyphase electric motors. The information is
simply presented so extensive engineering or electrical
knowledge is not necessary for a good understanding. It is
divided into four sections:
BASIC MOTOR PRINCIPLES ....................................... 2
THE AC MOTOR ........................................................... 3
POLYPHASE AC MOTORS .......................................... 4
SQUIRREL CAGE MOTORS .................................. 5-12
Squirrel cage motors are covered in detail because they are
the most common type motor used in industry. Typical
applications include blowers, fans, pumps, compressors,
machine tools, conveyors, mixers, crushers, and industrial
machinery of all kinds. Basic characteristics are as follows:
BASIC MOTOR PRINCIPLES
All motors can be classed into two categories, AC and DC.
The basic motor principles are alike for both the AC and
DC motor.
Magnetism is the basis for all electric motor operation. It
produces the forces necessary for the motor to run. There
are two basic types of magnets, the permanent magnet and
the electromagnet. The electromagnet has the advantage
over the permanent magnet in that the magnetic field can
be made stronger. Also the polarity of the electromagnet
can easily be reversed.
The construction of an electromagnet is simple. When a
current is passed through a coil of wire, a magnetic field is
produced. This magnetic field can be made stronger by
winding the coil of wire on an iron core (Fig. 1). One end
of the electromagnet is a north pole, while the other end is
a south pole. These poles can be reversed by reversing the
direction of current in the coil of wire.
Squirrel
Cage
Normal
or High
Starting
Torque
Constant
Speed
1/3
to 250
MACHINE
TORQUE
SPEED
USUAL
HP RANGE
HOW A MOTOR WORKS
The basic principle of all motors can be easily be shown
using two electromagnets and a permanent magnet.
Current is passed through coil #1 and coil #2 in such a
direction that north and south poles are generated next to
the permanent magnet, as shown in Figure 2. A permanent
magnet with a north and south pole is the moving part of
this simple motor. In Figure 2 the north pole of the
permanent magnet is adjacent to the north pole of the
electromagnet. Similarly, the south poles are adjacent to
each other. Like magnetic poles repel each other, causing
the movable permanent magnet to begin to turn. After it
turns part way around, the force of attraction between the
unlike poles becomes strong enough to keep the permanent
magnet rotating. The rotating magnet continues to turn
until the unlike poles are lined up. At this point the rotor
would normally stop because of the attraction between the
unlike poles (Fig. 3).
If, however, the direction of currents in the electro-
magnetic coils was suddenly reversed, thereby reversing
the polarity of the two coils, then the poles would again be
opposites and repel each other (Fig.4). The movable
permanent magnet would then continue to rotate. If the
current direction in the electromagnetic coils was changed
every time the magnet turned 180° or halfway around, then
the magnet would continue to rotate. This device is a
motor in its simplest form. An actual motor is more
complex than the simple device shown above, but the
principle is the same.
Figure 1
Figure 2
Figure 3
Figure 4
INTRODUCTION
Application Technology
Fundamentals of Polyphase Electric Motors
D 3
ALTERNATING CURRENT
In order to fully understand the AC motor, we must first
examine the fundamentals of alternating current.
Alternating current has several advantages over direct
current. One of the biggest advantages is economical
power transmission. Alternating current, after leaving the
generator, can be stepped up in voltage by means of a
transformer. This reduces the size of the wire needed to
transmit the current and, hence, lowers the cost. After the
power reaches its destination, it can be stepped down
again to the required voltage.
Another big advantage of AC over DC is the fact that AC
motors are simpler in construction, less expensive than DC
motors, and require less maintenance.
SINGLE-PHASE AC
Alternating current alternates or reverses many times each
second. The current increases to a maximum in one
direction, decreases to zero, and increases to a maximum
in the opposite direction. The number of times this process
occurs each second is called the frequency. The frequency
of most AC power systems is 60 Hertz (cycles per second).
Alternating current may be better understood by referring
to a hydraulic analogy (Fig. 5).
A belt drives the pulley, causing the crankshaft and piston
to move. As the piston moves back and forth in the water-
filled cylinder, it causes the water in the pipe to flow first
in one direction and then in the other. A flowmeter at G
registers the rate of water flow in the pipe as it reaches
peak speed, decreases to 0 and reverses direction.
Since the alternating electric current undergoes similar
changes, the sine curve will apply equally well to the
pump cycle as to the alternating current cycle.
THREE-PHASE AC
Industry uses, in addition to single-phase AC, a power
source called polyphase AC (poly meaning many).
The most common form of polyphase AC is three-phase.
Three-phase AC consists of three alternating currents of
equal frequency and amplitude, but differing in phase from
each other by one-third of a period. By adding two more
pistons to our hydraulic system, we can illustrate three-
phase AC (Fig. 6).
The cranks are placed 120° apart with the result that the
current in each cylinder reaches its maximum at a different
time. When any one of the currents is at its maximum, the
other two are at half their maximum value.
The biggest advantage in using three-phase power is in the
machines it supplies. Three-phase motors are much
simpler in construction than other types and, hence, require
less maintenance. A more powerful machine can be built
into a smaller frame and it will operate at a higher
efficiency.
All AC motors then can be classified into single-phase and
polyphase motors. Because polyphase motors are the most
commonly used in industrial applications, we shall
examine them in detail.
THE AC MOTOR
Figure 5
Figure 6
D
Application Technology
Fundamentals of Polyphase Electric Motors 4
POLYPHASE AC MOTORS
Polyphase motors make up the largest single type in use
today and usually are the first to be considered for the
average industrial application. There are several types of
polyphase motors. The most common type of motor in this
group is the squirrel-cage polyphase induction motor so
called because the rotor is constructed like a squirrel-cage
(Fig. 7). The squirrel-cage motor is the simplest to
manufacture and the easiest to maintain.
SQUIRREL-CAGE INDUCTION MOTOR
The operation of the squirrel-cage motor is simple. The
polyphase current produces a rotating magnetic field in the
stator. This rotating magnetic field causes a magnetic field
to be set up in the rotor also. The attraction and repulsion
between these two magnetic fields causes the rotor to turn.
Essentially this is all there is to the operation of this type
of motor.
The squirrel-cage motor is a constant speed motor with
either a normal or high starting torque. These
characteristics fulfill the requirements of the majority of
industrial applications, making the squirrel-cage motor
ideal for such applications including lathes, presses,
blowers, pumps, etc. Because of the importance of the
squirrel-cage motor to industry, and because of the fact
that the Lincoln motor is of this type, a more complete
analysis of this motor begins on page 5.
WOUND ROTOR INDUCTION MOTOR
The wound rotor or slip-ring induction motor differs from
the squirrel-cage motor only in the rotor winding. The
rotor winding consists of insulated coils, grouped to form
definite polar areas of magnetic force having the same
number of poles as the stator. The ends of these coils are
brought out to slip-rings. By means of brushes, a variable
resistance is placed across the rotor winding (Fig. 8). By
varying this resistance, the speed and torque of the motor
is varied. The wound rotor motor is an excellent motor for
use on applications that require an adjustable-varying
speed (an adjustable speed that varies with load) and high
starting torque.
SYNCHRONOUS MOTORS
Synchronous motors comprise the third group in the AC
polyphase group. Synchronous motors are motors that
always run at the same speed regardless of load.
Synchronous motors are somewhat more complex than
squirrel-cage and wound rotor motors and, hence, are more
expensive. There is no slip in a synchronous motor, that is,
the rotor always moves at exactly the same speed as the
rotating stator field. The speed is thus determined by the
design of the motor and frequency of the power supply.
The speed will remain constant with wide variations in
load. As the load increases, the motor will keep a constant
speed until the point is reached where the machine can no
longer take the load and maintain a constant speed. At this
point, the speed of the synchronous motor drops