INDUCTION MOTORS
The induction motor is the most commonly used type of ac motor. Its
simple, rugged construction costs relatively little to manufacture. The
induction motor has a rotor that is not connected to an external source of
voltage. The induction motor derives its name from the fact that ac voltages
are induced in the rotor circuit by the rotating magnetic field of the stator.
In many ways, induction in this motor is similar to the induction between the
primary and secondary windings of a transformer. Large motors and permanently
mounted motors that drive loads at fairly constant speed are often induction
motors. Examples are found in washing machines, refrigerator compressors, bench
grinders, and table saws. The stator construction of the three-phase induction
motor and the three-phase synchronous motor are almost identical. However,
their rotors are completely different (see fig. 1). The induction rotor is made
of a laminated cylinder with slots in its surface. The windings in these slots
are one of two types (shown in fig. 2). The most common is the squirrel-cage
winding. This entire winding is made up of heavy copper bars connected together
at each end by a metal ring made of copper or brass. No insulation
is required between the core and the bars. This is because of the very
low voltages generated in the rotor bars. The other type of winding contains
actual coils placed in the rotor slots. The rotor is then called a wound rotor.
fig(1) |
fig(2) |
Regardless of the type of rotor used, the basic principle is the
same. The rotating magnetic field generated in the stator induces a magnetic
field in the rotor. The two fields interact and cause the rotor to turn. To
obtain maximum interaction between the fields, the air gap between the rotor
and stator is very small.
As you know from Lenz’s law, any induced emf tries to oppose the
changing field that induces it. In the case of an induction motor, the changing
field is the motion of the resultant stator field. A force is
exerted on the rotor by the induced emf and the resultant magnetic
field. This force tends to cancel the relative motion between the rotor and the
stator field. The rotor, as a result, moves in the same direction as the
rotating stator field.
It is impossible for the rotor of an induction motor to turn at the
same speed as the rotating magnetic field. If the speeds were the same, there
would be no relative motion between the stator and rotor fields; without
relative motion there would be no induced voltage in the rotor. In order for
relative motion to exist between the two, the rotor must rotate at a speed
slower than that of the rotating magnetic field. The. difference between the
speed of the rotating stator field and the rotor speed is called slip. The
smaller the slip, the closer the rotor speed approaches the stator field speed.
The speed of the rotor depends upon the torque requirements of the load. The
bigger the load, the stronger the turning force needed to rotate the rotor. The
turning force can increase only if the rotorinduce emf increases. This emf can
increase only if the magnetic field cuts through the rotor at a faster rate. To
increase the relative speed between the field and rotor, the rotor must slow
down. Therefore, for
heavier loads the induction motor turns slower than for lighter
loads. You can see from the previous statement that slip is directly
proportional to the load on the motor. Actually only a slight change in speed is
necessary to produce the usual current changes required for normal changes in
load. This is because the rotor windings have such a low resistance. As a
result, induction motors are called constant-speed motors.
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