4-11
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 rotor-
induced 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.
Q8.
Why is the ac induction motor used more often than other types?
Q9.
The speed of the rotor is always somewhat less than the speed of the rotating field. What is the
difference called?
Q10.
What determines the amount of slip in an induction motor?
SINGLE-PHASE INDUCTION MOTORS
There are probably more single-phase ac induction motors in use today than the total of all the other
types put together.
It is logical that the least expensive, lowest maintenance type of ac motor should be used most often.
The single-phase ac induction motor fits that description.
Unlike polyphase induction motors, the stator field in the single-phase motor does not rotate. Instead
it simply alternates polarity between poles as the ac voltage changes polarity.
Voltage is induced in the rotor as a result of magnetic induction, and a magnetic field is produced
around the rotor. This field will always be in opposition to the stator field (Lenz’s law applies). The
interaction between the rotor and stator fields will not produce rotation, however. The interaction is
shown by the double-ended arrow in figure 4-10, view A. Because this force is across the rotor and
through the pole pieces, there is no rotary motion, just a push and/or pull along this line.
