Page 675 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 661

Conductor bars

FIGURE 8.36: Rotor of a squirrel-cage type AC induction
Shorting end rings motor. The conductor bars are shorted together at both ends
by two end rings.

for motors up to 100 KW (Figure 8.35). The air gap is even larger for motors which are
designed for applications that have very large peak torque requirements.
The number of phases of the motor is determined by the number of independent
windings connected to a separate AC line phase. The number of motor poles refers to the
number of electromagnetic poles generated by the winding. Typical numbers of poles are
P = 2, 4, or 6 (Figure 8.37). The coil wire for each phase is carefully distributed by design
over the periphery of the stator to shape the magnetic flux distribution.
The torque in any AC or DC electric motor is produced by the interaction of two
magnetic fields, with one or both of these fields produced by electric currents. In an AC
induction motor, the current in the stator generates a magnetic field which induces a current
in the rotor conductors. This induction is a result of relative motion between the stator
magnetic field (rotating electrically due to AC current) and the rotor conductors (which are
initially stationary). This is a result of Faraday’s induction law. The stator AC current sets
up a rotating flux field. The changing magnetic field induces EMF voltage, hence current,
in the rotor conductors. The induced current in the rotor in turn generates its own magnetic
field. The interaction of the two magnetic fields (the magnetic field of the rotor trying to
keep up with the magnetic field of the stator) generates the torque on the rotor. When the
rotor speed is identical to the electrical rotation speed of the stator field, there is no induced
voltage on the rotor, and hence the generated torque is zero. This is the main operating
principle of an AC induction motor.
In order to draw a visual picture of how torque is generated, let us consider a two-
phase AC induction motor (Figure 8.38). The principle for other numbers of phases is
similar. Let us consider the case where only phase 1 is energized and the current on the

θ m
N S

(a) (b) (c)
FIGURE 8.37: Stator windings of an AC induction motor: (a) two-pole (P = 2) configuration,
(b) four-pole (P = 4) configuration, (c) six-pole (P = 6) configuration.