4-9
together by copper rings. A low voltage is induced in these shorted windings by the rotating three-phase
stator field. Because of the short circuit, a relatively large current flows in the squirrel cage. This causes a
magnetic field that interacts with the rotating field of the stator. Because of the interaction, the rotor
begins to turn, following the stator field; the motor starts. We will run into squirrel cages again in other
applications, where they will be covered in more detail.
Figure 4-7.—Self-starting synchronous ac motor.
To start a practical synchronous motor, the stator is energized, but the dc supply to the rotor field is
not energized. The squirrel-cage windings bring the rotor to near synchronous speed. At that point, the dc
field is energized. This locks the rotor in step with the rotating stator field. Full torque is developed, and
the load is driven. A mechanical switching device that operates on centrifugal force is often used to apply
dc to the rotor as synchronous speed is reached.
The practical synchronous motor has the disadvantage of requiring a dc exciter voltage for the rotor.
This voltage may be obtained either externally or internally, depending on the design of the motor.
Q7. What requirement is the synchronous motor specifically designed to meet?
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. 4-8). 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. 4-9). The most common is the squirrel-cage winding. This entire winding is made up of
