Motors for alternating-current circuits may by either single-phase or polyphase (two-or three-phase).They may again be divided into two kinds, named respectively:1.Synchronous;2.Non-or asynchronous, ordinarily called induction motors.
The most widely used a.c. motor is the induction motor. It has two main parts: a)the stationary winding or stator, which sets up a rotating magnetic field, and b)the rotating part of the motor, i.e. the rotor. The rotor of a commercial a.c. motor consists of an iron core with large copper bars placed in sets around the circumference and connected at both ends to copper rings. This is called a squirrel-cage rotor. When a rotor is placed in a rotating magnetic field, a large current is induced in it.
A.c. motors are exactly similar in construction to a.c. generators and may be called inverted alternators, since the same machine may be used as either a generator or motor.
Synchronous motors are very suitable for large powers, where the machine can be started up without load, and once started run for long periods.
For supplying direct-current power networks, the supply comes first from an alternating-current source and is converted to direct current by synchronous convertors or motor-generator sets.