Induction Motor Characteristics

The induction motor has two major components: The Rotor and The Stator. In most motors, the Stator is in the outer part of the motor and comprises a stack of steel laminations and two or more windings. The inner part of the stator is hollow, and the windings are distributed around the inner surface of the stator imbedded in a number of slots. The windings are organized to form two or more electromagnetic poles.
The Rotor is a solid cylindrical stack of laminations with a series of conducting bars imbedded near the surface. The ends of these bares are shorted together by shorting rings.

When the supply is connected to the stator windings, a magnetic field is created which is rotating at the supply frequency. The field in a two pole machine will do one complete revolution per cycle of the supply. A Four pole machine requires two cycles for a complete revolution and a Six pole machine requires three cycles for a complete revolution.

The rotating magnetic field developed by the stator, causes a current to flow in the short circuited rotor winding in the same manner as the secondary current is caused to flow in a transformer. - infact the motor emulates a transformer with a short circuited secondary.
The rotor current in turn develops a rotating magnetic field which interacts with the stator field to develop a rotating torque field in the direction of the stator field rotation. The strength of the torque field is dependent on the interaction of the two magnetic fields, and is therefore dependent on the magnitude of the fields and their relative phase angle.
The full voltage start current and start torque curves vary tremendously between different motor designs due to the variations in rotor designs.

In designing a motor starting system, it is important to base the design on the actual motor being used. A design based on "typical" curves can yield very erroneous results.