Induction Motor Principle

Introduction to induction motor principle:

Induction motors are most common type of electrical motors. It is widely used because of its simple construction, economical cost and ruggedness.

As the name suggests, the motor works on principle of electromagnetic induction. In a way it can be called as rotating transformer because of the close similarity in principle of operation.

Let us study the what the induction motor principle is.


Description of an induction motor


A schematic diagram of an induction motor is shown above. Pairs of electromagnetic poles are housed in a casing called stator. The e electromagnetic poles are wound with conductors to produce magnetism which is called stator windings. A rotating part called rotor is placed in the annular gap of the stator by suitable mountings in such a way that the rotor can rotate freely. The rotor may be either wire wound or may simply be bars of metal. In the latter case, the rotor is called squirrel cage type.

The entire construction is as per the induction motor principle.


Induction motor principle


When a poly-phase AC supply is given to the stator windings, the electromagnets are energized and act as magnets. As the supply is alternating current, the polarity of the poles changes in tune to the frequency of supply. Thus the set of poles create a rotating magnetic field. Please note that there is no rotation actually exists but it is virtual. It is something like you feel the light is moving when a set of lamps glow and disappear in a sequence. The speed of rotation of the magnetic field is called the synchronous  speed  of the motor and is given by the formula,

synchronous speed in rpm = (120f)/p where f is the frequency of supply and p is the number of poles.

The rotating magnetic field, because of its change in magnetic flux induces current on the rotor. As per Lenz’s, the induced current starts opposing the cause by rotating, thereby trying to nullify the relative motion with respect to the magnetic field. This is how the rotor starts rotating in an induction motor.

The rotor can never attain the synchronous speed because if it does then there is no relative motion with respect to the rotating field  and the induced current in the rotor vanishes. The difference between the synchronous speed and the actual speed of the motor is called the slip of the induction motor.