SPEED CONTROL OF INDUCTION MOTORS

• Induction motor is a device which converts electrical energy into mechanical energy. It is an AC motor in which current is induced in the rotor winding by the magnetic field of the stator winding, by electromagnetic induction.

• They are classified as Single phase induction motors Three phase induction motors Synchronous motors Special purpose motors

• Out of these, three phase induction motors are widely used for their industrial drives because they are rugged, reliable and economical. They have self starting property, high power factor, good speed regulation.

INDUCTION MOTOR

Speed of induction motor• The synchronous speed of an AC induction motor is the theoretical speed at

which the motor should spin if the induced magnetic field in the rotor is perfectly followed by the magnetic field in the stator.

• The synchronous speed of an AC motor is given by N s = 120 * f p

• Slip is defined as the difference between synchronous speed and operating speed, at the same frequency.

S = (Ns – Nr) / Ns

• The shaft of an induction motor rotates because of the torque created by the interaction between magnetic field of the stator and the magnetic field of the rotor

T = (3/2*π*Ns) * (s*E22 *R2) / (R2

2 +(sX2)2) in Nm

How is the speed of induction motor controlled?

Cage induction motors were mainly used for fixed speed applications, as it was difficult to vary the frequency.

Wound rotor induction motors with slip rings connected to variable external resistance in the rotor circuit were used to bring about a considerable change in the speed of induction motor. They had many disadvantages such as cost ineffective, resistor losses, large slip rings.

Gradually there was development and it was learnt how to vary frequency and voltage which in turn affected the speed of induction motor. These methods overcame all the disadvantages of using a slip ring with rotor.

We know that the torque equation T ~ (s*E2

2 *R2) / (R22 +(sX2)2)

So as the parameters like R2 , E2 are changed then to keep the torque constant for the constant load condition, motor reacts by change in its slip. Effectively its speed changes.

Thus the speed of the induction motor can be controlled by two methods• From stator side• From rotor side

Stator side includes the following methods:• Supply frequency control , called V/f control• Supply voltage control• Controlling number of stator poles• Adding rheostats in stator circuit

Rotor side includes the following methods :• Adding external resistance in the rotor circuit• Cascade control• Injecting slip frequency voltage into the rotor circuit.

Although induction motor is a fixed speed device, by the advancement in science it is being increasingly used with variable speed drive. The speed can be varied by any of the ways given above.

Controlling the speed of induction motor helps in • Energy saving • It is cost effective• Rugged in construction• Reliable • Economical.

Thank you !