Torque

The torque of an electrical motor can be thought of as its ability to work against a load. The force and radius of the rotor determine the torque that is produced on the shaft of the motor.

The force depends on the:

• Resultant flux density in the air gap between the rotor and the stator
• Resistance of the rotor bars
• Length of the rotor bars

A motor can have different torque characteristics depending on the resistance (impedance) of the rotor bars and the inductive reactance of the rotor.

The impedance of the rotor affects the amount of induced current that flows in the rotor and the phase angle between the rotor current and induced voltage, which is crucial for determining the amount of torque produced.

There is considerable inductance in the rotor circuit because a laminated core surrounds the rotor bars.

• Inductance depends on frequency.
• Inductive reactance is greater when the motor first starts because the slip is the greatest.

Results in:

• Low lagging rotor power factor
• Low starting torque. The starting torque can be increased by adding resistance to the rotor circuit.

The torque decreases rapidly as speed increases beyond breakdown torque, reaching full-load torque at a speed slightly less than 100% synchronous speed. Full-load torque is the torque developed when the motor is operating with rated voltage, frequency, and load, and the speed at which full-load torque is produced is the slip speed or rated speed of the motor. It is important to match a motor’s characteristics with the characteristics of its load. The load characteristics of a fan, centrifugal pump, reciprocating compressor, conveyor, or grinder differ greatly.