Speed -Torque Characteristics of Polyphase Electric Motors

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Understanding the relationship between the load carried by an AC motor and the torque it develops during each stage of operation of the motor is crucial to selecting the right kind of electric motor for the right kind of application. AC Motors are therefore designed with torque-speed characteristics to match the requirements of common applications. The four standard NEMA motor designs, A, B, C, and D, have different characteristics. Below shows a typical torque/speed curve for an AC motor:

An AC motor develops four basic types of torques during operation. They are:

(a) Starting Torque (ST)/Locked Rotor Torque (LRT)
(b) Pull-up Torque (PUT)
(c) Breakdown Torque (BT)
(d) Full Load Torque (FLT)

Starting Torque/Locked Rotor Torque
Starting torque, also referred to as locked rotor torque, is the torque that the motor develops each time it is started at rated voltage and frequency. It is the torque produced when power is applied to a motor at rest, i.e. when the motor is energised at full voltage and the shaft is locked in place. This is the torque used to start accelerating the load. The starting torque is indicated on the torque/speed curve shown above.

Pull-up Torque (PUT)
This term is used for the lowest point on the torque speed curve for an electric motor which is accelerating a load up to full speed. As the motor picks up speed, torque decreases slightly to the lowest point shown on the curve above. The torque available at this point is called pull-up torque.

Breakdown Torque (BT)
Breakdown torque is the maximum torque that an AC motor develops with rated voltage applied at rated frequency without causing sudden drops in speed. This is also known as pull-out torque or maximum torque.

Full-Load Torque (FLT)
The torque required to produce rated power at full-load speed. Full-load torque is developed with the motor operating at rated voltage, frequency, and load. At full-load torque, the speed of the electric motor is slightly less than the synchronous speed as shown in the curve above.