Question 13.25: Motor Design What are possible modifications in the design o...

Here, to understand the different torques present in the motor, it is helpful to review the torque–speed characteristics of a squirrel cage induction motor as shown in Figure 13.53.

Starting torque,   also called locked-rotor torque, is the torque produced by the induction motor at start-up of the motor. Therefore, starting current of the motor is also called the locked-rotor current.

Pull-up torque   is also called the minimum torque developed by the induction motor.

Breakdown torque   is the maximum torque generated by the induction motor without stalling. Sometimes breakdown torque is called pull-out torque.

Full-load torque   is the torque which the motor produces at rated load.

Torque–speed characteristics of a squirrel cage induction motor can be altered to suit different applications by modifying the construction and design. Different torque–speed characteristics can be generated to suit different applications as illustrated in Figure 13.54.

The National Electrical Manufacturers Association (NEMA) has identified four such designs as part of their standard. Each is considered, along with its corresponding performance curve.

Design A   This design is characterized by a starting current close to 10 times that of the full-load current, high breakdown torque, and low slip. Starting torque is close to 150% of full-load torque and breakdown torque is close to 200% of full-load torque. Motors having this design are characterized by high efficiency.

Design B   This design has a high reactance obtained by narrower rotor bars, which limits the starting current to five times the full-load current. Starting torque and slip are the same as in the design A motor. Breakdown torque is lower than design A motors, which is a drawback of this design and consequently the design B motor cannot be used for loads which have high peak values. Motors of this design are used in applications with relatively low starting torque requirements like fans, blowers, centrifugal pumps, and so on.

Design C   This motor has the highest starting torque of any of the designs discussed so far, with values going up to 200% of full-load torque. However, the breakdown torque is lower than either design A or B. Full-load torque is, however, the same as in designs A and B. At rated load, the values of slip are higher and, as a result, these motors are less efficient at rated load. This motor is particularly used for accelerating heavy loads like conveyors, agitators, reciprocating pumps, and so on.

Design D   This design has the highest starting torque of any of the designs discussed with values approximately 280% to 300% of full-load torque. However, in this design, torque is inversely proportional to speed. Starting current is the same as in the motor of design B and slip is high. This design has the highest slip among any of the designs and is primarily used where the starting torque requirement is very high but the load is not continuous, such as in hoists and elevators.