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Rotor design variables will affect speed torque characteristics of motor

Publication date: 
November 2001
Cyndi Nyberg Esau, Former EASA Technical Support Specialist
Type of media: 

When manufacturers design a motor, there are many variables. For a given stator, the winding has to conform to some fairly rigid rules, and there is not a lot of variance, even among different manufacturers. However, the rotor design is wide open. The variables in the rotor design include: the number of bars, bar material, bar shape, end ring design, skew, air gap, and construction (cast or fabricated). All of these will ultimately have an impact on the speed torque characteristics of a motor. Unfortunately, because of the rotor design, it is difficult to alter the basic shape of the speed torque curve of a motor in for repair by modifying the stator winding. Modifications can be made to the rotor such as changing the bar material or size, and changing the end ring design, but it is difficult to determine the actual effect they will have on the operating characteristics. The NEMA Design letter of an induction motor is defined by the torque, current, and speed characteristics. The rotor resistance and reactance, along with the amount of stator flux, determine the speed-torque characteristics of the motor. These include starting, breakdown, maximum, and pull-up torque, as well as starting current. The minimum torques and maximum starting current for each Design letter can be found in NEMA MG-1-1998, Section 12, and the EASA Electrical Engineering Pocket Handbook.