Polyphase motors

  NEMA classifies polyphase induction motors according to locked rotor torque and current, breakdown torque, pull up
torque, and percent slip.
 • Locked rotor torque is the minimum torque that the motor develops at rest for all angular positions of the rotor at rated voltage and frequency.
 • Locked rotor current is the steady state current from the line at rated voltage and frequency with the rotorlocked.

 • Breakdown torque is the maximum torque that the motor develops at rated voltage and frequency, without an abrupt

drop in speed.
 • Pull up torque is the minimum torque developed during the period of acceleration from rest to the speed that breakdown torque occurs. Figure 4 illustrates typical speedtorque curves for NEMA Design A, B, C, and D motors.
 • Design A motors have a higher breakdown torque than Design B motors and are usually designed for a specific use. Slip is 5%, or less.
 • Design B motors account for most of the induction motors sold. Often referred to as general purpose motors,
slip is 5% or less.
 • Design C motors have high starting torque with normal starting current and low slip. This design is normally used where breakaway loads are high at starting, but normally run at rated full load, and are not subject to high overload demands after running speed has been reached. Slip is 5% or less.
 • Design D motors exhibit high slip (5 to 13%), very high starting torque, low starting current, and low full load
speed. Because of high slip, speed can drop when fluctuating loads are encountered. This design is subdivided into several groups that vary according to slip or the shape of the speedtorque curve. These motors are usually available only on a special orderbasis.