Squirrel-cage induction motor

The squirrel-cage induction motor is the most commonly used type in the industry. The rotor (rotating part of the motor) has copper or aluminum bars that are welded to the end rings. This configuration is similar to that of a squirrel cage; hence it has cquired the name “squirrel cage” induction motor. The NEMA standard [S1] has defined a set of motor characteristics, dimensions, and performance for motors rated up to 500 hp (375 kW).
 

Torque Characteristics

A typical speed–torque curve during starting (rest to full-load speed). Different types of torques developed by the motor are defined as follows: 


Locked-rotor or breakaway torque (point “a”): Locked-rotor torque of a motor is the minimum torque that it develops at rest for all angular positions of the rotor with rated voltage applied at rated frequency.

Speed–torque characteristic of an induction motor

Pull-up torque (point “b”): The pull-up torque of an AC motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs.

 
Breakdown torque (point “c”): The breakdown torque of a motor is the maximum torque that it develops with a rated voltage applied at a rated frequency, without an abrupt drop in speed.
Full-load torque (point “d”): The full-load torque of a motor is the torque necessary to produce its rated power at full-load speed.

The operating point is the intersection of the motor and load torque curves. Slip is the difference between the motor synchronous speed and the operating speed.

NEMA MG-1 [S1] has defined the minimum torques (a, b, c) and maximum locked-rotor currents for motors rated 1–500 hp (375 kW). Minimum torques developed by motors rated >500 hp (375 kW) are 60% locked rotor, 60% pull-up, and 175% breakdown.

NEMA Design Letters

NEMA has assigned design letters for motors rated up to and including 500 hp (375 kW) that define the torque characteristics. NEMA design letters shown in fig. 9.6 do not apply to motors rated above 500 hp (375 kW). The design letters and hp ranges are:

Designs A, B, and E: for motors rated 1–500 hp (375 kW)
Design C: for motors rated 1–200 hp (150 kW)
Design D: for motors rated 1–500 hp (375 kW)
Design E was introduced to accommodate the demand for a better than high-efficiency design, which resulted in a higher locked-rotor current and lower torques compared with design B motors.