Four Design Types for AC Induction Motors
The National Electrical Manufacturers Association (NEMA) standards mainly specify four design types for AC induction motors: Design A, B, C and D.
Their typical torque-speed curves of four design types for AC induction motors are shown in the Figure below.
Design A has normal starting torque (typically 150-170% of rated) and relatively high starting current. The breakdown torque is the highest of all the NEMA types. It can handle heavy overloads for a short duration. The slip is <= 5%. A typical application is the powering of injection molding machines.
Design B is the most common type of AC induction motor. Design B has a normal starting torque, similar to Design A, but offers low starting current. The locked rotor torque is good enough to start many loads encountered in the industrial applications. The slip is <= 5%. The motor efficiency and full-load PF are comparatively high, contributing to the popularity of the design. The typical applications include pumps, fans and machine tools.
Design C has high starting torque (greater than the previous two designs, say 200%), useful for driving heavy breakaway loads like conveyors, crushers, stirring machines, agitators, reciprocating pumps, compressors, etc. These motors are intended for operation near full speed without great overloads. The starting current is low. The slip is <= 5%.
Design D has high starting torque (higher than all the NEMA motor types). The starting current and full-load speed are low. The high slip values (5-13%) make this motor suitable for applications with changing loads and subsequent sharp changes in the motor speed, such as in machinery with energy storage flywheels, punch presses, shears, elevators, extractors, winches, hoists, oil-well pumping, wire-drawing machines, etc. The speed regulation is poor, making the design suitable only for punch presses, cranes, elevators and oil well pumps. This motor type is usually considered a "special order" item.
