AC Induction Motor Designs, Types
W orldwide popularity of ac induction motors in numerous applications has led to some standardized motor designs. Concentration on a finite number of motor types also brings design and manufacturing efficiencies, while helping to achieve attractive pricing.
The National Electrical Manufacturers Association, (NEMA, Washington, D.C.) has developed specifications for so-called NEMA design A, B, C, and D motor types. These designs are based on standardizing certain motor characteristics such as starting current, slip, and specified torque points (see below). Here’s a brief rundown on NEMA motor types:
Design A has normal starting torque (typically 150-170% of rated) and relatively high starting current. Breakdown torque is the highest of all NEMA types. It can handle heavy overloads for a short-duration. Slip &=5%. A typical application is powering of injection-molding machines.
Design B is the most numerous type of ac induction motor sold. It has normal starting torque, similar to Design A, but offers low starting current. Locked rotor torque is good enough to start many loads encountered in industrial applications. Slip &=5%. Motor efficiency and full load power factor are comparatively high, contributing to the popularity of the design. Typical applications include pumps, fans, and machine tools.
Design C has high starting torque (greater than previous two designs, say 200%), useful for driving heavy breakaway loads. These motors are intended for operation near full speed with-out great overloads. Starting current is low. Slip &=5%.
Design D has high starting torque (highest of all the NEMA motor types). Starting current and full-load speed are low. High slip values (5-13%) make this motor suitable for applications with changing loads and attendant sharp changes in motor speed, such as in machinery with flywheel energy storage. Several design subclasses cover the rather wide slip range. This motor type is usually considered a ‘special order’ item.
CE , July 1998, p. 94 provides an illustration of the comparative torque-speed characteristics of these motors. The diagram also shows graphically the following torque-speed points important to induction motor specifications. Locked-rotor torque (starting torque) refers to minimum torque generated with the rotor at rest, and rated voltage and frequency applied. Breakdown torque is maximum torque generated before an abrupt drop in motor speed occurs as rated speed is approached (at rated voltage and frequency). Pull-up torque is the minimum torque generated over the motor’s speed range from rest to the speed point where breakdown torque is developed
Motors manufactured for European and international markets conform to a different set of designs and specifications. The International Electrotechnical Commission (IEC, Geneva, Switzerland) defines these design characteristics. One type of IEC induction motor is called Design N. This motor type has operating characteristics comparable to NEMA Design A and B motors.
AC induction motors are manufactured with a variety of protective housings to suit specific applications. Besides the enclosure types mentioned in the ‘ Back to Basics ‘ article, induction motors are available in still other housing varieties; for example chemical duty, washdown, and explosion proof types.