NEMA Motor Designs
NEMA Motor Design classes are determined by their Locked Rotor Torque, Locked Rotor Current, Breakdown Torque, Pull-up Torque, and slip percentage.
NEMA Design A motors operated at a high efficiency and a breakdown torque than the other designs. The lower resistance of the windings draws more current during lock-rotor conditions. The higher breakdown torque extends the motor’s constant horsepower speed range. Starter, thermal overload, and short circuit protective devices require up-sizing. Due to the higher current draw, these motors often suffer from greater thermal and mechanical stress when used with across-the-line starters. Typically, reduce-voltage starters are used; their high lock-rotor current, low slip, and efficiency make these motors suitable for variable speed operation. The adjustable frequency control limits the torque to between the no-load and breakdown torques during start up. The machine’s high efficiency and very low slip may cause instability under light loading.
NEMA Design B motors are considered to have a standard locked-rotor torque. The higher impedance produces a higher starting torque with lower current. The locked-rotor current does not typically exceed 6.4 times the full load current, and the per unit locked-rotor torque decreases as the motor size increases. Design B motors are applied to variable torque, constant torque, and constant power applications. Their characteristics are what adjustable frequency control algorithms are optimized for. The motor’s low slip and high efficiency make these motors ideal for drives fans, centrifugal pumps and machine tools.
NEMA Design C motors are the only motor design to incorporate a double-cage rotor. The double-cage rotor helps to develop high starting torque, and low slip at full load. The outer cage (cage 1) has a lower inductive reactance and smaller conductors than the inner cage (cage 2). The double-cage design provides high resistance during starting periods, and low resistance while running at full-load. During starting periods, the low inductance and high resistance of cage 1 creates more torque. As the speed of the rotor increases the slip and frequency of the rotor current decreases. As the rotor approaches synchronous speed, the inductive reactance of both cages becomes negligible, and the rotor current is only limited by the low resistance of the inner cage (cage 2). During across-the-line starts the rotor speed is zero, the current waveform has a frequency equal to the line waveform, and current flows through the smaller bar of cage 1. The higher resistance of the outer winding draws more current, developing a higher starting torque. The higher currents during starting cause high I2R losses that can damage the smaller conductors of cage 1 when used with high-inertia loads. The rotor losses are can be magnified when the current waveform contains low order harmonics. Typical application start under load, such as pumps and piston compressors.
The NEMA Design D has the highest rotor resistance than any other NEMA design. The higher resistance produces a high locked-rotor torque, but turns at a slow speed at full-load torque. The high-resistance cage was designed for intermittent duty to prevent overheating, and protective devices are needed to ensure current draw at lower speeds does not exceed rated values. As the load increases, the speed decreases making these motors ideal for impact-type machines, and to accelerate high-inertia loads over a long acceleration period. The speed varies dramatically with the load due to the high amount of slip, typically between 5% and 8%. These motors have low efficiency, and runs hot.