NEMA Standard Design D Three-phase Asynchronous Motor

NEMA Three Phase Electric Motor Designs

Three phase induction motors are specified by their electrical design type. NEMA (National Electrical Manufacturers Association) has classified three phase AC motors into design A, B, C, D. A more recent design called E has been added. These designs are suited to particular classes of applications based upon the load requirements typical of each class.

As a motor operates from no load to full load, its torque varies with speed. The relationship between speed and torque is often shown in a graph, called a speed-torque curve. This curve shows the motor’s torque, as a percentage of full-load torque, over the motor’s full speed range, shown as a percentage of its synchronous speed. NEMA classification is based on the speed-torque curves. Typical speed-torque curve for designs A, B, C, and D is shown below:

NEMA Design Classes

NEMA classes categorize electric motors based on a motor’s starting-torque and its accelerating load. The four standard NEMA design classes are as follows:

NEMA Class A motors have a maximum 5% slip with a high to medium starting current, normal locked rotor torque, and normal breakdown torque. These characteristics make them well suited for a wide vareity of applications and they are often found powering fans and pumps.

NEMA Class B motors also have a maximum 5% slip and a normal breakdown torque but include a low starting current, and a high locked rotor torque. These motors are well suited for various applications that require normal starting torques and are commonly found in HVAC systems where they are used with pumps, blowers, and fans.

NEMA Class C motors have a maximum 5% slip, low starting current, high locked rotor torque and a normal breakdown torque. They work well with conveyors and positive displacement pumps that require a high starting torque and high inertia at startup.

NEMA Class D motors exhibit a maximum 5 to 13% slip with a low starting current and a very high locked rotor torque. Applications that require very high inertia starts, like hoists and cranes, often use Class D motors.

NEMA standard three-phase asynchronous motors (Design D, dedicated for oil well pump)

NEMA Oil Well Pump Motors are single speed, totally enclosed, self-ventilated, continuous duty, low voltage cage three phase induction motors which conform to NEMA MG1 design D Standard and have high slip ratio which is ≥5% , and large starting torque. The whole series motors adopt casting iron frames. The service factor is 1.15. And the motors have obtained Canada CSA Certification.