NEMA MG 1 assigns code letters to AC induction motor designs to indicate relationships between speed and torque. These relationships reflect the torque capabilities of various motor designs from zero (locked rotor) to synchronous speed. Why is knowing the torque characteristics of a motor important? Well, the most obvious reason is to ensure that the motor’s rated torque can supply the force needed to drive the load. This must be considered at all operating speeds – at start-up, the motor’s starting torque (also called “locked rotor” or “breakaway” torque) must be sufficient to move the load in order to avoid stalling the motor; and while running, the load torque requirements must not exceed the motor’s breakdown torque (also known as maximum or “pull-out” torque) or else the motor will see a steep drop in speed and rapid current and temperature rise.
For 3-phase motors up to 500 hp, there are basically three classes of speed/torque designs: A & B; C; and D. Let’s examine each in more detail; you can find representative curves and data in the figures below:
- A & B: these are similar in characteristics, differing principally in terms of starting current. Primarily due to the relatively low impedance of the rotor, Design A motors have a higher starting current than Design B. ‘B’ motors are by far the most common of all the designs and are considered general purpose because of their mid-level starting torque and low starting current draw. Depending on motor size and speed, minimum breakaway torque range is approximately 150-200% of rated torque. Slip, defined as the percentage difference between synchronous speed and rated speed and needed for the induction motor to generate any torque in the first place, is low (less than 5%). Typical applications for such motors include fans, blowers, centrifugal pumps and compressors, and motor-generator sets where starting torque requirements are not high.
- C: this motor uses a rotor winding configuration intended to increase starting torque while maintaining a low starting current. As a result, minimum breakaway torque requirements for these motors range from approximately 200-275% of rated torque. Slip is again low (less than 5%). Typical applications include conveyors, crushers, agitators, and reciprocating compressors.
- D: these motors are used where very high starting torque is required. Motors are selected based on the slip needed to match the load. Starting current is again low, and slip can range from 5-13%. Because of this, relative efficiency of these motors is the lowest of the group. You will find these motors supplying high-inertia loads such as punch presses, shears, cranes, hoists, and elevators.
It should be noted that the above classes define minimum ratings. In the competitive marketplace, many motor manufacturers have chosen to go NEMA one better by designing motors which exceed these ratings. For instance, many Design B motors actually have locked rotor torques equivalent to Design C motors. Manufacturers are willing to invest more in motor construction to hit target markets while not facing the need to meet all NEMA minimum classification requirements. It is prudent, therefore, to examine motor specifications closely before deciding on one based solely on classification.
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