4 motor designs identified in NEMA MG1

It's all about performance ...

Performance requirements for various types of induction motors intended for use on standard sine wave power supplies are identified in NEMA MG1. Some of these types of motors can be used in variable speed applications, depending on the type of application.

Performance requirements are also identified for motors for a specific purpose, invariable speed applications.

The rotational force that a motor develops is called torque. The amount of torque needed to start a load (starting torque) is usually different from that required to keep the load moving (full load torque).

Loads that have high breakout friction or that require additional torque to accelerate, must have a motor specified to have high starting torque.

 NEMA MG1 places no limit on the magnitude of locked rotor current on Design A motors other than the locked rotor current is greater than the upper limit on Design B motors.

They are generally used in situations where a higher locked rotor current is used in order to achieve higher operating efficiency and higher breaking torque.

Such motors generally require the use of voltage starting techniques for starting from the standard mains power source. However, the normal adjustable frequency control function limits the operation of the motor to the part of its torque speed characteristic between no-load and failure, even during starting.

Therefore, the higher locked rotor current of Design A motors is generally not a problem, and the motors are ideally suited for variable speed operation, exhibiting low slip and high efficiency.

The potentially higher breaking torque of a design A motor will extend its constant power speed range beyond what can be achieved by a design B motor. However, caution should be exercised in use. Design A motors in bypass mode, as their high locked rotor current can increase the sizing of starting, thermal overload and short circuit protection devices.

Design Motors may also experience greater thermal resistance and mechanical stress than other designs during start-up. Design A Very low slip motors can also exhibit instability under low load conditions.

NEMA design B motor

Design B motors are applied in variable torque, constant torque, and application of constant power.

Adjustable frequency control algorithms are generally optimized for the speed-torque-current characteristics of design B motors. They exhibit good efficiency and low slip and are suitable for bypass mode starting.

Very low slip design B motors can also exhibit instability under low load conditions.

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NEMA motor design C

The speed-torque-current characteristics of the Design C motor have been defined. Transverse applications requiring high starting torque (locked rotor) while maintaining Locked current, but slightly higher slip.

From a Design B motor operated from an adjustable frequency control can provide the same take-off torque as a Design C motor driven by a controller. It is generally preferable due to its standard availability and higher operating efficiency. In addition, since an adjustable frequency control driven motor normally operates at speeds above the breakdown speed, the high locked-rotor torque, and grip of a Design C motor are of no benefit in most applications. variable speed drive applications.

Because Design C motors typically achieve high starting torque with a double or pseudo-double cage rotor slot, they may exhibit higher rotor losses if the control output current waveform has content. significant harmonic of low order.

This can result in additional heating in Design C engines over that of Design B and a greater corresponding decrease in system efficiency. Design B motors may not be suitable for bypass operation in the application normally requiring the use of a Design C motor for a fixed frequency application.

NEMA motor design D

Design D motors have been developed specifically for high impact loads, high starting torque or high inertia.

They feature a very high locked rotor torque but suffer from operating efficiency due to their high slip characteristic. Using negative slip compensation with adjustable frequency control, a Design A, B, or C motor can be manufactured to emulate the speed-torque characteristic of a Design D motor while providing superior operating efficiency.

As a result, Design D motors are rarely used in general ASD applications.

Design A, B, or C motors cannot be used as a shunt in an application that normally requires a Design D motor for a fixed frequency application.

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