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4 Motor Designs Identified In NEMA MG1

4 Motor Designs Identified In NEMA MG1 (on photo: ABB's three-phase asynchronous electric motors via DirectIndustry.com)

It’s all about performance…

Performance requirements for various types of induction motors for use on standard sinewave power supplies

are identified in NEMA MG1. Some of these types of motors are suitable for use in variable speed applications,

dependent on the type of application.

Performance requirements are also identified for motors for specific use invariable speed applications.

The purpose of this section is to provide guidance on the selection of one or more of the types of motors identifiedin NEMA MG1 that may be appropriate for the particular variable speed application under consideration. See

Figure 1.

The turning force which a motor develops is known as torque.  The amount of torque necessary to start a loa

(starting torque) is usually different from the torque required to keep the load moving ( full load torque).

Loads which have a high breakaway friction or that require extra torque for acceleration, should have a motor 

specified to have high starting torque.

NEMA motor design A

NEMA MG1 does not impose any limits on the magnitude of the locked-rotor current  on Design A motors, othe

than that the locked-rotor current is greater than the upper limit for Design B motors.

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Figure 1 - Typical motor speed torque curves

They are usually used in situations where higher locked- 

rotor current is used  for the purpose of obtaining higher 

running efficiency and higher breakdown torque .

Such motors typically require the use of reduced voltage

starting techniques for starting across the standard utility

power source. However, normal adjustable frequency

control function limits motor operation to the portion of its

torque speed characteristic that lies between no-load andbreakdown, even during starting.

Because of this, the higher locked rotor current of 

Design A motors is generally of little concern and the

motors are well suited for variable speed operation,

exhibiting low slip and high efficiency.

The potentially higher breakdown torque of a Design A

motor will extend its constant horsepower speed range

beyond that achievable by a Design B motor. However,

caution should be used when applying Design A motors in

by-pass operation, as their high locked-rotor current can

increase starter, thermal overload, and short circuit

protection device sizing.

Design A motors may also suffer greater thermal and

mechanical stress thanother designs when started across-the-line. Design A motors with very low slip may also

exhibit instability under lightly loaded conditions.

NEMA motor design B

Design B motors are applied in variable torque, constant torque and constant horsepower applications.

 Adjustable frequency control algorithms are generally optimized to the speed-torque-current characteristics of 

Design B motors. They exhibit good efficiency and low slip,and are suitable for across-the-line starting in bypass

mode.

Design B motors with very low slip may also exhibit instability under lightly loaded conditions.

NEMA motor design C

Design C motor speed-torque-current characteristics were defined to address  across-the-line applications

requiring high starting(locked-rotor) torque while generally maintaining Design Blocked-rotor current , but

slightly higher slip.

Since a Design B motor operated from an adjustable frequency control can provide the same breakaway torque

as a Design C motor operated from a control, it is usually preferred because of its industry-standard availability

and higher running efficiency. Also, since an adjustable frequency control driven motor  normally operates at

speeds above the breakdown speed, the high locked-rotor and pull-up torque of a Design C motor serves no

benefit in most adjustable speed drive applications.

Because Design C motors usually 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 significant low order harmonic

content.

This can result in additional heating in Design C motors over that in Design B and a corresponding greater 

http://electrical-engineering-portal.com/ac-adjustable-speed-drive-asd-an-overview

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decrease in system efficiency. Design B motors may not be suitable for bypass operation in an application that

normally requires use of a Design C motor for fixed frequency application.

NEMA motor design D

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

They exhibit very high locked-rotor torque but suffer in running efficiency due to their high slip characteristic. By

employing negative slip compensation with an adjustable frequency control, a Design A, B or C motor can be

made to emulate the speed-torque characteristic of a Design D motor while providing higher running 

efficiency .

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

Design A, B, or C motors cannot be used for bypass operation on an application that normally requires a Design

motor for fixed frequency application.

References

1. NEMA Standards Publication – Application Guide for AC Adjustable Speed Drive Systems

2. BALDOR (A member of the ABB Group) – Specifier Guide

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for 

design of LV high power busbar trunking (

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