This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Design A: The Forgotten Motor
by Johnny Douglass

According to MotorMaster+, there are fewer than 1,000 Design A motors in use, compared to about 23,000 Design B motors. In fact, Design A motors may be posing as Design B motors.

Although these motors are similar, they have some significant differences. The most commonplace of all models, the Design B motors must comply with certain specifications laid out in National Electrical Manufacturers Association Standard MG1. These limit the design to no more than 5 percent slip and place minimum limits on torque during starting and acceleration. The standards also define a maximum allowable locked rotor current, also known as starting current. The Design A specification is identical except the motors are not limited to any maximum locked rotor current.

With starting current, less is better. If starting torque and all other performance characteristics are identical, purchasers will choose motors with a lower starting current. This allows for lower rated circuit protectors and less voltage sag on the circuit during across-the-line starts. It accounts for the predominance of Design B models over Design A.

Recently, a motor manufacturer representative tipped the MotorMaster+ software team that some manufacturers appear to be fudging in their labeling. A follow-up query of the MotorMaster+ catalog database found 8,090 NEMA Premium motor models labeled as Design B. Of these, 3,120 listings omitted starting current. For the 4,970 models where the manufacturers provided starting current data, 117 had a Design A starting current.

Why should you care? If you specify and pay for a Design B motor you should get a motor that does not exceed Design B locked rotor current limits. You should also be able to see the manufacturers’ stated locked rotor current so you can verify it on receiving inspection.

One might argue that even if locked rotor current is omitted from MotorMaster+ it is coded on the nameplate. It is indeed coded on the nameplate with a bracketing letter code system that NEMA has used for other parameters like insulation temperature rating and, at one time, efficiency. This system obscures the actual current because the nameplate labels the rating simply as “Code” without designating it as locked rotor or starting current. You have to refer to a table in NEMA Standard MG1 to convert the letter code to a range of kVA per horsepower. Then you have to divide by the kV and multiply by the rated horsepower to finally get a range in which the locked rotor current presumably falls. Most people don’t bother.

You may not need a motor that holds locked rotor current within the Design B limit. Locked rotor current is irrelevant if the motor is powered by an electronic adjustable speed drive or started with a soft starter.

Even with across-the-line starting, the higher Design A starting current may present no problem for smaller motors and motors powered by low impedance “stiff” distribution systems. A common misperception is that motors with higher starting current cause higher demand charges. Actually, the duration of high current during acceleration is only a few seconds within a typical demand interval of 15 minutes or more.

A portable power analyzer can measure the voltage and even capture a trace of the locked rotor current when a motor starts. If you do not have a problem with starting current, do not limit your search for replacement NEMA Premium motors to Design B.

At this point, it may sound like Design A motors are cheaper and more efficient. It is easier to design an extremely efficient motor if you do not have to design additional resistance into the rotor and stator to limit starting current.

However, so much emphasis has been placed on improving the Design B motor that the relatively smaller Design A population will not necessarily offer the best efficiency. If you broaden your search to include both Design A and B you may find the best price and efficiency combination in a Design A motor. This likelihood may increase if the number of Design A models increases.

For several reasons, the number of NEMA Premium Design A motor models may increase soon. Manufacturers and the U.S. Department of Energy have become aware that some motors with Design A performance have been misidentified as Design B. The DOE has modified motor systems trainings to emphasize consideration of Design A motors. Also the next revision of MotorMaster+ will modify the listing query to allow users to list Designs A and B in a single query or just Design B. Currently, a user must exclude Design B in order to list Design A motors, leading most users to simply ignore Design A.

Design A motors can be NEMA Premium, severe duty, ODP, TEFC, Explosion Proof, Inverter Duty, or vertical. Include them in your search along with Design B in your next application where starting current is not a concern. If starting current is a concern, limit your search to Design B but check with MotorMaster+ or the manufacturer for the actual starting current, since it varies by model even within the design categories.

(Note: Douglass is a registered professional engineer with the Energy Services Clearinghouse.)