Barks Publications, Inc.

Magazines and Books for Business and Industry

Internet questions (and answers) reveal motor misunderstandings

For anyone with a question, the Internet has become the place to seek answers. Several open forum or message board web sites feature postings dealing with electrical technology including motors and control. The questions being asked though legitimate, are sometimes surprising in the ignorance they reveal on the part of engineers and technicians who could be expected to know better. More surprising are the misunderstandings apparent from the answers being given.

New technology or changes in standard practices are seldom involved. Rather, what's apparent is that misconceptions about basic motor behavior remain widespread.

For example, a-c motor specifiers and users often have questions about "starting current." Standards define no such term. The current drawn by an accelerating motor takes on several different values during the starting period, rather than being constant as is often assumed.

A motor develops the same torque during acceleration regardless of the load being accelerated. Use of a "soft" or reduced voltage starting method offers several benefits, but they do not include any reduction in thermal stress within the motor (the reverse is often true) or any significant energy saving. If a motor's accelerating torque is inadequate to start the load, specifying a high torque NEMA Design C motor can be self-defeating, because such a motor often has less thermal capacity for acceleration heating than a standard design. The NEMA D alternative has that disadvantage plus low efficiency at full speed. Simply oversizing the motor means higher current -- and greater voltage drop -- during starting.

Also widely misunderstood is motor power factor, which is unaffected by the driven load; by the power factor in the rest of the plant; or by the power factor on the utility feeder. Concern is sometimes expressed that "overcorrection" can boost motor power factor above 100 percent, which is a physical impossibility.

Many other misunderstandings that continue to hamper proper motor selection and application can readily be found on Internet forums. Anyone using that source of information needs to proceed with caution.

Industry news...

 
Copyright © 2010 Barks Publications, Inc.
400 N. Michigan Avenue, Suite 900
Chicago, IL  60611
312-321-9440 tel  ...call for our email addresses...  fax 312-321-1288