Watch out for energy-efficient motors with insufficient torque to start some loads.

If you recently replaced a standard induction motor with a new energy-efficient motor of the same size, you could run into various motor-starting problems; especially if youchose a Design E unit. Why? Although this type of motor offers the highest efficiency in its class, it also comes with real deficiencies.

Torque, current, and energy-efficient motors. Certain early energy-efficient motors didn't meet required standards. In some instances, if you replaced a standard Design B motor with an energy-efficient design, the replacement motor could not develop enough torque to start the load. In others, the starting current was so high, the existing motor controller/disconnect could not handle the new motor.

The good news is: most U.S. manufacturers are now aware of these problems and are trying to correct such deficiencies. Some have made excellent progress; others need further improvements. Most problems relate to the smaller hp motors (those rated 5 hp or less), although some entire product lines barely meet standards.

Design E motors are generally the most efficient of the new designs. NEMA standards for Design E motors reveal they have very high efficiencies. However, some have starting torques too low for direct replacement and starting currents excessively high for existing starting equipment. (See Figs. 1 and 2, original article) As a result, always use caution when directly replacing existing motors with new Design E types. In new installations, good design practices and the selection of Design E motors will result in a successful application. Be particularly careful when selecting foreign-made motors, because many are not tested in accordance with standards set up by the Institute of Electrical and Electronics Engineers, Inc. (IEEE). As a result, the characteristics of these energy-efficient motors are inconsistent and tend to relate to poorer design values of certain Design E motors.

Are energy-efficient motors cost-effective? After all, they tend to have higher starting currents, torque problems, and even lower power factor in some instances.

In the long run, energy-efficient motors are well worth the frustration of new selection factors, different values to plug into designs, and other installation requirements. Recent surveys show significant savings, depending on the application. Typical data on the cost of energy alone for a 10-hp motor running at $0.05/kwhr shows an energy-efficient motor will save nearly $4000 yearly over a standard motor. For a 50-hp motor, savings can approach $20,000 per year. This data is an average of different types of motors and installation parameters. In addition, it relates only to direct energy costs. The cost of money, interest, inflation, increased motor life, rebate programs, increased process efficiency, etc., are not included.

(The January and March 1999 issues of EC&M discuss the key characteristics of standard induction motors; particularly torque, speed, and current. They also point out that torque, speed, and current play a major role in the proper operation of motor and in its initial and running costs.)

Sidebar: EPACT and NEMA Influence on Motors

With the passing of the Energy Policy Act of 1992 (EPACT), most new motors manufactured are required to be of energy-efficient design. The law applies to any general-purpose, T-frame, single-speed, foot-mounted, polyphase induction motor of Design A and B configurations continuously rated and operating at 230/460V, 60 Hz, as defined in the NEMA standard MG1 -1987. Affected motors include those rated from 1 hp to 200 hp and of drip-proof and totally enclosed design. (NEMA published additional data and tables in 1994 and 1995 to clarify applicable regulations.)

Note: Most available energy-efficient motors are still designated as Design B, although a few Design A may be available. The Design E motor has the highest efficiency.