Selecting Torque for Top Motor Efficiency

Jan 1, 1999 12:00 PM, By Robert J. Lawrie, Senior Editorial Consultant

Sure, horsepower is an important motor parameter. However, torque is just as important. That said, what exactly is torque? It really depends on your point of view.

If you're the motor, your shaft exerts the torque's turning effort. As such, we call it motor torque. We commonly measure it in pounds-feet (lb-ft.). If you're the load, torque is the turning effort you require at your input shaft for your proper operation. In this context, we call it load torque, also measured in lb-ft. Basically, you should consider torque both as it pertains to the demands of the load you want to drive, and as it applies to the capabilities of the driving motor.

Regardless of your point of view, selecting the best and most efficient motor for the job (based on first-cost and operating costs) depends directly on its torque. Remember, a motor's operating efficiency falls off rapidly at lower values of torque loading.

Motor ratings and nameplates are standardized in terms of horsepower and speed, with no mention of torque. However, you can relate horsepower and torque with the following equation: Horsepower = Torque (lb-ft) x rpm/5250

Cost differentials.

Up-front cost. It's the design torque rating of the motor that most directly affects its material cost. For example, a 10-hp, 1200-rpm motor costs more than a 10-hp, 1800-rpm motor, because the former's rated torque is nearly 50% greater. Likewise, a 10-hp, 900-rpm motor costs even more, because its rated torque is almost 100% greater than that of an 1800-rpm motor.

This also helps explain why you don't see motors with synchronous speeds lower than 900 rpm very often; it's usually more economical to use an 1800-rpm motor instead, and add any necessary gearing or pulleys to obtain the desired speed.

Operating cost. The operating cost of a motor depends on its efficiency at a given condition of loading. This applies to existing standard induction motors as well as energy-efficient (EE) motors.

For competitive manufacturing-cost reasons, manufacturers generally design motors to have peak efficiency at or near the full-load nameplate rating. You can see this in the accompanying diagram, on page 44. Notice how rapidly efficiency deteriorates below the approximate 50% level of nameplate load. The reason for this decline is: About one-third of the losses in a typical motor will vary with load torque, while the remaining losses are essentially constant.

Because of these fixed losses, a large cost penalty occurs if you operate a motor at low values of loading.

Selection guidelines. From the foregoing cost factors, you should focus on two important selection and application guidelines:

Buy only as much motor as you need: Avoid oversizing.
Minimize prolonged operation at light-load conditions.

In some manufacturing plants, preexisting conditions may exist, which may prevent you from applying these rules. In any case, you must be aware of the cost penalties involved if you cannot "load-match," size appropriately, or operate the motor you select according to guidelines.

You can use many different techniques to comply with these two guidelines. It's important you research the numerous considerations applying to proper motor selection and operation. We'll cover these in the March issue. Future Motor Facts articles will discuss how steady-state, acceleration, and starting torque reduce motor operating costs.