Power and efficiency drive motor selection

June 1, 2000
Methods for sizing motors, selecting drives and controllers, and using design rules for reducing cogging torque, help motion control engineers build more accurate, efficient, and reliable systems.

Edited by John R. Gyorki, Lawrence Kren, Kathleen Franzinger

This family of three-phase brushless dc motors are 30% more efficient, use 20% less power, and are 2/3 the size of ordinary BLDC motors. They come in sizes from 60 to 110 W and include onboard PWM drive electronics with tachometers to lower distortion.


Output power and overall efficiency are key parameters to consider when sizing motors. In the case of dc brushless and brush-type motors, the first step determines the motors' power output from

Po = 0.00074ST,

where S = speed, rpm, and T = torque, oz-in.

Most catalogs list the motor's output power, so matching the results of the calculation to an available motor is straightforward.

The second step determines the motor efficiency, represented as the ratio of output to input power and expressed as a percent from

Eff = 100Po/Pi,

where Po = output power, W, and Pi = input power, W.

Peak efficiencies range from about 55 to 65% for brush-type motors and from 70 to 80% for brushless motors, depending on the magnetic materials used. Motors with higher efficiencies tend to be more expensive, but can reduce total system size and cost over the long haul.

For example, consider a requirement for a 24-Vdc motor to run 2,500 rpm under a load of 10.8 oz-in. First, calculate output power,

Po = 0.00074ST
= 0.00074(2,500)(10.8) = 20 W.

Next, calculate the input power for a brushless and a brush-type motor using ceramic magnets, where Pi = Po/E. For the brush-type motor, Pi = (20/0.55) = 36.4 W, and for the brushless motor, Pi = (20/0.70) = 28.6 W.

Last, calculate the run current for each motor where I = Pi/V. For the brush-type motor, I = 36.4/24 = 1.52 A, and for the brushless motor,

I = 28.6/24 = 1.19 A.

Although brush-type motors usually cost less than brushless motors, brush types require 28% more current and a larger power supply. In the final analysis, brushless motors with a smaller power supply might be a less expensive solution for the system. Add that to the fact that a brushless motor can last up to three times longer, and the brushless motor becomes the optimum choice.

Peak efficiency is used in the example as the prime factor, and should be considered when working with a motor manufacturer to customize a unit for a specific application. Moreover, the motor maker may review the required speed, torque, and voltage, and decide to modify the motor's winding or torque constant to come as close as possible to theoretical peak efficiency.

Information for this article was contributed by Walter Kalwara, Product Manager, Thomson Airpax Mechatronics, 7 McKee Place, Cheshire, CT 06410, (877) 924-7729, www.thomsonind.com

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