Brushless DC vs. brushless AC motors

Written by j.t. barett
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Brushless DC vs. brushless AC motors
A brushless DC motor sits next to the engine of this hybrid car. (Detailansicht Hybrid Motor image by 3rdTwin from Fotolia.com)

Some simple electric motor designs use brushes. The brushes sit on the fixed part of the motor, feeding current to a set of contacts fixed to the rotating part. While this works, the brushes' friction lowers efficiency, and they eventually wear out. Alternating current (AC) motor designs don't need brushes. They exploit AC's undulating nature. Engineers have developed brushless direct current (DC) motors that use electronic controls in place of the brushes.

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History

Nikola Tesla invented AC power and motors in the late 1800s. While engineers have developed new ones since, many of the operating principles remain the same. DC motors, on the other hand, traditionally had a design that used brushes. The brushless DC motor had to wait until the 1950s and '60s for the development of sophisticated electronic sensors and controls.

Sensors

To know when to turn its coils on and off, a brushless DC motor uses Hall Effect sensors, or optical or magnetic encoders. Part of the sensor turns with the rotor, while another part remains stationary. Though this is similar to the brush type, the brushless sensors make no physical contact. The sensors connect to electronic circuits that govern the motor's coil power cycles.

AC motors, on the other hand, don't use sensors to time their coils. Alternating current, especially the three-phase power produced by electric utilities, is itself a source of timing. All an engineer needs to do is design a motor with the correct connection and placement of coils, and the AC does the rest.

Simplicity

Because a typical AC motor uses the 60-cycle frequency of the electric power to drive its coils, it has a simpler design. While the electronic sensors used by brushless DC motors are reliable, the extra pieces drive up cost.

Power

Brushless DC motors, for the most part, fit a niche at the lower end of the power spectrum, under a hundred watts. Typical applications include computer hard drive motors, small pumps, and fans. Hybrid cars use larger, more advanced brushless DC motors. AC motors find their way into higher range of power, from devices that use 100 watts to many thousands of watts, such as home appliances and commercial and industrial equipment.

Speed Control

The 60-cycle electric power from the utilities determines the speed of most AC motors. Though the motor's speed is fixed, engineers have developed systems of pulleys or gears to drive machines at a few different speeds. More recently, AC motors have been powered by electronic speed control units. These mimic the AC coming from the utility but give the operator a range of speeds from which to choose.

Vary the speed of DC motors easily by changing the voltage of the electric power. The electronic sensors open up other avenues of speed control, especially where precise control is important.

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