Brushless DC, the high-flying overachiever

Brushless DC motors have been gaining in popularity in the last several years. This is because they provide a “no compromise” solution to servo control for many applications. Brushless DC motors are relatively smooth and quiet, and yet do not require mechanical brushes for commutation. In addition, brushless DC motors do not drive current through the rotor. Instead current is driven through the stator, which is solidly connected to the motor case allowing heat to be rapidly dissipated. This in turn allows the brushless DC motor to generate high torque for a given package size.

Finally, brushless DC motors are available in a wide variety of power ranges up to and beyond 1 kilowatt and they can be made to operate at very high speeds. Some motors can go up to 30,000 RPM and beyond.

Maximum torque

Despite these important advantages the brushless motor has two main disadvantages. The first is that they are expensive compared to DC servo or step motors. This is due to the fact that the manufacturing volume of brushless motors is not as high as for the other two motor types, and also due to the cost of the rare-earth magnetic materials that are used in the rotor. The second disadvantage of brushless DC motors is that they must be commutated externally. This increases the complexity of the controls and also requires the installation of Hall sensors, or equivalent phasing tracks in an optical encoder disk. This increases the cost and complexity of the motor.

Now that we have been introduced to the characteristics of positioning motors let us examine some important issues related to how these motors are controlled. By developing a better understanding of motor control techniques we will be able to make more informed decisions about the best motor type for a given application.