DC brush, the workhorse

Typical waveforms

DC brush motors are available in a large variety of sizes up to a kilowatt and beyond. They can operate at speeds of 10,000 RPM and even higher. Finally, DC servo motors are smooth and relatively quiet.

DC brush motors have two primary disadvantages. The first is the very fact that they require a mechanical device to commutate the motor. The brushes of this type of motor can wear out, or cause electrical arcing which generates electro magnetic interference (EMI).

Another disadvantage is that the DC servo motor has a relatively low torque output for a given size. This is due to the fact that the DC brush motor has current driven through its coils which are located in the rotor. The rotor is not “anchored” to the motor frame from a thermodynamic standpoint and therefore the total amount of energy that can be extracted from the coil is limited. This in turn limits the available torque output of DC servo motors.