DC machines and DC motors

A DC machine is an electromechanical device that operates on direct current and can act either as a motor, turning electrical energy into rotation, or as a generator, doing the reverse. As a motor it produces torque by passing current through armature windings sitting in a magnetic field; the resulting force on the conductors turns the shaft. DC motors became popular because their speed and torque are easy to control simply by varying the applied voltage or current, long before electronic drives made AC motors equally controllable.

In a conventional brushed DC motor the armature windings are on the rotor. To keep producing torque in one direction, the current in those windings must be reversed at the right moment as the rotor turns. This is the job of the commutator, a segmented ring on the shaft, together with carbon brushes that press against it. The arrangement works well but the brushes and commutator wear over time and can generate sparking and electrical noise.

The brushless DC motor avoids these drawbacks by moving the windings to the stator and the permanent magnets to the rotor, then switching the currents electronically based on rotor position. Speed in both types is commonly regulated with pulse-width modulation, which rapidly switches the supply on and off to set an average voltage. DC motors of both kinds appear in servos, robotics, vehicles and countless small drives where precise control is required.

Frequently asked questions

What does the commutator do in a DC motor?

The commutator, working with the brushes, reverses the current in the armature windings at the correct moments so that the torque always acts in the same direction of rotation.

How is the speed of a DC motor controlled?

Speed is set by varying the voltage applied to the armature, often using pulse-width modulation, which switches the supply rapidly to produce a controllable average voltage.

What advantage does a brushless DC motor offer?

By switching currents electronically instead of through brushes, a brushless motor avoids brush wear and sparking, giving higher reliability, efficiency and lower electrical noise.