The Direct-Current Motor

A direct-current motor converts DC electrical energy into rotary mechanical motion. Its essential parts are a stationary field that supplies a steady magnetic field (from permanent magnets or field windings), a rotating armature carrying current-carrying coils, and a commutator with brushes that feeds current into the armature. When current flows through an armature coil sitting in the field, the coil experiences a force that produces torque and turns the shaft.

The commutator is what makes continuous rotation possible. As the armature turns, the segmented commutator reverses the direction of current in each coil at the right moment, so the torque always pushes the rotor the same way around. Without this switching the rotor would simply line up with the field and stop. Brushes, usually made of carbon, press against the rotating commutator to deliver the current while allowing the shaft to spin freely.

As a DC motor speeds up, the rotating coils generate a voltage of their own that opposes the supply, known as the back electromotive force. This back-EMF rises with speed and limits the current the motor draws, which is why a stalled motor draws a large current while a freely spinning one draws little. Speed can be controlled by varying the supply voltage or the field strength. Animations are well suited to showing the moment-by-moment current reversal at the commutator and the way force on each coil combines into smooth, continuous torque.

Frequently asked questions

What does the commutator do in a DC motor?

It reverses the current in each armature coil as the rotor turns, keeping the torque acting in one direction so the shaft rotates continuously instead of stalling at alignment.

What is back-EMF?

It is the voltage generated by the spinning armature that opposes the supply. It increases with speed and naturally limits the current the motor draws when running.

How is the speed of a DC motor controlled?

Speed depends on the applied voltage and the field strength. Reducing the armature voltage slows the motor, and weakening the field can increase its speed.