DC Electric Motor Brushes

DC electric motors with brushes. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats. In a dynamo, the contact point of where a pair of brushes touch the commutator is referred to as the commutating plane. In this diagram the commutating plane is shown for just one of the brushes.

When a current passes through the coil wound around a soft iron core, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force. According to Fleming's left hand rule, the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, "direct current" commutator make the current reverse in direction every half a cycle thus causing the motor to rotate in the same direction. With brushes supplying the contact with current.

The problem facing a simple two-pole design motor with brushes, is when the plane of the coil is parallel to the magnetic field; i.e. the torque is ZERO-when the rotor poles or displaced 90 degree from the stator poles. The motor would not be able to start in this position, but the coil can continue to rotate by inertia.

There is a secondary problem with a this design; at the zero-torque position, both commutator brushes are touching across both commutator plates, resulting in a short-circuit that uselessly consumes power without producing any motion. In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful, but if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brushes damage, and potential welding of the metallic brushes to the commutator.

Unlike the demonstration motor, above, DC motors are commonly designed with more than two poles, are able to start at any position, and do not have any position where current can flow without producing electromotive power.