A simple d.c. electric motor consists of a coil of wire placed in a magnetic field. When current flows through the coil, a torque is produced. The brushes and commutator conduct the current from the supply to the coil.

Each of the carbon brushes makes contact with one half of the commutator. The commutator rotates with the coil.

This arrangement ensures that the torque produces a constant sense of rotation. In the diagram below, the force on side a – b of the coil will be directed downwards (Fleming’s left hand rule) so the rotation is anti-clockwise (viewed from the front).

When the coil has rotated 180°, side d – c is on the left but, as the commutator has also rotated, the torque is still in the same sense.

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