The d.c. motor, relays and loudspeakers

Last time

Before we start: last time, in The magnetic effect of a current, the motor and the relay, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Machines built on one rule

A fan, a washing machine drum, a car starter motor. Each one spins because of the force on a wire carrying a current in a magnetic field. Add a clever switch to keep it turning, and you have the d.c. motor. Then change what the current does, and you have a loudspeaker.

The coil in a field

A rectangular coil sits between the poles of a magnet. The current goes along one side one way, and back along the opposite side the other way. The forces on the two sides are in opposite directions: one side is pushed up, the other down. These two forces make a turning effect, and the coil rotates.

The parts of a d.c. motor

The coil turns between the poles of the permanent magnets. At each end of the coil is a half of a split ring, called the commutator. Carbon brushes press on it and connect the supply. Every half turn, the split ring swaps the connections, so the current in the coil reverses. The forces then keep pushing the same way, and the coil keeps spinning.

Why a split ring?

Without a commutator, the coil would turn through 90 degrees and then settle in the vertical position, and stop. The commutator reverses the current just as the coil reaches the vertical position, so the forces carry it round. The brushes are made of carbon, which conducts and is smooth, so the commutator can slide against them.

The speed and direction

A motor turns faster with a larger current, a stronger magnet, or more turns of wire on the coil. A lower supply voltage means a smaller current, so a slower turn. Reverse the direction by reversing the current or the magnetic field, but not both.