11.5 A simple a.c. electric motor

How does a simple a.c. AC electric motor work?

Diagram of a simple a.c. electric motor

The a.c. power supply is connected to the rotating coils of the armature by two slip rings and carbon brush contacts.

Note the difference - two slip rings in an a/c. motor, instead of a split-ring commutator in a d.c. motor.

As the coil rotates, the direction of the current passing through the coil reverses direction.

When the coil has moved through half a turn, the direction of the current has reversed.

This ensures the force, produced by the interaction of the two magnetic fields, always operates in the same net direction, so the rotation of the coil is always in the same direction (clockwise or anticlockwise, depending on the poles of the permanent magnet).

The usefulness of electric motors (d.c. or a.c.)

Where do we start!

There are hundreds of uses in the home, in industry and transport systems.

They speed up many processes once done by manual labour, and increase the efficiency of machines once powered by coal (eg steam locomotives) or wind (eg windmills).

Electric motors power semi- or fully-automated production lines e.g. packaging of food, car assembly lines etc.

You can build electric motors of any required power from little motors in toys to powerful machines in heavy industry.

Powerful electric motors (ac or dc) drive electric trains.

Electric cars are slowly replacing fossil fuelled cars.

Appliances in the home powered by electric motors:

microwave cooker, food mixer, hair dryer, toothbrush, CD player,

we would be rather lost without all our electrically powered 'gadgets'!

This British Railways 'INTERCITY' electric locomotive has a maximum speed of 140 mph (225 km/hour) derived from a 4700 kW a.c. electric motor powered from 25 kV overhead power lines. Note that electric motors can be a.c. or d.c. current driven.