12.3 The alternator a.c. AC generator - producing p.d. with an alternating current

Diagram of a simple ac ALTERNATOR generator

Reminders: All generators must have a source of power to rotate the coil of wire.

The copper coil of wire is rotated from some external power source.

As the coil spins, it cuts through the magnetic field and a current is induced in the coil.

Dynamos are d.c. generators and alternators generate an a.c. current.

Here, for this simple design of an a.c. generator, the direction of rotation is predicted from Fleming's right-hand rule.

Explaining how a simple ac alternator generator works

The construction is very similar to that of a simple electric motor.

The coil is rotated through the magnetic field by some external power source of kinetic energy.

Here the magnetic field is produced from permanent magnets.

As the coil rotates, cutting through the magnetic field, a current is induced in the coil.

The current will change direction after every half-turn.

To 'extract' the electrical current i.e. connect with the external circuit, ac generators use a system of slip rings and brushes (NOT a split-ring commutator, as in the DC dynamo).

This means the contacts don't swap every half-turn and so an alternating current (ac, alternating p.d.) is produced.

See the oscilloscope traces below of p.d. versus time - note the full oscillating wave shape of the trace.

This is different from the split-ring commutator used in the simple electric motor and the dynamo generator described in Parts 4 to 6

Brush contacts allow continuous electrical connection without inhibiting the movement of the commutator.

 

Comparing the output from an a.c. alternator and d.c. dynamo generator

CRO oscilloscope traces from generators

An oscilloscope can show how the p.d. across the coil of a generator varies with time.

Three examples of oscilloscope traces from generators are shown above (x axis = time, y axis = pd).

1. This trace shows an alternating current i.e. the p.d. is changing from +ve to 0 to -ve values in a continuous cycle.

You can tell its an a.c. trace because it goes up and down of the horizontal axis of p.d. 0 V.

The height of the trace above 0 V at any point tells you the p.d. generated at that point.

Note the full oscillating wave shape of the trace.

2. This is also a trace from an alternator generator, but the rotation of the coil is greater than for 1.

The higher the peak from the 0 V horizontal axis, the greater the potential difference generated.

Note the full oscillating wave shape of the trace.

Note the full oscillating wave shape of the trace.

3. This is a trace from a dc dynamo generator

You can tell it is not an alternating current because the trace consists of a succession of half-cycles.

The a.c. generator describe above will produce traces 1. and 2.

Large a.c. alternator generators are used in power stations producing electricity for power lines of a national grid system which can carry power with a p.d. of up to 400 kV. See National Grid notes

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