Physics

8.10 - Alternators & Dynamos

5 min

note 1 of 3

Alternators generate alternating current

A generator uses movement to induce a current, doing the reverse of a motor.
An alternator is a generator that produces an alternating current.

How an alternator works

A coil of wire rotates in a magnetic field, or a magnet rotates inside a coil of wire.
As the coil (or magnet) turns, it cuts through the magnetic field lines, inducing a potential difference across the ends of the coil.
If the coil is part of a complete circuit, this drives a current.
The induced potential difference changes direction every half turn, because each side of the coil reverses the direction it moves through the field.
The output is an alternating potential difference, which drives an alternating current.

Slip rings and brushes

An alternator uses slip rings and brushes to keep the output alternating.
A slip ring is fixed to each end of the coil and rotates with it.
Each slip ring stays in continuous contact with a brush, which doesn't rotate and connects to the rest of the circuit.
The brushes stay in contact with the same end of the coil throughout, so the connection is never swapped over.

A rectangular coil turning between north and south pole blocks, its shaft carrying two rings touching flat contacts wired to a meter, labelled magnet, magnetic field, coil, slip rings, half-turn, brush and voltmeter.

note 2 of 3

Factors that increase the generator output

The size of the induced potential difference (and current) can be increased by:

Spinning the coil or magnet faster, so the field lines are cut at a greater rate
Using more turns of wire on the coil
Using a stronger magnetic field
Increasing the area of the coil
Adding a soft iron core inside the coil

Reversing the direction of rotation reverses the direction of the induced potential difference, but doesn't change its size.

note 3 of 3

Dynamos generate direct current

A dynamo is a generator that produces a direct current.
It works the same way as an alternator, but uses a split-ring commutator instead of slip rings.

How the commutator gives direct current

As the coil rotates, the induced potential difference reverses direction every half turn.
The split-ring commutator swaps the connections to the circuit every half turn.
Because the swap happens in step with the reversal, current always flows the same way through the circuit, producing a direct current.

A rectangular coil turning between north and south pole blocks, its shaft carrying a two-piece blue ring pressed between contacts wired to a meter, labelled magnet, magnetic field, coil, brush, voltmeter and split-ring commutator.

key terms

TermDefinition

GeneratorA device that uses movement to induce a current.

AlternatorA generator that produces an alternating current.

Slip ringA continuous ring fixed to the end of a rotating coil that stays in contact with a brush.

DynamoA generator that produces a direct current.

Split-ring commutatorA split ring that reverses the connections between a rotating coil and the circuit every half turn, which keeps a motor turning one way or makes a dynamo give direct current.

OscilloscopeAn instrument that displays how a potential difference changes with time, as a graph of potential difference against time. It is also called a cathode ray oscilloscope.

end of notes

Key points to remember

Alternators generate alternating current

A generator uses movement to induce a current, doing the reverse of a motor.

An alternator is a generator that produces an alternating current.

How an alternator works

A coil of wire rotates in a magnetic field, or a magnet rotates inside a coil of wire.

As the coil (or magnet) turns, it cuts through the magnetic field lines, inducing a potential difference across the ends of the coil.

If the coil is part of a complete circuit, this drives a current.

The induced potential difference changes direction every half turn, because each side of the coil reverses the direction it moves through the field.

The output is an alternating potential difference, which drives an alternating current.

Slip rings and brushes

An alternator uses slip rings and brushes to keep the output alternating.

A slip ring is fixed to each end of the coil and rotates with it.

Each slip ring stays in continuous contact with a brush, which doesn't rotate and connects to the rest of the circuit.

The brushes stay in contact with the same end of the coil throughout, so the connection is never swapped over.

Factors that increase the generator output

The size of the induced potential difference (and current) can be increased by:

Spinning the coil or magnet faster, so the field lines are cut at a greater rate

Using more turns of wire on the coil

Using a stronger magnetic field

Increasing the area of the coil

Adding a soft iron core inside the coil

Reversing the direction of rotation reverses the direction of the induced potential difference, but doesn't change its size.

Dynamos generate direct current

A dynamo is a generator that produces a direct current.

It works the same way as an alternator, but uses a split-ring commutator instead of slip rings.

How the commutator gives direct current

As the coil rotates, the induced potential difference reverses direction every half turn.

The split-ring commutator swaps the connections to the circuit every half turn.

Because the swap happens in step with the reversal, current always flows the same way through the circuit, producing a direct current.