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A Dynamo is a machine for converting rotary motion into electricity.

The usual form of dynamo consists of a circular metal frame which is magnetised so as to create a strong magnetic field across the internal space.

The frame is made of a ring of magnetisable iron or steel alloy

Inside the ring is the 'rotor', a cylinder of metal which can rotate.

Pole pieces made of magnetisable iron are also fixed to the inside surface of the ring


Coils of insulated electric wire are wound on the pole pieces


When an electric current is passed through the coils, it cause the pole pieces to become magnetised. The magnetism flows easily through the ring which joins the backs of the pole pieces, but from the face of each pole piece it has to jump an air gap to the rotor and across to the opposite pole.

This means that the rotor is in a very strong magnetic field, but is still free to rotate.

The rotor is wound with a coil of insulated wire which rotates with it.

(The way a connection is made to this rotating coil wiil be explained later)

As the coil rotates in the magnetic field, the magnetism passes through the coil
first from A to B, then from B to A. The coils sees a constantly changing magnetic field, even though it is the coil which is moving, not the field..

If we have two electrical terminals fixed to the rotor at A and B, we can connect the two ends of the coil to them.
When the magnetism in the coil changes, it will cause a voltage to appear between the terminals.
This voltage depends on the rate of change of the magnetism, not on its actual value

Strong magnetism
but no change
Greatest rate of change
Still changing
Strong magnetism
but no change
Changing the other way
Greatest rate of change
Still changing back


This voltage is said to be Alternating, it regularly cycles from one direction to the other.

Alternating current is useful for some things and Direct current is useful for others.
Depending on how we make the connection to the rotating coil, either alternating or direct current can be supplied.


The current can be picked up by carbon brushes rubbing on a set of copper slip-rings. Each brush is continuously connected to the same end of the coil


The current can be picked up by carbon brushes rubbing on a commutator, which is a drum made of copper bars (insulated from each other). Each brush is connected first to one end of the coil, then to the other, as the rotor revolves

With slip-rings making the connection, the positive supply alternates from terminal to terminal, giving AC

With commutator bars making the connection, the connections are constantly being swapped so that the positive terminal is always connected through a brush to whichever end of the coil happens to be positive at any particular moment. This gives DC

Alternating current

Direct Current

Direct current is preferred for tramways for, a number of reasons


Commercially-built dynamos for tramways have several pole pieces, many coils on the rotor and a complex system of brushgear for picking up the current





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