|
|
HOW A DYNAMO IS CONSTRUCTED
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 |
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. |
|||||||
but no change |
|
|
|
but no change |
|
|
|
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 |
|
|
|