12.4 The dynamo d.c. DC generator - producing a direct current of constant p.d

Diagram of a simple d.c. DC DYNAMO

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

A 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.

Explaining how a simple dc dynamo generator works

A dynamo works like an alternator in that the coils are rotated through the magnetic field by some external source of kinetic energy.

Here the magnetic field is produced from permanent magnets.

However, unlike the alternator, but like the electric motor, it uses a split-ring commutator and NOT slip rings, to connect with the external circuit.

The split ring commutator swaps the connections every half-turn so the current keeps flowing in the same direction - direct current generation (dc).

However, because of the rotation of the dynamo coil, you do not get a constant p.d. and you get peaks as with the output of an a.c. alternator, but no change from a +ve to a -ve p.d as get peaks and troughs with an a.c. output.

See the oscilloscope traces below of p.d. versus time - note the lack of oscillating wave shape of the trace, its just a series of oscillating 'humps' or half-waves.

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

 

Comparing 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 = p.d.).

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.

Therefore trace 2. shows a greater a.c. frequency than 1.

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

The maximum p.d. is greater in CRO trace 2 than trace 1 - you can tell from the greater amplitude.

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 d.c. generator describe above will only produce trace 3.