The AC dynamo

Figure 13.20 shows an AC dynamo. A rectangular coil ABCD, wound with many turns of insulated copper wire, is fixed to a spindle so that it can turn about its axis. North and south magnetic poles on either side set up a strong magnetic field through the coil.

Figure to be added

Figure 13.20 — a current induced in a coil turning in a magnetic field: coil ABCD between N and S permanent magnets on a spindle; end A goes to slip ring P and end D to slip ring Q; carbon brushes X and Y press on the rings and lead to a galvanometer G; beside it, a right hand applying the rule to side AB.

Part	What it is
Coil ABCD	Many turns of insulated copper wire; turns in the field
Slip rings P and Q	Two whole copper rings on the spindle; A joined to P, D to Q
Brushes X and Y	Carbon blocks pressing on the slip rings
Armature	The coil, slip rings and spindle together

As the coil turns, the sides AB and CD cut across the magnetic field, so an emf is induced in them. The circuit is complete, so a current flows in AB and CD, and its direction is given by Fleming's right-hand rule. Turning the coil anticlockwise as in figure 13.20, AB moves upwards so the induced current flows from A to B; CD moves downwards so its current flows from C to D. The currents in AB and CD therefore flow the same way round the coil, ABCD. In the outside circuit the current flows through the galvanometer from Y to X, and its pointer deflects to the left.

Why slip rings, not split rings

A dynamo looks like a motor run backwards, and it is — but notice one difference. The motor needs a split-ring commutator to reverse the current every half turn. The AC dynamo uses whole slip rings, so each end of the coil is always connected to the same brush. Whatever the coil produces is passed straight to the outside circuit. As the next page shows, the coil's current reverses every half turn, so the outside circuit receives a current that keeps changing direction: an alternating current.

A coil turning in a field

In an AC dynamo a coil turns between magnetic poles; slip rings and brushes carry the induced current, ABCD, to the outside circuit.