# An electric motor used as a generator

##### Demonstration

You can generate an alternating current with a fractional horsepower motor.

#### Apparatus and materials

Power supply, low-voltage, variable

Demonstration meter with dial (2.5–0–2.5 mA DC)

#### Health & Safety and Technical notes

For convenience, the motor should be mounted on a board, as shown, with 4 mm sockets allowing connections to the rotor and stator windings.

A video demonstrating an experiment of this kind is freely available from the National STEM Centre e.library.

#### Procedure

a Connect the armature (rotor) windings to the demonstration meter.

b Connect the field (stator) windings to the low-voltage supply.

c Set the supply to 2 V d.c. and switch on.

d Turn the armature by rotating the pulley wheel on the shaft by hand.

e Reverse the direction of rotation to see any difference.

f Repeat without any voltage applied to the field terminals.

#### Teaching notes

1 Only small dynamos have permanent magnets to make the magnetic field; large ones have electromagnets (whose coils are usually fed by a little of the dynamo's own output current).

2 The very large a.c. generators in power stations are called alternators. In them, the assembly of field coils rotates, driven by a turbine, and is called the rotor. The armature coils, in which the output voltage is generated, are held in a frame outside the rotor and remain stationary; this is the stator.

This arrangement is convenient for big machines because it does not need brushes and commutator or slip-rings to carry out the large output current. The spinning rotor's electromagnets are supplied with the small direct current they need from a small d.c. dynamo on the same spinning shaft as the big generator.

3 A dynamo spinning at constant speed with its field magnet kept at a constant strength produces a constant potential difference (the e.m.f.), like a well-behaved battery of cells. Even if there is no output current, a dynamo still produces an e.m.f. It is ready to drive a current. When you let it drive a current, by connecting something to its output terminals, the amount of current depends on the resistance of the thing which you connect (and the internal resistance of the dynamo coils).

This experiment was safety-checked in April 2006