A-level Physics (Advancing Physics)/Generators

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Fleming's right-hand rule

We have seen that a change in flux induces an electric current in a coil of wire. One way of changing the flux is to move the magnet. Alternatively, we can move the coil relative to the magnet. Generators work on this principle - a non-electrical source of energy is used to rotate something (known as the rotor), which induces an electric current in either the rotor or the stator (the stationary part of any electromagnetic machine). For a generator, the relationships between the directions of current, field and motion are given by Fleming's right-hand rule (right).

Moving Coil[edit]

AC Generator[edit]

Attaching carbon brushes to a slip-ring commutator results in an AC generator.

If a coil of wire is placed in a magnetic field and rotated, an alternating (sinusoidal) current is induced. As it rotates, sometimes it is 'cutting' through lots of flux, and so lots of current is induced. At other times, it is moving parallel to the flux, and so no flux is cut, and no current is induced. In between, some current is induced. This creates an alternating current.

Either end of the coil can be connected to wires outside of the generator in order to use the current elsewhere. This would be fine for the first few rotations, but after this, the wires would get tangled up and the generator would be useless. To avoid this, we use a commutator. In an AC generator, this is a pair of rotating conducting 'slip rings' attached to either end of the coil. Carbon brushes bring these into contact with the outside world.

DC Generator[edit]

Attaching wires to a split-ring commutator results in an DC generator.

If we replace the slip-ring commutator in an AC generator with a pair of brushes which the ends of the coil rotate inside, the generator creates direct current (DC) instead. Halfway through the rotation, the brushes come into contact with the other end of the coil, and so the AC changes direction every half a rotation. This approximates to a direct current. This direct current is not perfect since it consists of a series of positive-voltage pulses. These pulses can be smoothed out using a capacitor or a complex system of commutators.

Moving Magnet[edit]

Simple AC Generator[edit]

An alternative method of generating an alternating current is to rotate a permanent magnet in a gap between two coils. This has the advantage of not requiring a commutator (the coil is the stator), but often a coil is lighter than a magnet, and so it is more efficient to use a rotating coil.

Three-Phase Generator[edit]

If we place three pairs of coils, evenly spaced, around the rotating magnet, then three different alternating currents, with three different phases, will be generated. This is a more efficient method of generating electricity, since current is always being generated. The sum of all three currents is zero, so three different cables must be used to transport the currents. Three-phase power is often used in motors with three coils in the stator.

Questions[edit]

1. Draw diagrams of an alternating current, the 'direct current' produced by a DC generator, and this current once it has been smoothed with a capacitor.

2. What is the phase difference (in radians) between the voltages produced by a three-phase generator?

3. According to Faraday's law, what three things will increase the amplitude of the emf created by a generator?

4. If an albatross touched two power cables carrying AC in phase, what would happen?

5. What would happen if the two cables carried three-phase power?

Worked Solutions