The electric generator and alternating current

How does a power station make electricity?

In a power station, a huge coil is turned in a magnetic field. The turning is done by a turbine, which is driven by falling water (a hydroelectric station), by steam made from burning coal or from nuclear fuel, or by the wind. The coil makes the electricity that goes to our houses. This is the electric generator.

The AC generator

A rectangular coil is turned between the poles of a magnet.

• As the coil turns, the magnetic field through it changes, and a current is induced in it.
• The two ends of the coil are joined to two complete rings, the slip rings, which turn with the coil.
• Two fixed brushes touch the slip rings, and they take the current to the outside circuit.

During the first half of a turn, the induced current goes one way in the coil. In the next half turn, the sides of the coil change places, and the current goes the other way. So the direction of the current changes after every half turn. This is an alternating current (AC).

Alternating current

The graph of an alternating voltage against time is a wave, which goes up and down about zero.

• One full turn of the coil is one cycle. The number of cycles in one second is the frequency, in hertz (Hz).
• In India, the mains supply has a frequency of $50\ \text{Hz}$. Some countries use $60\ \text{Hz}$.
• At $50\ \text{Hz}$, the time of one cycle is $\dfrac{1}{50\ \text{Hz}} = 0.02\ \text{s}$. The direction of the current changes $2 \times 50 = 100$ times in each second.

Direct current

A direct current (DC), for example the current from a battery, always goes in the same direction.

A DC generator is made by replacing the two slip rings with a split ring (a commutator), as in the motor. The split ring reverses the connections to the brushes every half turn, so the current in the outside circuit has one direction. It is a direct current, but it is not steady. It pulses from zero to a maximum and back, as in the right-hand graph.

You should find that doubling the speed halves the time of one cycle and doubles the largest voltage, and that the split ring turns the wave into humps.

Why do we use AC for mains?

• An alternating current is easy to change to a higher or a lower voltage with a transformer. At a high voltage, a current of a given power is small, so less energy is lost as heat in the long cables from a power station to a town.
• An AC generator is simple and strong, with no commutator.

A worked example

A coil of a generator turns $2400$ times in a minute. What are the frequency, the time of one turn and the number of reversals in a second?

Step 1. The frequency is the number of turns in a second: $f = \dfrac{2400}{60\ \text{s}} = 40\ \text{Hz}$.

Step 2. The time of one turn is $T = \dfrac{1}{f} = \dfrac{1}{40\ \text{Hz}} = 0.025\ \text{s}$.

Step 3. The current reverses twice in each turn, so $2 \times 40 = 80$ times in a second.

Practise with new numbers

Try it in Python

The program finds the voltage of a generator in $2\ \text{ms}$ steps over one cycle.

Remember these

• A generator changes mechanical energy into electrical energy. It uses electromagnetic induction.
• An AC generator has slip rings, and the current changes direction after every half turn. The mains has a frequency of $50\ \text{Hz}$.
• A DC generator has a split ring, and the current has one direction but pulses.
• A motor and a generator are opposites: a motor changes electrical energy into mechanical energy, and a generator does the reverse.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many times does the direction of a $50\ \text{Hz}$ current change in one second?

2. What is the difference between slip rings and a split ring?

3. Name two sources of energy that can turn the turbine of a power station.

4. A generator's coil turns $1200$ times a minute. What is the frequency, and how long is one cycle?