S4.13.6 Chapter 6
Generators and dynamos
A bicycle lamp lights as you pedal, with no battery. A power station lights a whole city. How do they make electricity?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Explain how a bicycle dynamo or a power station generator works
- Compare the output of an alternator with that of a dynamo
A generator is a machine that changes movement energy into electrical energy. It has the same parts as a motor: a coil and a magnet. But now something turns the coil, and a voltage is induced. A generator is a motor run backwards.
Look at a coil turned between the poles of a magnet. We look at it from the end of its axle, as in the chapter on motors. Each long side of the coil cuts across the field lines as it moves. A voltage is induced in each side.
The induced voltage depends on how fast the sides cut the field lines. It is largest when the coil lies along the field, because then the sides move straight across the field lines. It is zero when the coil stands across the field, because then the sides move along the lines and cut none.
Draw it first
One half turn of a generator coil
1 The coil lies along the field. The sides cut across the field lines, so the current is largest. Side A, on the right, has the current into the page.
2 A quarter turn later the coil stands across the field. The sides move along the field lines and cut none, so the current is zero.
3 Half a turn from the start, the two sides have swapped places. Side A is on the left now, and its current is out of the page. In side A the current has reversed.
The current in the coil reverses twice in every turn. To take it out to a lamp, we need contacts that do not tear the wires. There are two ways to do it, and they give two different kinds of output.
With slip rings, each end of the coil is joined to its own full ring on the axle. A brush presses on each ring all the time. The lamp is joined to the brushes. The current that comes out reverses every half turn, so the output is alternating current, AC. A generator like this is an alternator.
With a split ring, called a commutator, the brushes swap ends every half turn, as in the motor. A generator like this gives a current that always goes the same way. It is direct current, DC, but it comes in pulses. A generator like this is a dynamo.
The graph shows both outputs for a model coil with a peak of 6 V. The voltage is given against the angle of the coil from the field. The alternator, A, goes positive and negative. The dynamo, D, is always positive, in humps, and falls to zero twice a turn.
One full turn of this coil is one cycle of the alternating current. The frequency is the number of cycles in one second, in hertz, Hz. So a coil that turns 25 times a second gives 25 Hz. The period is the time for one cycle: period = 1 ÷ frequency.
Worked example
A simple coil turns 25 times a second. What is the frequency of the AC it makes, and the time for one cycle?
- One turn is one cycle, so the frequency is 25 Hz.
- The period is 1 ÷ frequency = 1 ÷ 25 = 0.04.
- So one cycle takes 0.04 s.
Answer: 25 Hz and 0.04 s
Worked example
A generator makes AC at 25 Hz. How many cycles does it make in 8 seconds?
- The frequency is 25 cycles every second, so 8 seconds hold 25 × 8 cycles.
- 25 × 8 = 200.
- So in 8 seconds the generator makes 200 cycles.
Answer: 200 cycles
A bicycle lamp “dynamo” does it the other way round. A small magnet spins inside a fixed coil, turned by the wheel. The changing field induces a voltage in the coil. There is no ring to wear out. By our rule, it is really a small alternator, because its output is AC.
A power station generator works in the same way. A turbine, turned by steam, water or wind, spins a big electromagnet inside fixed coils. The fixed coils carry the large current, so it needs no moving contact. Only the small current to the electromagnet goes through slip rings.
A generator does not make energy. The energy comes from the fuel, the water or the wind that turns it. The generator changes some of that energy into electrical energy. Power station generators make AC at 50 Hz in most of the world and at 60 Hz in some places.
To make a generator give more voltage, do what you do for induction. Turn it faster. Use a stronger magnet. Use more turns of wire. Turning faster also raises the frequency, because there are more cycles each second.
Try it
Make a motor into a generator
You need: a small motor from an old toy, a small LED from an old toy or torch, two short pieces of wire, a piece of string.
Be safe: Hold the motor firmly and keep fingers, hair and clothes away from the string and the shaft. Use only a motor and an LED. Never connect anything made this way to mains electricity.
- Join the two wires from the motor to the two legs of the LED.
- Wind the string round the motor’s shaft 15 to 20 times. Hold the motor firmly.
- Pull the string sharply, so the shaft spins fast. Watch the LED.
- If the LED stays dark, pull the string the other way, or swap its two wires: an LED lights only one way.
- Try a slow pull, and then a fast one.
What you will see: The motor now works as a generator, and the LED may flash while the shaft spins fast. A faster spin gives a brighter flash, and a slow one gives little or none. A toy motor has a split ring, so as a generator it is a small dynamo and gives DC. That is why the LED lights for one way of turning only. The motor turned by hand makes the voltage, with no cell at all.
A wrong idea many people have
A generator makes electrical energy from nothing.
Why it feels right: The lamp lights with no battery, so it seems the energy appears out of nowhere.
The test that breaks it: Pedal a bicycle with the lamp on, then switch it off. It becomes easier to pedal. With the lamp on, you do extra work against the push of the induced current. That work is the energy the lamp gets.
What is true: A generator changes energy from one form to another. The energy of movement becomes electrical energy, with some lost as heat.
A wrong idea many people have
Alternating current flows both ways, so the two ways cancel and it does no work.
Why it feels right: The current goes forward and then back, so it seems it gets nowhere.
The test that breaks it: Every lamp in a home runs on AC from the mains, and it glows. A wire heats up whichever way the current goes through it. Energy is transferred in every half cycle.
What is true: The current reverses, but energy still flows to the lamp in each half cycle. A lamp, a heater or a motor can work on AC.
A wrong idea many people have
A bicycle lamp has a battery inside that the dynamo charges.
Why it feels right: Most things that light up have a battery, and the lamp stays on for a moment when you slow down.
The test that breaks it: Lift the back wheel and spin it by hand. The lamp lights while the wheel spins and goes dark as it stops. There is nothing to store the energy.
What is true: A simple bicycle lamp is lit straight from the generator. It glows only while the wheel turns.
Key words
- generator
- a machine that changes movement energy into electrical energy by turning a coil and magnet relative to each other.
- alternator
- a generator with slip rings. Its output is alternating current.
- dynamo
- a generator with a split ring. Its output is direct current in pulses.
- slip rings
- two full rings on the axle, each with a brush, that carry the current out without swapping it.
- frequency
- the number of cycles each second, in hertz, Hz. The period is 1 ÷ frequency.
Back to your guess
The answer is B: They turn a coil in a magnetic field, or a magnet in a coil, and induce a voltage.
A generator is a motor run backwards. Something turns a coil in a magnetic field, or a magnet inside a coil, and the changing field induces a voltage. The generator changes the energy of movement into electrical energy. It does not make energy from nothing.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What does a generator do?
See the answer
It changes movement energy into electrical energy.
Something turns the coil in the field, or the magnet in the coil, and a voltage is induced. The movement energy becomes electrical energy.
- If you chose B: No machine makes energy from nothing. A generator changes the energy of movement into electrical energy.
- If you chose C: That is what a cell or battery does. A generator makes a current only while it is being turned.
- If you chose D: That is a motor. A generator works the other way round.
What is the output of a dynamo, which has a split ring?
See the answer
Direct current, always the same way, in pulses.
The split ring swaps the brushes every half turn, so the current outside the coil always goes the same way. It is DC, but it comes in pulses.
- If you chose A: That is the output of an alternator, which has slip rings. A split ring makes the current go the same way each time.
- If you chose B: The ring is split on purpose, so that the brushes swap every half turn. A current does flow.
- If you chose D: One coil gives pulses, which fall to zero twice every turn. Many coils at different angles smooth them out.
What is the output of an alternator, which has slip rings?
See the answer
Alternating current, which reverses every half turn.
The current in the coil reverses every half turn. With slip rings, the ends of the coil stay joined to the same brushes, so the current outside reverses too.
- If you chose A: That is the output of a dynamo. With slip rings, each end of the coil stays on its own brush, so the current reverses every half turn.
- If you chose B: That is the output of a dynamo with a split ring. Slip rings give a current that goes both ways.
- If you chose C: Unbroken contact is what lets the current out. The coil makes a voltage as it turns, so a current flows.
Why does a power station generator spin the electromagnet and keep the large coils fixed?
See the answer
So that a very large current does not have to pass through slip rings.
The fixed coils carry the large output current directly. Only the small current to the electromagnet goes through slip rings.
- If you chose A: Slip rings give AC, not DC. The reason for fixed coils is the size of the output current.
- If you chose C: The way it is arranged does not make energy. The energy comes from the turbine.
A simple generator is turned faster. What happens to its output?
See the answer
The voltage is bigger and the frequency is higher.
A faster turn changes the field more quickly, so the voltage is bigger. There are also more turns each second, so more cycles each second.
- If you chose A: A faster change of the field induces a bigger voltage, not a smaller one.
- If you chose B: The voltage rises too, because the sides cut the field lines faster.
- If you chose D: The faster the coil turns, the bigger the voltage and the higher the frequency.
When is the current in a turning generator coil zero?
See the answer
When the coil stands across the field.
With the coil across the field, its sides move along the field lines and cut none. No voltage is induced for that moment.
- If you chose A: There the sides cut straight across the field lines, so the current is largest.
- If you chose B: Slow turning gives a small voltage, but not zero. It is zero only at certain angles, however fast the coil turns.
- If you chose D: It falls to zero twice every turn, when the sides move along the field lines and cut none.
From an earlier chapter
A magnet is held still inside a coil. What current is induced in the coil?
See the answer
None, because the field is not changing.
A voltage is induced only while the field through the coil is changing. A still magnet gives none.
A generator coil turns 40 times a second, one cycle for each turn. What is the time for one cycle, in seconds?
See the answer
0.025 s
The frequency is 40 Hz. The period is 1 ÷ 40 = 0.025 s.
A generator makes AC at 40 Hz. How many cycles does it make in 12 seconds?
See the answer
480 cycles
There are 40 cycles each second, so in 12 seconds there are 40 × 12 = 480 cycles.
The AC from a generator takes 0.01 s for one cycle. What is its frequency, in hertz?
See the answer
100 Hz
The frequency is 1 ÷ period = 1 ÷ 0.01 = 100 Hz.
Keep it
Copy this into your exercise book.
A generator turns a coil in a field and induces a voltage. Slip rings: AC (alternator). Split ring: DC pulses (dynamo). One turn is one cycle. Period = 1 ÷ frequency. Faster gives more voltage.
Draw: A coil between N and S poles with two slip rings, and a wave beside it.