4.2.5Magnetism and Magnetic Fields
Generators and Dynamos
5 min read 4 parts Lesson 11 of 13 Unit quiz
Section 1 of 4Principle of a Generator
A generator turns kinetic energy into electrical energy using electromagnetic induction.
Rotating coil
- A coil of wire is turned in a magnetic field, so its sides repeatedly cut through the field lines.
Induced potential difference
- As the coil cuts field lines a potential difference is induced between its ends.
Slip rings and brushes
- Metal rings fixed to the coil touch carbon brushes, giving a continuous connection to the external circuit.
Energy change
- The energy transferred is kinetic energy of rotation, which is converted to electrical energy.
Section 2 of 4Alternator and Dynamo
Alternator and Dynamo
Alternator
- Uses two slip rings, so the output potential difference reverses direction every half turn.
Alternating current
- The current flows one way for half a turn and then the opposite way, giving a sine-shaped graph.
Dynamo
- Uses a split-ring commutator that swaps the connections every half turn, keeping the output in one direction.
Direct current
- The dynamo output rises from zero to a maximum and back to zero twice per turn, but never changes sign.
| Feature | Alternator | Dynamo |
|---|---|---|
| Connection to coil | Two slip rings | Split-ring commutator |
| Output | a.c. (reverses) | d.c. (one direction) |
| Graph | Positive and negative | Always one sign |
Section 3 of 4One Rotation of the Coil
Follow an alternator coil as it turns through a complete circle.
1
When the coil is at right angles to the field its sides move along the field lines, so none are cut and the output is zero.
2
As the coil turns away from this position its sides cut field lines faster, and the output rises to a maximum when the coil is parallel to the field.
3
At the next position at right angles to the field the sides again move along the lines, the output falls to zero and half a rotation is complete.
4
In the second half the sides move in the opposite directions through the field, so the induced potential difference is reversed.
5
Turning the coil faster raises both the frequency and the peak value of the output potential difference.
Section 4 of 4Exam Checklist & Tips
4.2.5 – Key Points for the Exam
- Alternator A generator that produces alternating current using slip rings.
- Dynamo A generator that produces direct current using a split-ring commutator.
- Slip ring A continuous metal ring that connects a rotating coil to brushes.
- Commutator A split ring that reverses the connections every half turn.
- Alternating current Current that repeatedly changes direction.
Common Mistakes
- Confusing generators with motors;
- a generator takes in motion and gives out electricity, a motor does the reverse.
- Saying a dynamo produces a.c. because the coil rotates; the commutator keeps the output d.c.
- Drawing the dynamo graph below the axis, when it never becomes negative.
- Forgetting that faster rotation increases both the frequency and the size of the output.