A motor and a generator are the same arrangement used in opposite directions. A motor turns electrical energy into motion, and a generator turns motion into electrical energy.
This lesson is part of SPM Physics electromagnetism. It uses the force rule from determining magnetic-force direction and induction from explaining electromagnetic induction.
How do they compare?
| D.c. motor | A.c. generator | |
|---|---|---|
| Energy change | electrical to kinetic | kinetic to electrical |
| Principle | force on a current in a magnetic field | induced e.m.f. from changing flux |
| Input | current from a supply | rotation of the coil |
| Output | rotation | alternating e.m.f. |
| Contact parts | split-ring commutator | two slip rings |
| Direction rule | Fleming’s left-hand rule | Lenz’s law, or the right-hand rule your course uses |
Worked explanation: why the commutator?
In a coil in a field, the force on one side is up and on the other side is down. The coil turns until its plane is at right angles to the field.
If the current stayed in the same direction, each side would be pushed back after passing that position, and the coil would stop. The commutator reverses the coil’s connections at that moment. The force on each side therefore still turns the coil the same way.
Worked explanation: what slip rings allow
A generator coil turns through the field, and the e.m.f. changes direction every half turn. Slip rings keep each end of the coil connected to its own brush.
The external circuit therefore sees current that alternates. Replacing them with a commutator would flip the connection at each half turn and give a one-direction, pulsing output.
The mistake that loses marks
A common slip is to write “a generator uses a commutator to produce a.c.”, which swaps the parts.
The fix is to tie the part to the aim. A motor needs continuous rotation, so it uses a commutator. An a.c. generator needs a continuous connection, so it uses slip rings.
Check yourself
A generator’s coil is rotated twice as fast. State what happens to the size and frequency of the output, with a reason.
Answer
The flux changes twice as fast, so the peak e.m.f. doubles. The coil completes twice as many turns each second, so the frequency also doubles.
Both follow from the rate of change of flux and the rotation rate.
What to study next
Go on to calculating transformer relationships, then test the machines in the practice set.
Use the mistake log to track where parts are swapped. For a teacher to go through the diagrams with you, see online one-to-one Physics tuition or the one-hour trial class (from RM50).