4.5.2 The a.c. generator
- Syllabus
- 0625–2026–2027
- Topic
- 4.5.2
- Level
- —
Learning objectives
4.5.2.1Simple form of a.c. generator• Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings and brushes where needed4.5.2.2And interpret graphs of e.m.f. against• Sketch and interpret graphs of e.m.f. against time for simple a.c. generators and relate the position of the generator coil to the peaks, troughs and zeros of the e.m.f.
How a simple a.c. generator works
A generator transfers mechanical energy to electrical energy by electromagnetic induction. In a simple design, a coil rotates between magnetic poles so its sides cut magnetic field lines.
| Part | Job |
|---|---|
| magnetic poles | provide the magnetic field |
| rotating coil | its changing flux linkage produces an induced e.m.f. |
| two slip rings | each remains connected to one end of the rotating coil |
| stationary brushes | press on the slip rings and connect the rotating coil to the external circuit |
During the next half-turn, each side of the coil moves across the field in the opposite direction, so the induced e.m.f. and current reverse. Repetition produces an alternating output, with one complete a.c. cycle per complete coil revolution.
A generator may instead rotate a magnet beside a fixed coil. The field linking the fixed coil then changes; because the coil and its output wires are stationary, slip rings are not needed for that arrangement.
Slip rings preserve continuous contact; they do not reverse the external connections. A split-ring commutator is a different component used when the external output must keep one direction.
Read an a.c. generator e.m.f.–time graph
At steady rotation in a uniform field, the output alternates smoothly between equal positive and negative peaks. One complete revolution of the coil produces one complete wave cycle.
| Coil position relative to the field | Rate of cutting / flux change | Graph value |
|---|---|---|
| plane of coil parallel to the field | greatest | positive or negative peak |
| quarter-turn later: plane perpendicular to the field | momentarily zero | zero crossing |
| another quarter-turn: plane parallel again | greatest, opposite direction | opposite peak |
| another quarter-turn: plane perpendicular again | momentarily zero | next zero crossing |
In the common end-view diagram with a horizontal N-to-S field, a horizontal coil line is parallel to the field and corresponds to a peak; a vertical coil line is perpendicular and corresponds to zero. Always use the field direction rather than relying on the words horizontal and vertical alone.
| Graph feature | Meaning |
|---|---|
| peak height | maximum magnitude of induced e.m.f. |
| time between matching peaks | period of one revolution |
| cycles per second | frequency, equal to revolutions per second for this simple generator |
| sign above or below zero | opposite polarity / current direction |
Faster rotation makes the linkage change faster, so the peaks are larger and the cycles are closer together: both peak e.m.f. and frequency increase.
Zero e.m.f. does not mean zero magnetic flux. At a zero crossing the flux linkage is at a maximum or minimum but is momentarily changing at zero rate; at a peak the flux linkage passes through zero most rapidly.