Learning outcomes
- Describe a simple a.c. generator.
- Explain why rotation induces an alternating e.m.f.
- Relate coil position to waveform.
- State factors increasing output.
- Distinguish slip rings from a split-ring commutator.
17.1 Converting mechanical energy
A generator converts mechanical energy to electrical energy by electromagnetic induction. A coil is rotated in a magnetic field, changing magnetic flux linkage. An induced e.m.f. drives current when the external circuit is complete.
The mechanical input may come from a turbine driven by steam, falling water, wind, combustion or human effort. The generator does not create energy; it transfers energy while losses occur through heating, friction and sound.
17.2 Main components
A simple a.c. generator has a rotating coil between magnetic poles, an axle, two slip rings and stationary carbon brushes. Each end of the coil connects to one slip ring. Brushes maintain electrical contact while allowing rotation.
The magnetic field may be produced by permanent magnets or electromagnets. In large generators, design details differ, but changing flux linkage remains the essential principle.
17.3 Why output alternates
During the first half-turn, one side of the coil moves through the field in one direction and the induced e.m.f. has one polarity. During the next half-turn, motion reverses relative to the field, so the e.m.f. reverses. The external current is therefore alternating.
Slip rings do not reverse connections mechanically; they allow the naturally alternating coil output to reach the circuit. A split-ring commutator, used in a simple d.c. motor, exchanges contacts every half-turn for a different purpose.
17.4 Shape of the waveform
For steady rotation in a uniform field, output is approximately sinusoidal. Induced e.m.f. is zero when the rate of flux change is zero and greatest when the coil cuts field lines at the greatest rate.
Exact position descriptions depend on how the coil angle is defined, so use rate-of-cutting or rate-of-flux-change reasoning rather than memorising a potentially ambiguous diagram orientation.
17.5 Increasing maximum e.m.f.
Maximum induced e.m.f. increases if rotation is faster, the magnetic field is stronger, coil area is larger, or the number of turns is increased. These changes increase the rate of change of flux linkage.
Increasing speed also increases output frequency because more revolutions occur each second. Increasing field strength or turns changes amplitude but not frequency if rotation rate is unchanged.
17.6 Frequency and rotational speed
For a simple two-pole generator with one repeating cycle per revolution, output frequency equals revolutions per second. More complex generators can produce several cycles per revolution depending on pole arrangement.
On an oscilloscope, faster rotation makes cycles closer together horizontally and usually taller vertically because both frequency and maximum e.m.f. increase.
17.7 Generator loading
When a load draws current, the magnetic effect of induced current opposes rotation according to Lenz’s law. More mechanical force or torque is required to maintain speed. This is why turning a generator becomes harder when it supplies a larger load.
Electrical output power cannot exceed mechanical input power. Efficiency is reduced by winding resistance, eddy currents, magnetic hysteresis, friction and air resistance.
Worked examples
Changing speed
Rotation speed doubles. For a simple generator, frequency doubles and peak e.m.f. also increases because flux changes more rapidly.
More turns
The number of coil turns triples while speed and field remain unchanged. Peak induced e.m.f. approximately triples; frequency is unchanged.
Energy explanation
Connecting a lamp makes the generator harder to turn because induced current creates an opposing torque. Extra mechanical work supplies the lamp’s electrical energy.
Practical focus
Investigation
Rotate a small hand generator connected to an oscilloscope or data logger. Compare slow and fast rotation. Record changes in period and amplitude. Connect a lamp and compare the turning effort with open-circuit operation.
Examination guidance
- A generator converts mechanical energy to electrical energy.
- Slip rings are used in the simple a.c. generator.
- Faster rotation increases frequency and induced e.m.f.
- Use changing flux linkage in explanations.
Check your understanding
- Why does generator e.m.f. reverse every half-turn?
- Give four ways to increase peak output.
- Why is the generator harder to turn when loaded?
Answers
- The direction in which coil sides cut the field reverses.
- Increase speed, field strength, turns or coil area.
- Induced current produces an opposing torque, so more mechanical work is needed.
