A.C. Generator (A Level)


ac generator

An alternating current (A.C.) generator is an important application of electromagnetic induction. A.C. generator is an electromagnetic device which transforms mechanical energy into electrical energy. It consists of a rectangular coil of wire which can be rotated about an axis. The coil is located between the poles of two permanent magnets. As the coil rotates, the magnetic field through the coil changes, which induces an electromotive force (e.m.f.) between the ends of the coil.

Note: The induced current does not flow UNLESS the generator is electrically connected to an external circuit with an electrical load, such as a light bulb as shown in the figure above.

Purpose of slip rings:

The slip rings allow the transfer of alternating e.m.f. induced in the rotating coil to the external circuit. Each ring is connected to one end of the coil wire and is electrically connected to the external circuit via the conductive carbon brushes.

Note the difference between A.C. generator and D.C. motor. D.C. motor uses split-ring commutator, which reverses the current direction in the coil every half a turn and allows the coil to always turn in the clockwise direction.

ac generator emf graph

Using the figure above, we will investigate the workings of an a.c. generator. Note that the coil is being turned in a clockwise manner and the magnetic field is pointing towards the right.

Steps in the operation:

  • Coil starts in reference position $0^{\circ}$: The plane of the coil is perpendicular to the magnetic field lines. This means that the sides of the coil are moving parallel to the magnetic field lines and not “cutting” through any magnetic field lines. Hence, no e.m.f. is induced.
  • Coil gets turned to reference position $90^{\circ}$: The plane of the coil is parallel to the magnetic field lines. The sides of the coil are moving perpendicularly to the magnetic field lines and will be “cutting” through the magnetic field lines at the greatest rate. Hence, the induced e.m.f. is the maximum at this position. Using Fleming’s right hand rule, the direction of force at A is upwards (due to clockwise motion), while the magnetic field lines are pointing rightwards. This will give an induced current pointing into the screen. You can do the same analysis for B, which will be carrying an induced current pointing out of the screen.
  • Coil gets turned to reference position $180^{\circ}$ and $360^{\circ}$: Same as the analysis in reference position $0^{\circ}$.
  • Coil gets turned to reference position $270^{\circ}$: Same analysis as in reference position $90^{\circ}$ BUT the e.m.f. is in the opposite direction. This is due to the position of A and B switching places and by the Fleming’s right hand rule, the inwards current will be carried by B and outwards current will be carried by A.

The frequency of rotation is related to the period T by:

$$f = \frac{1}{T}$$

Ways to increase emf in a.c. generator:

  1. Decrease distance between magnet and coil. (To increase magnetic field strength experienced by coil)
  2. Use a stronger magnet.
  3. Increase frequency of rotation of the coil. (Double freq. = double max. e.m.f. and halving T)
  4. Increase number of turns in the coil. (Double no. of turns = double max e.m.f.)

Turning the magnets instead of the coil

ac generator (turning magnets)

For the generation of large currents, it is more practical and advantageous to keep the coil fixed and to rotate the magnetic field around the coil. In this case, the magnetic field cuts the coil to produce the induced e.m.f., instead of the coil cutting the magnetic field. Note that the slip rings and carbon brushes (incapable of carrying large currents) are absent in this design.


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