V=-N(dΦ/dt)
Electricity and Magnetism

The motor as a dynamo

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

This simple experiment shows the reversibility of motor and dynamo effects.

Apparatus and Materials

For each student group

  • Electric motor, small e.g. one constructed by student group in earlier activity
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA, 10 ohm resistance (see note below)
  • Crocodile clips, 2
  • Leads, 4 mm, 2
  • Copper wire, insulated with bare ends, 200 cm

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Choose electric motors which have an axle which can be readily turned by hand. They must also have two contacts where the crocodile clips can be attached.

Read our standard health & safety guidance


See the experiment:

The electric motor


Procedure

  1. Connect the electric motor to the galvanometer.
  2. Spin the motor; observe the galvanometer. You can spin the motor with a finger and thumb on the axle; alternatively, wrap a length of cotton once round the axle and pull from either end, as illustrated.
  3. Investigate the factors which affect the galvanometer reading. What happens if you reverse the direction of spin?
  4. If you are using a model electric motor, you can convert it so that it will generate alternating current (AC) as follows.
  5. Remove the rubber bands and undo the commutator wiring.
  6. Insulate both ends of the aluminium tube by wrapping adhesive tape around them.
  7. Bring out the ends of the leads at opposite ends of the armature.
  8. Bare the leads for two or three centimetres, and wind the bare ends tightly around the aluminium tube.
  9. Make a brush at each end and connect up to the meter.
  10. Investigate the factors which affect the galvanometer reading. What happens if you reverse the direction of spin now?

Teaching Notes

  • Turning a motor by hand makes the motor into a dynamo. The potential difference can be measured on a sensitive galvanometer. Frequently it is enough just to use a sensitive ammeter, as long as confusion does not arise. The dynamo effect produces an e.m.f. (a potential difference}, and not a current.
  • Students should note that turning the dynamo faster produces a greater deflection of the meter. Reversing the spin of the dynamo generates a deflection in the opposite direction.
  • The wave form of this DC dynamo can be observed on a C.R.O. It will not be a flat graph but rather follow the changes in magnitude of the e.m.f. throughout the cycle. However, it will be uni-directional. If either brush fails to make a connection as it rotates, there will be a considerable AC potential difference between the input terminals of the C.R.O. This will cause a misleading trace of AC mains on the tube. Connecting a 10 kΩ resistor across the input terminals of the C.R.O. will prevent this.
  • If you use a home-made model electric motor which has run for an appreciable time, the brushes and commutator will be dirty and have a high resistance. Strip down and scrape the brushes and commutator with emery-paper to clean them. Avoid finger grease. Taking these precautions may increase the galvanometer deflection several times.
  • AC version: As an alternative to the slip rings, it is possible to make a temporary dynamo that will work for a few turns. Bring out a pair of leads of thin wire from the coil, and let the leads twist up as the coil is turned.
  • The AC wave form produced by the dynamo is best seen by connecting the dynamo to a C.R.O. The trace should resemble the traditional sine wave.

This experiment was safety-tested in July 2007

  • A video showing a similar electromagnetic induction practical:

V=-N(dΦ/dt)

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