Fractional horse-power motor

Demonstration

If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed.

Apparatus and Materials

• Electric motor, fractional horsepower

• String, length of
• Power supply, low-voltage, variable
• Demonstration forcemeter (5 kg / 50 N)
• Retort stand, boss, and clamp

Health & Safety and Technical Notes

Read our standard health & safety guidance

The fractional horse-power motor should operate from approximately 12 volts DC, which is conveniently obtainable from the variable low-voltage supply. The field and armature connections should both be connected, in parallel, to the voltage supply.

It is helpful to use a motor with a removable plate which can be taken off to reveal the commutator and brushes.

Procedure

1. Show the motor in operation.
2. Remove the plate on the end to show the brushes and internal movement. Allow students to look at this so that they can identify the parts. Replace the plate.

3. Attach the string to the spindle of the motor. Do this by tying it to a spoke of the pulley wheel and winding it several times round the spindle.
4. Attach the other end to the demonstration force meter which is suspended from a retort stand.
5. Increase the voltage of the supply gradually. Observe the force with which the motor pulls on the force meter. (Note that the motor should not be in this condition for long, as it is being heated with many watts of electrical power. Raise the voltage carefully to avoid overloading the power supply and the motor.)

Teaching Notes

• If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed. In most cases the magnets are actually electromagnets. (Beware: most commercial motors are induction motors rather than moving coil motors!) The armature is likely to be wound in slots in a soft iron block so that it acts as several armatures placed at an angle to each other. This is so that the motor runs more smoothly.
• An ammeter placed in the armature circuit will show how the current changes when the motor is doing a job such as hauling up a load.

This experiment was safety-tested in July 2007