The electric motor
A motor makes use of the catapult field.
Apparatus and materials
For each student group
Copper wire, PVC-covered, 150 cm with bare ends
Magnadur magnets, 2
Split pins, 2
Bicycle valve rubber tubing
Health & Safety and Technical notes
Kits are commercially available for making model electric motors like this.
The armature is made from a wooden or plastic block with an aluminium tube through the clearance hole drilled through the wood. The base is also of wood or plastic, with holes positioned to take the rivets which hold the wires.
This experiment is linked to the experiment 'Using ammeters' and 'Moving coil meter'.
A video demonstrating a much simpler electric motor is freely available from the National STEM Centre eLibrary.
a To make the commutator, first insulate one end of the aluminium tube with Sellotape. Then cut two slices off the length of valve-rubber tubing to make two rubber bands which are slid over the end of the tube.
b Bare one end of the PVC-covered copper wire, loop it as shown in the detail of the illustration, and fix it in place with the rubber bands. Note that the bared wires are in the same plane as the coil, not at right angles to it. This is essential.
c Wind about ten turns on the armature and cut off the wire, leaving enough to finish the other side of the commutator. Bare this end and loop it.
d Slide this end against the Sellotape on the opposite side of the aluminium tube from the original loop, under the rubber bands, to retain both ends of the coil in position.
e Pass the knitting needle through the aluminium tube in the armature. Support it above the wooden base using the two split pins. Check that the knitting needle is firmly held, and that the armature can turn freely. (The split pins can be rotated so that the knitting needle jams in the eye of the split pin; this encourages rigidity.)
f Take two further lengths of insulated wire, with bare ends. These will form the brushes and leads to the supply. Bend the ends so that they will press against the commutator.
g Put the rivets in the holes in the wooden base. Wind the wires from the brushes around the rivets and connect the free ends to the power supply. (Don't switch on yet.) Make sure that the brush wires press gently against the commutator wires.
h Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. This provides the magnetic field.
i Slip the yoke into place under the wooden base.
j Switch on the power supply. Give the armature a gentle push to start it turning.
1 This goes much better if you have already built a model and then challenge students to produce their own. It can be a bit fiddly, especially for large fingers, but once one student has got it working then others soon follow.
2 The motor will run very easily on DC, but if it is spun to begin with at the synchronous speed then it will also run on AC. The commutator enables the current in the armature to reverse, so that the current in the right-hand side of the armature, for example, is always in the same direction, forcing that side either up or down and so continuing the rotary motion.
3 The looped ends of the wire forming the commutator are not strictly necessary - a straight end would suffice - but the loop enables contact with the brushes to be maintained over a greater part of a revolution, with a consequent increase in power.
4 Contact with the brushes when the coil is in the horizontal plane is particularly important, as this is when the force on the coil is maximum.
5 When the motor has been running for a long time, the brushes will become dirty and have a high resistance from the sparking. The commutator should be stripped down, and the wires emery-papered and cleaned before being reassembled.
6 For those with poor manipulative ability, there is an easier way to make the commutator. It is not as good as the first version, as the frictional torque is greater with a commutator of larger diameter. In place of the Sellotape, rubber tubing is slid over the aluminium tube to provide insulation. Larger rubber bands are needed to secure the bared wire loops, and the assembly proceeds as before.
7 Success with this motor is really impressive and students may want to construct one at home with odds and ends. The only problem may be acquiring the magnets. Many science centres now sell the magnets on their own, or even complete motor kits.
This experiment was safety-checked in July 2007