IGCSE Physics / Electric motors and electromagnetic induction
The simple d.c. electric motor revision notes
Edexcel International GCSE Physics (4PH1), chapter 21: Electric motors and electromagnetic induction, specification point 6.12, 6.14. Free lesson.
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A simple d.c. motor uses forces on a current-carrying coil in a magnetic field to make it turn. The split-ring commutator reverses the current every half turn, keeping the coil turning in the same direction.
What parts make up a d.c. motor?
A rectangular coil of wire sits in the magnetic field between the north and south poles of a magnet. The field runs across the coil, which can spin on an axle. Two carbon brushes press lightly on the split-ring commutator and carry current from the supply into the spinning coil. Each half of the split-ring commutator is connected to one end of the coil.
How do forces make the coil turn?
Current flows along one side of the coil and back along the opposite side, so the two sides carry current in opposite directions. In the magnetic field, this produces opposite forces. By the left-hand rule, one side is pushed up and the other down. These forces act on opposite sides of the coil, creating a turning effect that makes it rotate.
How does the commutator keep the coil turning?
When the coil reaches the vertical position, the gaps in the split ring pass the brushes. The commutator reverses the current in the coil every half turn. This reverses the forces on the coil's sides, so the side that was pushed up is pushed down, and the coil continues turning in the same direction. Without the commutator, the coil would rock back and forth and stop at the vertical.
How can you change the speed or direction?
A bigger force on each side makes the motor turn faster. Reversing the current or the magnetic field changes the direction of the turning effect, so the motor turns the other way. To make it turn faster: increase the current, use a stronger magnet, or add more turns of wire to the coil. To reverse its direction: reverse the current by swapping the supply leads, or swap the magnet's poles round. More turns make the motor faster but do not change its direction.