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The electric motor is one of the most useful machines ever invented. It turns electrical energy into rotational movement, and it is everywhere — spinning the drum of a washing machine, driving the wheels of an electric car, turning the fan in a laptop, and powering the pump in a fish tank. Every one of these motors works on the motor effect you met in the last lesson: a current-carrying coil sitting in a magnetic field is pushed by forces, and a clever piece of engineering turns that push into continuous rotation. This lesson, part of Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A, explains how a simple direct-current (d.c.) motor works, what the split-ring commutator does, and how the speed and direction of the motor can be changed.
By the end of this lesson you should be able to describe the structure of a simple d.c. motor, explain how the forces on the two sides of the coil produce a turning effect, explain the job of the split-ring commutator, and state how to change the speed and direction of rotation.
A simple direct-current (d.c.) motor is built from a small number of parts:
The diagram below shows the layout.
When current flows around the coil, each of its two long sides is a current-carrying wire sitting in the magnetic field — so each side experiences a force by the motor effect. The trick is that the current flows in opposite directions along the two sides of the coil (it goes up one side and down the other). Because the current directions are opposite, Fleming's left-hand rule gives forces in opposite directions on the two sides: one side is pushed up while the other is pushed down.
Two equal forces pushing in opposite directions on either side of the axle create a turning effect (a moment) that rotates the coil. As the coil spins, it carries the axle and whatever the motor is driving round with it.
This is exactly the moment idea from forces: each force acts at a distance from the axle (the pivot), and because the two forces act on opposite sides of the axle in opposite directions, their turning effects add together rather than cancel — both work to spin the coil the same way round. The wider the coil, the larger the distance from the axle, and so (for the same force) the larger the turning effect — which is one more reason a real motor uses a coil rather than a single straight wire.
Exam Tip: The coil turns because the two sides carry current in opposite directions, so by the motor effect the forces on them are in opposite directions — one up, one down — giving a turning effect. Apply Fleming's left-hand rule separately to each side.
There is a problem. After the coil has turned half a turn, the side that was being pushed up is now on the other side, where the force would push it back down — so without some clever fix, the coil would simply rock back and forth and never keep spinning. The job of the split-ring commutator is to solve this.
The commutator is a ring split into two halves, fixed to the axle so it turns with the coil, with each half connected to one end of the coil. The brushes stay still and press against it. Every half-turn, the gaps in the split ring pass the brushes, so each half of the ring swaps over to the other brush. This reverses the direction of the current in the coil every half turn.
Because the current reverses just as the coil passes the vertical, the side of the coil nearest the north pole is always pushed in the same direction (say, always up), so the coil keeps being pushed the same way round and rotates continuously instead of rocking back and forth.
graph LR
A[Coil turns half a turn] --> B[Split-ring gaps reach the brushes]
B --> C[Current direction in coil reverses]
C --> D[Forces keep coil turning the same way]
D --> A
Exam Tip: The split-ring commutator reverses the current in the coil every half turn, so the force on each side always pushes the coil the same way round, keeping it rotating continuously. Without it, the coil would only rock back and forth.
The d.c. motor is easy to control, which is part of why it is so useful.
To make the motor turn faster (more turning effect):
Each of these increases the force on the sides of the coil, and so the turning effect, making the coil spin faster.
To reverse the direction of rotation:
Either change reverses the forces on the coil (by Fleming's left-hand rule) and so reverses the direction of spin. Note that reversing both at once would leave the direction unchanged.
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