Split ring commutator

A split ring commutator is a metal ring cut into two halves, fixed to the coil of a DC electric motor so that it turns with the coil. Two brushes press against it and carry the current in. Every half turn, the split ring swaps which end of the coil is connected to the positive terminal. That flips the direction of the current through the coil, and the forces on it keep turning it the same way instead of rocking it back and forth.

What is a split ring commutator?

A simple DC motor has a coil of wire between the poles of a magnet. Each end of the coil is joined to one half of the split ring. The two halves are separated by small gaps, so they do not touch. Two fixed contacts called brushes, often made of carbon, press on opposite sides of the ring and connect it to the power supply. As the coil turns, the ring turns with it and slides past the brushes.

How a split ring commutator works

AQA GCSE Physics says a current-carrying coil placed in a magnetic field tends to rotate, and that this is the basis of an electric motor. The forces come from the motor effect, and their directions follow Fleming's left hand rule. Follow one turn of the coil:

1. Current flows into the coil through one brush and half-ring, round the coil, and out through the other half-ring and brush.
2. The current goes one way along one long side of the coil and the opposite way along the other, so the field pushes one side up and the other side down. The coil starts to turn.
3. When the coil is vertical, the brushes touch the gaps in the ring, and for a moment no current flows. The coil keeps moving because of its momentum.
4. Now each brush touches the other half of the ring, so the current in the coil is reversed.
5. The side that was on the left is now on the right, but its current has flipped too, so the force on it still turns the coil the same way.
6. This repeats every half turn, so the coil keeps spinning in one direction.

Making the motor spin faster

The coil turns faster with a bigger current, a stronger magnetic field, or more turns on the coil, because each of these increases the force on the sides of the coil. Reversing the current or swapping the magnetic poles makes the motor spin the other way.

Check your understanding.

The split ring commutator in a dynamo

Turn the motor by hand and it works backwards, as a generator. A coil turning in a magnetic field has a potential difference induced across it, and that p.d. naturally reverses every half turn. AQA GCSE Physics says the generator effect is used in an alternator to generate AC and in a dynamo to generate DC, and asks you to draw and interpret graphs of the p.d. against time.

Here x stands for the time (or how far the coil has turned) and y for the induced p.d. With slip rings, the output follows the coil: positive for half a turn, then negative.

In a dynamo, the split ring commutator swaps the connections at the moment the p.d. in the coil reverses, so the output always has the same sign. It still rises and falls, but it is direct current. The peaks come at the same times in both graphs, when the sides of the coil cut across the field lines fastest.

Split ring commutator vs slip rings

Common mistakes

• Saying the commutator reverses the magnetic field. It reverses the current in the coil; the magnet stays the same.
• Saying it reverses the current every full turn. It is every half turn.
• Saying it changes the supply from AC to DC in a motor. A DC motor already runs on DC; the commutator reverses the current in the coil to keep it turning.
• Mixing up split rings and slip rings. Split ring: DC motor and dynamo. Slip rings: alternator.
• Forgetting the brushes. They are the fixed contacts that let a spinning coil stay connected to the circuit.

Practise with a short timed paper

Motors and generators come up in GCSE Physics, and electromagnetic induction in the second year of A-level Physics. The daily GCSE Physics Higher and A-level Physics: Year 2 papers are short timed papers of 8 questions, with a new one every day. You can explore magnetic fields in the Field Lines Simulator.

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