S4.13.4 Chapter 4

Electric motors

A coil in a magnetic field is pushed at both sides. Why does it keep turning round and round, and not just flip over and stop?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A split ring reverses the current every half turn, so the force always turns it the same way.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The two forces on the coil are equal and opposite, so it spins freely with no force on it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The coil swings back and forth like a pendulum, and the magnet keeps it moving.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a split ring keeps a motor turning one way
Name ways to make a motor turn faster

A motor is a coil of wire that can turn between the poles of a magnet. A current flows round the coil. We look at it from the end of its axle. The long sides of the coil are wires at right angles to the field, so each feels a force.

The current goes one way along one side and the other way along the opposite side. By the left hand rule, the forces are opposite. One side is pushed up and the other is pushed down. Two equal and opposite forces in different places turn the coil.

The coil has turned 30 degrees clockwise past lying along the field. One side is pushed down, the other up, and the coil goes on turning clockwise.

The turning effect depends on the angle of the coil. When the coil lies along the field, each force is at right angles to the line from the axle. The turning effect is then the largest. As the coil turns, the effect gets smaller.

Draw it first

The turning effect as the coil turns

1 The coil lies along the field. The forces are at right angles to the coil, as far from the axle as they can be. The turning effect is largest.
2 The coil has turned 60 degrees clockwise. The forces are the same size, but they pull more along the coil, so the turning effect is smaller.
3 The coil has turned 90 degrees clockwise and stands at right angles to the field. The forces now act along the line through the axle, so they cannot turn it. The turning effect is zero.

Without anything else, the coil would swing past this upright place and then be pushed back, like a pendulum. It would rock to and fro and settle upright. To keep it turning one way, the current in the coil must reverse as it passes the upright place.

A split ring does this. It is a ring cut into two halves with a gap between them, one half joined to each end of the coil. Two brushes are fixed on either side and press on the ring. They carry the current from the cell into the ring. The ring turns with the coil.

The coil lies along the field. Each brush touches a half of the ring, so a current goes round the coil. The turning effect is largest.

When the coil stands across the field, the brushes sit on the gaps. For a moment no current flows, and there is no turning effect. The coil’s momentum carries it past this place. That is why a motor can start badly if it stops just there.

The coil stands across the field. Both brushes touch the gaps, so no current flows and the coil turns on by its momentum.

Once past the upright place, each half of the ring has moved to the other brush. The current now goes round the coil the other way. The side that was on the left is now on the right. It carries the current the other way, so it is pushed down. The coil goes on turning clockwise.

Half a turn later. The hatched half of the ring is now on the left, so each brush is on the other half. The right side still carries the current into the page and is pushed down, so the coil keeps turning clockwise.

The graph shows the turning effect against the angle of the coil from the field, in a model with a steady field. With the split ring it is always on the same side of zero. Without it, it changes sign after the upright place and pushes the coil back.

Curve A, solid, is a motor with a split ring. Curve B, dashed, is the same coil with no split ring. The ring turns the dips below zero into humps. The turning effect still falls to zero at 90 and 270 degrees. From 0 to 90 degrees and from 270 to 360 degrees the two curves are the same line.

A real motor has many coils at different angles and a ring with many segments. Then there is always some coil with a large turning effect, and the motor runs smoothly. The field is made radial, with curved poles, and a soft iron core strengthens it.

Each turn of a coil gives its own force. A coil of N turns has N wires on each side, each with the same current. So the force on one side is N × B × I × L. More turns means more force.

Worked example

A coil has 50 turns. Each side is 0.08 m long in a field of 0.2 T, and the current is 1.5 A. What is the force on one side?

Use N × B × I × L, with N = 50, B = 0.2 T, I = 1.5 A and L = 0.08 m.
50 × 0.2 × 1.5 × 0.08 = 1.2, so the force is 1.2 N.

Answer: 1.2 N

Worked example

The force on one side of a coil must be 2.4 N. Each side is 0.1 m long in a field of 0.4 T, and the current is 3 A. How many turns are needed?

Rearrange: N = F ÷ (B × I × L).
B × I × L = 0.4 × 3 × 0.1 = 0.12.
N = 2.4 ÷ 0.12 = 20 turns.

Answer: 20 turns

So what makes a motor turn faster? Anything that makes the force on the coil bigger. Use a stronger magnet, so B is bigger. Use more turns of wire. Use a bigger current, for example from more cells. The motor speeds up until friction and the load balance the turning effect.

To reverse a motor, reverse the direction of the force. Swap the connections to the cell, so the current reverses. Or turn the magnet round, so the field reverses. If you reverse both, the motor turns the same way as before.

Try it

Test a toy motor

You need: a small motor from an old toy, one or two torch cells, two short pieces of wire, tape, 1 m of thread and a paper clip, a clock that shows seconds.

Be safe: Keep hair, fingers and loose clothes away from the turning shaft and the thread. Use cells only, never mains. If the thread tangles or the motor stalls, take the wire off at once: a stalled motor gets hot.

Tape the wires to the two terminals of the motor. Tape one end of the thread to the motor’s shaft, and tie the paper clip to the other end.
Hold the motor up so the clip hangs. Touch the wires to one cell. The thread winds up on the shaft and lifts the clip. Time how long it takes to wind 1 m.
Swap the wires on the cell. The shaft turns the other way and lets the thread out.
Join two cells end to end. Wind 1 m again, and time it.

What you will see: Swapping the wires reverses the turn. Two cells give a bigger current, so the motor turns faster and winds up the thread in less time. The numbers in the table are one learner’s results. Yours will differ, but the pattern should be the same.

If you cannot try it, this is what happens.

A wrong idea many people have

The split ring changes the magnetic field.

Why it feels right: The ring is part of the magnet side of the picture, and it seems to switch something over each half turn.

The test that breaks it: The magnet is fixed and does not move. The ring is on the axle and carries the current from the brushes to the coil. Swap the cell connections and the coil reverses its turn. That shows it is the current that is changed.

What is true: The split ring reverses the current in the coil, not the field. The magnet stays the same.

A wrong idea many people have

The two forces push the whole coil sideways.

Why it feels right: A force pushes a thing in its own direction, so two forces seem to push the coil somewhere.

The test that breaks it: The forces are equal and opposite, so they cancel as pushes. The coil does not move sideways. They act on opposite sides of the axle, so together they turn the coil.

What is true: Equal and opposite forces at different places give a turning effect, and no overall push.

A wrong idea many people have

Without a split ring, the coil would still spin round, just slower.

Why it feels right: The forces still act on the coil, so it seems it would still go round.

The test that breaks it: Past the upright place, the same current would push the sides back the way they came. The coil would swing like a pendulum and stop upright.

What is true: With a steady current in one direction, the coil would rock and stop. The reversal of the current is what keeps it turning.

Key words

split ring: a ring in two halves that reverses the current in the coil every half turn. Also called a commutator.
brush: a fixed contact that presses on the ring and carries current to it.
turning effect: how strongly a force or forces try to turn something about an axle.

Back to your guess

The answer is A: A split ring reverses the current every half turn, so the force always turns it the same way.

The forces on the two sides are opposite, and that makes the coil turn. Each time the coil stands across the field, the turning effect falls to zero. The split ring reverses the current just there, so the next half turn is pushed the same way as the last.

Keep it

Copy this into your exercise book.

Opposite forces on the two sides turn the coil. The split ring reverses the current every half turn. Faster: stronger magnet, more turns, bigger current. Reverse the current or the magnet to reverse the turn.

Draw: A coil end on between N and S poles, with arrows on its sides and a split ring below.