The electric motor

What is inside a fan?

Open the case of a small toy motor. You find a coil of wire, a magnet, and two small brushes that touch a split metal ring. A fan, a mixer, a washing machine and an electric train all have an electric motor. A motor changes electrical energy into mechanical energy.

The idea

A rectangular coil is placed between the poles of a magnet. A current flows in the coil.

• On one long side of the coil, the current goes into the page, and the force on it is down.
• On the opposite long side, the current goes out of the page, and the force is up.

The two forces are equal and opposite, but they act on two different sides, so they make the coil turn. A pair of forces that turns a body is a couple.

The parts of a motor

• The coil (armature), a rectangular coil of insulated copper wire, on an axle, so it can turn. In a real motor, the coil is wound on a soft iron core, and it has many turns.
• The magnet, which makes the magnetic field. A strong permanent magnet, or an electromagnet.
• The split ring (commutator), a metal ring cut into two halves. The two ends of the coil are joined to the two halves. It turns with the coil.
• The brushes, two fixed strips of carbon, which touch the two halves of the split ring and are joined to the battery. They give the current to the coil.

How does it work?

1. The current goes into the coil from the battery, through brush 1 and the first half ring. The two long sides feel opposite forces, and the coil turns.
2. After a half turn, the coil is upright, and each half of the split ring moves to touch the other brush. The current in the coil is now reversed.
3. The side that was on the left is now on the right, and the current in it is reversed too. So the force on each side is still in the same turning direction.
4. The coil keeps turning in the same direction.

You should find that the forces always turn the coil the same way, and that a reversed current turns it backwards. More turns or a larger current give a larger turning effect.

The commutator is the key. Without it, the coil would turn only half a turn, and then it would swing back, because the forces would turn it the other way.

What makes a motor stronger?

The turning effect is larger when

• the current is larger,
• the magnetic field is stronger,
• the number of turns in the coil is larger,
• the area of the coil is larger.

For a coil of $N$ turns, each of area $A$, the largest turning effect is $\tau = N\,B\,I\,A$, in newton metres (N m).

The turning effect (the torque) also depends on the position of the coil. It is the largest when the plane of the coil is parallel to the field. It falls to zero when the plane of the coil is at right angles to the field. In a real motor, many coils at different angles are used, so that the turning effect stays steady.

Practise with new numbers

Try it in Python

The program finds the turning effect of a coil of $100$ turns for different angles between the plane of the coil and the field.

Remember these

• A motor changes electrical energy into mechanical energy.
• The two long sides of the coil feel opposite forces, and the coil turns.
• The split ring (commutator) reverses the current every half turn, so the coil keeps turning in one direction.
• A larger current, a stronger field, more turns and a larger area give a stronger motor.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the job of the commutator?

2. Name two ways to make a motor turn with more force.

3. A food mixer's motor gets warm after a while. Which energy changes take place in it?

4. A coil of $50$ turns, each of area $0.003\ \text{m}^2$, carries $2\ \text{A}$ in a field of $0.2\ \text{T}$. What is its largest turning effect?