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Electric Motors (for 14 to 16 year-olds)

The roles of copper

Copper is an excellent electrical conductor. This is why it is used in so many electrical applications - including electric motors.

We use electric motors in the home, the garden and at work. Here are some places where electric motors are used:

• An electric lift: One electric motor moves the lift up and down. Another operates the doors.
• A car: Cars have several electric motors. The starter motor turns the petrol engine to get it going. Other motors work the windscreen wipers. Some cars have electric motors to operate the windows and even the wing mirrors.
• An electric train: An electric train has a powerful motor to drive it.

In this section, we'll look at how electric motors work and how they can be made efficient.

Looking Inside

You can take a motor apart to see how it is made. Inside you will find the following components:

• Coil: The coil is made of copper wire - because it is such an excellent conductor. It is wound onto an armature. The coil becomes an electromagnet when a current flows through it.

• Armature: The armature supports the coil and can help make the electromagnet stronger. This makes the motor more efficient.

• Permanent magnets: There are two permanent magnets. They produce a steady magnetic field so that the coil will turn when a current flows in it. Some motors have electromagnets instead of permanent magnets. These are made from more coils of copper wire.

• Commutator: Each end of the coil is connected to one of the two halves of the commutator. The commutator swaps the contacts over every half turn.

• Brushes: The brushes press on the commutator. They keep contact with the commutator even though it is spinning round. The current flows in and out of the motor through the brushes.

• Steel former: The former made of magnetic material links the two permanent magnets and, in effect, makes them into a single horseshoe shaped magnet. Commercial motors often use a horseshoe magnet.

How does it work?

The motor is connected to a battery. When the switch is closed, the current starts to flow and the coil becomes an electromagnet. In this case the current is flowing anti-clockwise in the top of the coil. This makes the top a north pole. This north pole is attracted to the south pole on the left. So the top of the coil turns towards the left. Notice that the bottom of the coil is a south pole and is attracted to the magnet on the right.

Once the coil gets to the upright position, there is no turning force on it because the electromagnet of the coil is lined up with the permanent magnets. If the current in the coil were constant, the coil would stop in this position. However, to keep it spinning, the commutator breaks contact in this position. So the current stops for an instant. The momentum of the coil keeps it going and the contacts are reconnected. However, they are now the other way around. So, the side of the coil that used to be a south pole is now a north pole.

The commutator will keep swapping the contacts every half turn (when the coil is in the upright position). In this way, the motor keeps spinning.

The Motor Effect

The electric motor effect is what makes a motor spin. We can see it work on a single piece of copper wire.

Electric catapult

Look at the picture. It shows a loose piece of copper wire on some rails. The loose piece of wire is between the poles of a magnet. The rails are attached to a power supply. What will happen if we switch on the voltage?

The wire is catapulted to the right. The magnetic field made it move but only when there was an electric current in the wire.

Which way are they facing?

The magnetic field points from the north pole of the magnet to its south pole. Notice that the field is at right angles to the current. This arrangement produces the biggest force and makes the wire move out.

A current in a wire at right angles to a magnetic field produces a force on the wire.

Which way does it move?

The wire moves at right angles to both the magnetic field and the current. We can remember which way it moves using Fleming's Left Hand Motor Rule. Arrange your left hand like the one in the digram.

The three digits represent the three quantities as shown.

• First finger = Field
• SeCond finger = Current
• ThuMb = Motion

Making it spin

We can understand a motor in the same way. As the current flows round the coil:

• one side of the coil feels an upward push
• the other side feels a downward push.

Together, these two forces make the coil turn on its axis.

When to swap the current

When the coil is in the upright position, there is no turning force trying to push it round. The two forces are trying to pull the two sides of the coil outwards. It is at this point that the commutator swaps over the contacts.

If the coil is already spinning, its momentum will carry it through this upright position. When the contacts are reconnected, the commutator has reversed the current. So the side of the coil that was being pulled up before is now being pulled down. And vice versa.

Therefore the coil keeps spinning in the same direction.

Making the Most of Energy

Electric motors get hot when in use. This is a waste of energy. We want motors to make things move; we don't use them as heaters!

Energy transfers

We supply electrical energy to an electric motor. An efficient motor transfers most of this energy as kinetic energy (useful work). Only a small fraction is wasted as it heats up the surroundings. We can show this in a Sankey diagram. The size of the arrows represents the amount of each type of energy.

Energy is lost as the electric current flows through the motor's coils. The wire coils have electrical resistance; the greater the resistance, the harder it is for the current to flow and the more energy is wasted.

Copper is a good metal to use for a motor's coils because:

• it has less resistance than almost any other metal
• it is easily made into wires
• it is not too expensive
• it can survive to a high temperature
• it can be easily recycled when the motor is replaced.