Electric Motor Effect

The electric motor is at the heart of many devices, electrical and electronic. It turns electrical energy into mechanical energy. You may have seen this experiment:

• The carbon rod is NOT magnetic.
• When no current flows, the rod is stationary
• When we turn on the current, the rod experiences a force that makes it move.
• The direction of the force is determined by Fleming' Left Hand Rule.

[In an exam, you can always tell that there is a question on the motor effect because all the students start to twist their left hands about, and several their right. Make sure you aren't one of these latter!]

This does not make an efficient motor. However, if we make lots of coils of wire, and mount them onto a rotating shaft, then we have a viable motor.

You may well have made a motor like this in your GCSE course.

We can see how the motor works if we view the motor head on.

Look at side 1 of the armature. When a current flows, it moves upwards. Side 2 must move downwards.

 

 

 

Now, half a turn later, side 1 is on the left, and moves downwards. Side 2 moves upwards. The whole motor spins anticlockwise in this diagra

The maximum turning effect (torque) occurs when the current is at 90o to the magnetic field. If the coil is at less than 90o, the turning effect is less.

So in figures (a) and (c) of the diagram, you can see where the maximum torque occurs. In (b) and (d) it is less. If the coil is lying parallel to the field, the torque is zero. If you want to investigate the matter further, refer to the Physics section of this site (still under construction) or any A level Physics textbook, such as A level Physics by Keith Gibbs.

The motor you saw above was very inefficient. Practical motors are constructed like this:

This motor can run off alternating current or direct current. It is called a universal motor. It can be controlled easily, it is powerful, light, and can be designed to run at any speed you like. Its disadvantage is that it is noisy and the brushes wear out. The commutator gets worn, which shortens the useful life of the motor. Sparking at the commutator can also be dangerous if there are flammable gases in the air. You will find a universal motor in many power tools, as well as hoovers and lawn mowers.

Many machines that run for long periods at a time use an induction motor. This works of ac only, its speed cannot be varied, is heavy, and can be inefficient. However there is no commutator, so there is not the intrusive whine of the universal motor. It is therefore much quieter.


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