Sunday, January 2, 2011

Motor Coil Forces

Electric Motor

Look at the single coil of wire below. It is connected to a DC supply. Current flows, as shown by the red arrows.

Use Fleming's Left Hand Rule to show that the left and right hand sides of the coil experience a force up and down.

Using the Left Hand Rule, you should work out that the wire experiences the following forces:

As a result, it will rotate clockwise. Let's see what will happen next.

When the coil moves into the vertical postition (as long as the current is still flowing) the forces acting on the wires remain :

The forces act against each other: the coil will stop rotating! How then can a real motor keep going in the same direction? And how does it not twist its wires into a mess?

The Commutator

The answer is to use a commutator. This device prevents the wires from twisting. More importantly it actually allows the coil to keep rotating in one direction!

The one shown is a split-ring commutator. It has two halves, each connected to the power supply, completing the coil circuit. The coil is able to turn smoothly around it.

The commutator is made from two round pieces of copper, one on each side of the spindle. A piece of carbon (graphite) is lightly pushed against the copper
to conduct the electricity to the armature. The carbon brushes against the copper when the commutator spins.

In the vertical position there is no current as the coil has lost its connections to the commutator. Since the coil was already moving, it keeps turning. Think about riding a bike - stop pedalling and you don't stop moving for quite some time.

As the motor rotates, first one piece of copper, then the next connects with the brush every half turn. The wire on the left side of the armature always has current flowing in the same direction, and so the armature will keep turning in the same direction.

The pieces of copper are held apart in the centre and do not touch each other.
They look like a ring of copper which is split down the middle. This is why it is called a split - ring commutator.

Summary points: split-ring commutators reverse the direction of the current in the coil each half turn. This allows the motor coil to rotate continuously in one direction.

To increase the turning effect of the coil in the motor:

1.Insert a soft Iron core or cylinder into coil - to concentrate the magnetic field lines.

2. Increase the number of turns in the coil.

3. Increase the current in the coil.

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