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Motor

A motor rotates. You find it everywhere - in fans, in computers, in washing machines, in microwave ovens, and so on. How does it really work?

On another page, I have described how a wire that carries a current in a magnetic field experiences a force.  This is the effect that is used in motors.  A simple motor contains a coil of wire in between two magnets.  The magnet produce a magnetic field in between.  When a current flows through the coil, the coil experiences forces.  These forces cause the coil to rotate.

To see why the forces on the coil give rise to rotation and not just push the coil to one side, we need to look at the current in the coil.  We also need to recall the left hand rule.

To make it easier, lets imagine a coil of wire that is rectangular in shape.   At first, the coil is horizontal.  It lies in between the north and south poles of two magnets.  The magnetic field is also horizontal.  Suppose that a current flows around the coil.  This current comes from two wires that are connected to a battery.

Consider the left side of the coil in the figure below.  Curent flows along this side in a direction that is perpendicular to the magnetic field.  If you apply the left hand rule, you will find that there is an upward force on this side of the coil.  On the right side of the coil, the current is in the opposite direction.  So the force here is downwards.  Because the forces on two sidesare in opposite directions, there is a turning effect.  So the coil rotates.

Now remember the two wires that lead away from the coil.  If the coil rotates, won't the two wires get twisted round and round?  Also, when the left side of the coil rotates to the right side, the force on the right side now points upward.  So the coil rotates back!  This is clearly not right.  There must be something more to the motor.

The two problems are solved with a clever setup called a split ring.  This is literally a ring that is split into two, as in the picture above.  Each half of the split ring is connected rigidly to one of the two wires leading away from the coil.  When the coil rotates, the split ring rotates with the coil.

The split ring is in contact with two pieces of carbon, one on each side of the ring.  As the ring rotates, it rubs against the carbon.  Because of this, the carbon here are called carbon brushes.  These carbon brushes are fixed and do not move with the ring.  The brushes are connected to wires that finally lead to the battery.  These brushes obviously solve the problem of twisting wires from the coil, since the wire leading from the coil rotates with the split ring.  But how do they solve the problem of the coil rotating back?

The answer is that when the left side of the coil rotates to the right side, each half of the split ring also switches carbon brush.  As a result, the current on the left side of the coil changes direction when it comes to the right side.  So the force on the left side also change direction and point downwards.  In this way, the coil will rotate in the same direction.