When current flows in a conductor, an electromagnetic field is generated around it. If the conductor is placed so that it cuts across a stationary magnetic field, the conductor will be forced out of the stationary field. This happens because the lines of force of the stationary field are distorted by the electro-magnetic field around the conductor and try to return to a straight line condition.
Reversing the direction of current flow in the conductor will cause the conductor to move in the opposite direction. This is known as the motor effect and is greatest when the current carrying conductor and the stationary magnetic field are at right angles to each other.
A conductor loop which can freely rotate within the magnetic field is the most efficient design. In this position, when current flows through the loop the stationary magnetic field is distorted and the lines of force try to straighten. This forces one side of the loop up and the other side of the loop down.
The motor effect causes the loop to rotate until it is at ninety degrees to the magnetic field. To continue rotation, the direction of current flow in the conductor must be reversed at this static neutral point.
A commutator is used for this purpose.
An example commutator consists of two semi-circular segments which are connected to the two ends of the loop and are insulated from each other. Carbon impregnated brushes provide a sliding connection to the commutator to complete the circuit and allow current to flow through the loop.
Rotation commences with both sides of the conductor loop cutting the stationary field. When the loop passes the point where the field is no longer being cut, the momentum of rotation carries the loop and the commutator segments over so that the brushes maintain current flow in the same direction in each side of the loop relative to the stationary field.
This process will maintain a consistent direction of rotation of the loop. In order to achieve a uniform motion and torque output, the number of loops must be increased. The additional loops smooth out the rotational forces. A starter motor armature has a large number of conductor loops and so has many segments on the commutator.
