The purpose of a dc motor is to produce movement from a battery. If a current is passed through a coil positioned in a magnetic field, the coil will experience a force according to Fleming's Left Hand Rule. The left hand of the coil AB will experience an upwards force and the right hand side CD of the coil will experience a downwards force. When the coil reaches the vertical position the commutator changes polarity. This means that current will reverse direction in the coil, but since the coil has undergone a half turn, CD is on the left and AB is on the right. Now CD experiences and upwards force, AB experiences a downwards force and the coil continues to turn in the same sense -clockwise above.
The rate of rotation can be increased by
Increasing the number of turns onthe coil. Each wire experiences an upwards force, so doubling thewire double the force.
Increasing the strength of themagnetic by using stronger magnets.
Putting more cells in the battery.
The gap between the two halves of the split-ring commutator is so wide that a carbon brush can
only touch one half of the split-ring at any time. This protects the circuit. It also means that
sometimes the motor will not start when switched on, if the carbon brushes are not in contact with the commutator.