DC motors.

I learnt about parts of a direct current motor, which include Shaft, armature, windings, bearings, commutator and brushes as the picture below;

A DC motor converts electric current into mechanical movements, i.e Rotational movement known as torque or turning effort therefore can be used to drive a number of systems. One example is a starter motor which drives the car Engine to start or a window winding motor to close and open car windows.

Brushes are always in contact with the commutator which is attached to the ends of the individual windings around the armature which are insulated to avoid short circuits. The commutator is a split slip ring so that when the motor is running, it should not produce reverse current to induce drag against direction of rotation.
Bearings hold the shaft in a true rotational position to avoid it touching the outer casing and magnets.

When the motor is given current the armature becomes an electro-magnets which react to the permanent magnets on the casing and this causes rotation of the armature shaft which can drive any desired component. When there is no current flow, electro magnetic force collapses and the motor stops. To change the direction of rotation only polarity of the current is reversed and the more the current the more the speed of the motor. the relationship of speed and torque is that torque is maximum at start and continues dropping as speed increases, But as current is increased torque increases. Efficiency on the other hand increases sharply after idling speed and starts dropping slowly as speed continues to increase.

Nowadays motors are driven electronically from pulse generators which operate transistors which in turn switch the motor ON and OFF at a high frequency, this allows a small current to be used in the control circuit and to vary the speed only the switching time is varied. This allows the system to be automated and self monitoring.