The working principle of starter can be explainned by the operating principle of its main component dc motor.A direct current motor is a device that converts electrical energy into mechanical energy,based on the principle that electrically charged conductors are subjected to electromagnetic force in a magnetic field.Its working principle is shown in the figure.Because the toque generated by a coil is too small and the rotational speed is unstable,in face, there are a lot of coils around the armature of the motor,and the number of commutator segments also increases with the increase of the coil.The ensures sufficient torque and steady speeed.