The electro-magnets are formed by current flow through heavy strip copper windings, wound around iron pole shoes which are fastened to the yoke. Permanent magnets are located similarly, but occupy less space. The yoke is made of iron and serves to concentrate the magnetic field produced by the field magnets.
Starter motors with electro-magnetic field windings for light vehicle applications are series wound motors. Because the resistance of the field and armature windings is low, the current flow is high when the motor starts under load and this generates a strong magnetic field that will produce a high turning effort at low speeds. This high initial torque drops sharply as motor speed increases due to the back EMF induced in the armature windings which opposes current flow and reduces torque output.
Some series wound motors have the field windings in parallel with each other but then in series with the armature. These are referred to as a series-parallel field, series motor. By connecting the field windings in this way, more current can flow in the circuit and an overall increase in torque is obtained.
Source: CDX Global
