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The standard starter motor has only series field coils. They carry the same current as the armature. This makes a motor that draws the same current stalled or turning fast but also produces the same torque independent of speed. So when run unloaded the speed can get unreasonably high. So high the armature windings are thrown from the slots jamming the armature. A bigger series motor run without load accelerates like a siren but stops in a fraction of a turn with a large bang. Anyway the current and the torque is pretty much set by the resistance of the windings in a series motor. A pure shunt field DC motor works more like an AC motor with a nearly constant speed with varying load. And the armature current rises with an increased load. Adding shunt coils will increase the magnetic field strength so the armature current can be more effective at producing more torque. Torque is essntially a product of armature current times field strength. You wanted more torque, the shunt fields give you more torque. The also limit the unloaded motor speed and may give the starter a tendency to draw more current when turning slowly. Depends on which field source is strongest, the shunt or the series. With a dual field motor, called a compound motor you often have characteristic of both a series and a shunt motor. If he put in shunt fields in place of the series fields, the starter can have more torque than with the original series fields, but will draw a whole heap more current from the battery which may make brush and armature life short.
Gerald J.
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