Question

Discuss different methods of speed control of a d.c. series motor by adjusting field ampere turns.

Change of current through field pole windings is easy to achieve by use of a rheostat (variable resistance) in series with a shunt motor field (Figure a) or a diverter for the field of a series motor (Figure b).

Figure a. Shunt motor with field control of speed through a rheostat

Figure. b. Series motor with field control of speed using a diverter

The weakening of the magnetic fields of the poles by reducing field current has the inverse effect of causing the motor to speed up. The explanation for this is that a weaker field causes less back e.m.f. to be generated in the armature conductors as they cut the flux. The resulting larger difference between supply voltage and back e.m.f. means that there is more effective voltage to increase armature current. This method is useful for changing speed at the top end of the range, but it is not possible to reduce speed below that which is obtained when full shunt-field current flows. One limitation is that weakening of the magnetic fields of the poles permits them to be more easily distorted by the rotating armature (armature reaction). Greater advantage can be taken of field control if the machine has interpoles to limit the armature reaction.