What happens if DC series motor is connected to an AC supply?

     An AC supply will exert unidirectional torque because the direction of the armature current and field current reverses at the same time and the direction of rotation will be as it is. Due to the presence of alternating current, eddy currents are induced in the yoke and field cores which causes excessive heating of the yoke and field cores. The power factor becomes low because of the high inductance formed by the field and armature circuit. There is sparking at the brushes of the DC series motor.

     The field winding of a dc series motor has fewer turns and larger wires than the field winding of the dc shunt motor. Therefore, its inductive reactance is less. The strength of the magnetic field is about the same as when the motor runs on dc power.

     The field winding and the armature winding are connected in series. Therefore, the current lags behind the potential difference by the same amount in both windings. As a result, the current in the two windings is in phase. The current in the armature winding is in phase with the magnetic field produced by the field winding.

     However, a dc series motor does not operate well on ac power. The motor has a low-efficiency power factor and runs with considerable sparking and a low brush. The low efficiency is caused by high hysteresis loss and high eddy current loss. The power factor is caused by the inductive reactance of the field and armature windings.