The series motor responds to increased load by slowing down; the current increases and the torque rises in proportional to the square of the current since the same current flows in both the armature and the field windings. If the motor is stalled, the current is limited only by the total resistance of the windings and the torque can be very high, but there is a danger of the windings becoming overheated. Series wound motors were widely used as traction motors in rail transport of every kind, but are being phased out in favour of power inverter - fed AC induction motors. The counter emf aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate, the counter emf is zero and the only factor limiting the armature current is the armature resistance. As the prospective current through the armature is very large, the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter emf. As the motor rotation builds up, the resistance is gradually cut out.
factors on which the speed of dc motor depends
armature resistance armature resistance