DC series motor operated from ac supply

A DC series motor can run on AC because the torque direction does not depend on the direction of the supply current.

• In a series motor the same current flows through the field winding and the armature, so flux $\Phi\propto I$ and the torque is

$$T\propto\Phi I_a\propto I^2$$

• When the AC supply reverses, the current in the armature and the field both reverse together. The product $\Phi I_a$ keeps the same sign, so the torque is always in one direction (pulsating at twice the supply frequency, smoothed by the rotor inertia).

Such a motor is called a universal motor (works on both DC and AC). For good performance on AC the motor needs these changes:

1. The field core and yoke are laminated to reduce eddy-current and hysteresis loss.
2. The field winding has fewer turns and the armature has more, to reduce the reactance voltage drop and improve power factor.
3. A compensating winding is used to neutralise armature reaction and reduce commutation sparking.

It is used in vacuum cleaners, mixers, drills and sewing machines.