SOLUTION

For a given applied voltage and current, the speed of a universal motor will be higher in dc excitation than in ac excitation. When the motor is operated on DC, the current is limited by resistance only, but in AC operation, the current is opposed by impedance which is built up of resistive and reactive components. During AC operation of the universal motor, the inductive reactance of the armature and field windings opposes the current. In effect, a reactance voltage is produced in both the armature and field windings, consuming some of the applied voltage. Consequently, the speed of an AC series motor is less than that of the identical DC series motor.