The universal motor is nothing but dc series motor.
When dc motor is supplied with AC supply then torque generated is positive for both half cycles. Because during the negative half cycle, armature current reverses and flux also reverses so that resultant torque is in the same direction as that of the positive half cycle.
Some changes must be made to use the dc motor as a universal motor are :
- An entire magnetic circuit is laminated in order to reduce eddy current loss. So universal motor has expensive construction than dc series motor.
- Series field winding uses few turns as possible to reduce reactance of field winding to a minimum. It reduces vtg drop across field winding.
- A high field flux is obtained by using a low reluctance magnetic circuit.
- There is considerable sparking between brushes and commutator when a motor is used on ac. Because alternating flux establishes high currents in the coils short-circuited by brushes. When the short-circuited coils break contact from commutator, excessive sparking is produced. This can be eliminated by using high resistance leads to connect coils to commutator segments.