Some single phase motors can be run not only with DC circuit but also with AC universal motor circuit and for that reason they are known as Universal Motors. These motors are also known as commutation type motors as they continue to run in the same direction even if the polarity of the line terminals of their DC circuit is reversed.

The polarity of both the fields and the direction of current in the armature can help in determining the direction of the running motor because armature current and fluctuation are proportional to torque. If, a DC motor is connected to AC universal motor circuit the reversal of flow of AC current from negative to positive will affect direction of current in armature and flux polarity but not the direction of the motor.

The change in AC universal motor circuit will not change the direction of rotation of the motor if the direction of the torque developed is positive. But the frequency of the torque will be twice the frequency of the line because torque is pulsating by nature.

In this way a universal motor works with AC universal motor circuit as well as DC circuit with equal efficiency. But a motor that is quite specifically designed to work on DC can show following issues if connected to single phase AC supply.