The universal motor can run on both direct and alternating current. The notable difference between universal motor and direct current motor is that if the universal motor is supplied with an alternating source, it will not reverse the direction of rotation if the source polarity is reversed (as with the DC motor). It will continue to spin in the same direction forever. If it is necessary to reverse the direction of rotation of a universal motor, the connections on the stator electromagnets must be inverted to reverse their poles.
Due to the use of the universal motors, there will be wear on the carbon brushes, which can be replaced, in a very simple operation. A true direct current electric motor does not accept AC power and this will reverse thrust bearings the current sense every half cycle and this would cause shaking. Likewise, a true AC motor does not accept continuous power, because it does not offer the appropriate changes of the current direction for the correct operation of the motor. However, if we change the permanent magnets of the stators of the motors of direct current by electromagnets and we connect these electromagnets in the same circuit of the rotor and switch, we will have a universal motor. The universal motor will ‘turn’ correctly powered by AC or DC.