The universal motor can run on two feeds, either DC or AC, but a DC electric motor does not accept AC power, Gates AX44 Tri-Power Belts because the current would be inverted every half cycle and this would only cause the machine to shake and not rotate. Likewise, an AC motor will not accept DC power, as it will not offer the appropriate changes in the current direction for the correct operation of the electric motor. However, if we change the permanent magnets of the stators of the DC motors by electromagnets and we connect in series these electromagnets, in a same circuit of the rotor and commutator, we will have a universal motor.

This motor will rotate correctly whether it is powered by direct current or alternating current. The remarkable difference between universal motor and DC motor is that if we feed the universal motor with DC source, it will not reverse the direction of rotation if you reverse the polarity of the source, as with the DC motor, it will always continue to rotate in the same sense. If you really want to reverse the direction of rotation of a universal motor you should reverse the connections on the stator electromagnets to reverse their poles.