When the single-phase universal motor is powered by AC, it is called AC universal motor; when it is powered by DC, it is called DC universal motor.

For a single-phase DC universal motor to change its power supply polarity, the direction of the torque and speed of the motor does not change (constant steering), this is because the excitation winding and the armature winding are connected in series, and the excitation current and armature current change at the same time for the sake of direction. If it is connected to the AC power supply, it can also make the direction of the torque and speed of the motor unchanged (constant steering). This is because the positive half-wave of the alternating current is equivalent to the positive polarity of the DC power supply, while the negative half-wave of the alternating current is equivalent to the negative polarity of the direct current.

In summary, single-phase universal motors is a dual-purpose motor.