Why is a single-phase series motor a dual-purpose motor?

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

For a single-phase DC series motor to change its power supply polarity, the direction of torque and speed of the motor does not change (constant steering), this is because the excitation winding and 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.

To sum up, the single-phase series motor is an AC and DC fan motors.