In Chapter 1, you have read about electromagnet-based DC motors. You have seen various connection methods there such as shunt, series, and separate field. Among these motors, what do you think will happen if AC power is applied to a series motor?
Assume that when the motor is rotating in the condition shown in Fig. 2.33 (a), the polarity of the voltage applied to the motor is reversed. If the polarity of the electromagnet remains the same as shown in Fig. 2.33 (b), then the current flowing through the rotor is reversed and, as a result, rotational direction of the rotor is reversed.
However, with a series motor, the polarity of the electromagnet is also reversed as shown in Fig. 2.33 (c). So the rotation direction remains unchanged.
Namely, this motor has a feature of being capable of continuing the rotation with AC power, too.
However, if you apply AC power to a series motor designed to be used with DC power, there occurs a problem. If AC power is applied when cast metal or soft steel is used for the stator core, current is generated due to electromagnetic induction (this current is called eddy current), generating a large amount of heat. Therefore, just like with an induction motor, a silicon steel-based insulation lamination structure is employed in the core of a commutator motor that operates on AC.
These motors are called universal motors since they are operable both on AC and DC.
However, since these motors are mostly operated on AC in practical use, they are called AC commutator motors.