ABSTRACT
A “universal” motor is one that runs on AC or DC. Consider the DC series motor, as shown in Figure 11.1a. Observe that the field polarity depends on the direction of the field current. But the field current is the armature current! Hence if the armature current reverses direction, as shown in Figure 11.1b, the field synchronously reverses polarity. If both the field and armature current reverse polarity, the developed torque (and hence rotation) is in the same direction. Thus such a motor is “universal” in that it runs on AC or DC.
