Which of the following lists the three types of three-phase motors?

• A. A DC motor, universal motor, and shaded-pole motor.
• B. A linear motor, servo motor, and stepper motor.
• C. A squirrel-cage motor, synchronous motor, and a wound-rotor induction motor.
• D. A brushless DC motor, induction motor, and reluctance motor.

Answer: (C)

Three-phase motors fall into two main families: induction motors and synchronous motors. Induction motors are typically built with different rotor designs, the most common being squirrel-cage rotors and wound-rotor rotors. Synchronous motors, on the other hand, run at a constant speed defined by the supply frequency and do not rely on slip like induction motors. So a list that includes two induction motor rotor styles (squirrel-cage and wound-rotor) plus a synchronous motor covers the three typical categories of three-phase machines seen in practice. The other options mix in motors that aren’t standard three-phase categories (such as DC or single-phase types, or devices focused on control rather than motor class), which is why they aren’t the best fit.