Which of the following lists all three types of three-phase motors commonly used?

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

Answer: (C)

The concept being tested is the common types of three‑phase motors used in typical systems. Among three-phase motors, the main categories you’ll encounter are two forms of induction motors and a synchronous motor.

A squirrel-cage induction motor is the most widely used three-phase motor. Its rotor consists of bars shorted by end rings, which makes it rugged, simple, and inexpensive—ideal for reliable pump and fan service.

A wound-rotor induction motor is another induction motor variant where the rotor windings are connected through slip rings to external resistances. This arrangement allows control over starting torque and speed, which is useful in applications needing adjustable or high-torque starts.

A synchronous motor operates at the same speed as the electrical supply (no slip) and requires separate DC excitation. It’s valued for maintaining a precise speed, which is important in processes where exact timing or synchronization is critical.

The other options mix motors that aren’t typically categorized as three-phase motors—for example, DC motors, shaded-pole or universal motors (which are usually single-phase), and piezoelectric or stepper motors. Those don’t represent the standard trio of widely used three-phase motor types.