What are the two classes of AC motors?

• A. Induction and DC
• B. Rotary and Linear
• C. Series and Shunt
• D. Synchronous and Asynchronous

Answer: (D)

AC motors are categorized by how the rotor interacts with the stator’s rotating magnetic field. In a synchronous motor, the rotor locks to the speed of that rotating field and runs at the exact synchronous speed set by the supply frequency and the motor’s pole count. It needs proper excitation and may require a starting method to reach speed, but once running, its speed stays constant regardless of load (within design limits).

In an asynchronous motor, also known as an induction motor, the rotor always runs a little slower than the rotating field due to slip. Currents induced in the rotor create torque, and the speed adjusts with the load. These motors are typically rugged, inexpensive, and self-starting, which is why they dominate many applications.

Thus, the two classes of AC motors are synchronous and asynchronous. The other options mix in DC motors or describe motion forms or winding configurations that aren’t the primary way AC motors are classified. Induction motors are the same concept as asynchronous motors, just another name.