The rotation of a single-phase motor is determined by

• A. The direction of current through the start winding
• B. The rotor inertia
• C. The speed of the rotor
• D. The run winding polarity

Answer: (A)

In a single-phase motor, the direction of rotation is set by how the auxiliary start winding is driven relative to the run winding. The run winding provides the main magnetic field, while the start winding (with a capacitor) creates a second, phase-shifted field. The combination of these two fields produces a rotating magnetic field. The direction of that rotation depends on the direction of current through the start winding; reversing that current changes the relative phase, which reverses the rotating field and thus reverses the motor’s rotation. Rotor inertia or rotor speed don’t determine direction, and the run winding’s polarity is fixed in normal operation, so it doesn’t set the direction by itself.