What is the purpose of a capacitor in a capacitor-start motor?

• A. To create a phase shift between the run winding and the start winding
• B. To boost motor current
• C. To increase running speed
• D. To store energy for cooling

Answer: (A)

The starting action in a capacitor-start motor relies on creating a rotating magnetic field to kick the rotor into motion. The start winding by itself would produce a magnetic field that isn’t effective at starting because it doesn’t create the needed rotating effect. Placing a capacitor in series with the start winding shifts the current in that winding relative to the run winding. This phase-shifted current causes the start winding’s magnetic field to be out of phase with the run winding’s field, typically creating a near-90-degree lead. The combination of the two fields produces a rotating magnetic field, which generates a high starting torque and starts the motor. Once the motor reaches a sufficient speed, the start winding is disconnected (often by a centrifugal switch or a device like a PTC), and the motor runs on the run winding alone. The capacitor’s job is not to boost current during running, not to increase speed, and not to store energy for cooling.