CSLB C-20 HVAC ContractorControls, Electrical and SafetyEasy
What is the primary purpose of a start capacitor in a single-phase motor circuit, and when is it typically removed from the circuit?
- AIt provides extra starting torque during startup and is switched out of the circuit once the motor reaches speed
- BIt protects the motor windings from overheating and stays in the circuit at all times
- CIt filters electrical noise for the thermostat and is unrelated to motor torque
- DIt smooths voltage ripple continuously and remains energized while the motor runs
Show answer & explanationAnswer & explanation
Correct answer: A. It provides extra starting torque during startup and is switched out of the circuit once the motor reaches speed
A start capacitor is wired in series with the start winding to create a phase shift that boosts starting torque. Once the motor reaches roughly 75% of its rated speed, a relay (potential or current relay) opens and disconnects the start capacitor, since it is not designed for continuous duty and would overheat.
Why the other options are wrong
- B. Overload protection is a separate function performed by thermal overloads, not capacitors.
- C. Start capacitors directly affect motor torque, not electrical noise filtering.
- D. Start capacitors are not continuous-duty devices and are disconnected after startup.
Start Capacitor Function
A start capacitor provides a large capacitance boost to increase starting torque in single-phase motors and is disconnected by a relay once the motor reaches running speed.
- Wired in series with the start winding
- Disconnected by potential or current relay near 75% speed
- Not rated for continuous duty, unlike a run capacitor
Memory trick: Start gives the push, then takes a rest.