Compressor Capacitor (Single-Phase AC Compressor – Run Capacitor)
Two Core Functions
1. Starting Function (Phase Shift and Starting Torque)
The capacitor is connected to the compressor's start winding, causing the current in the start winding to lead the current in the run winding. This creates a rotating magnetic field that provides the torque needed to overcome resistance and start the rotor.
If the capacitor fails, the outdoor unit may produce a humming sound while the compressor fails to start. After a few seconds, the compressor may overheat and trigger thermal protection.
2. Continuous Running Function (Active Throughout Operation)
After the compressor starts successfully, the capacitor remains connected and continuously provides phase compensation:
- Reduces motor operating current and winding heat
- Improves compressor output torque and energy efficiency
- Stabilizes the current waveform and reduces electrical stress on the circuit
Note: Older equipment may also use a Start Capacitor, which operates only for approximately 2–5 seconds during startup. Once the motor reaches the required speed, a relay disconnects the start capacitor. Most modern air-conditioning outdoor units use a run capacitor that remains connected during operation.
Simple Working Principle
A compressor has two sets of windings:
- Run winding (main winding)
- Start winding (auxiliary winding)
Without a capacitor: The currents in the two windings are in phase, so a rotating magnetic field cannot be properly generated. The motor may only vibrate and cannot start rotating.
With a capacitor: The current in the auxiliary winding leads the current in the main winding, creating a rotating magnetic field that allows the rotor to start and continue rotating.
Typical Signs of a Failed Capacitor
Capacitance degradation:
The compressor has difficulty starting, reduced output, poor cooling performance, higher operating current, and frequent thermal protection trips.
Open circuit / Bulging / Leakage:
The outdoor unit may only produce a humming sound, while the compressor fails to start completely.
