WiremanWireman · Compressor won’t start or trips the breaker: troubleshooting for electricians · 2023 NEC ·
Compressor won’t start or trips the breaker
Capacitor, voltage on start, the C-S-R windings, then windings to ground.
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How it works
A single-phase compressor has a run winding and a start winding joined at a common terminal, marked C, S and R. The run capacitor sits between the HERM terminal and the start winding to give it starting torque. An internal overload inside the shell opens the circuit when the compressor gets too hot, and it can take an hour or more to reset. On start, the compressor pulls its locked-rotor amps (LRA on the nameplate); if it pulls that and doesn’t turn, it trips on overload.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What does the compressor do when the contactor pulls in?
- Hums, then clicks offTest: Test the HERM side of the capacitorExpect: Within the tolerance on the label, often ±5% or ±6%. A 40 µF section at ±6% should read about 37.6 to 42.4 µF.
- Low, open or bulgedWeak or failed run capacitor
- Within the ratingTest: Measure voltage while it tries to startExpect: No more than about a 10% drop from the running voltage, and not below the nameplate minimum.
- Drops more than about 10%Voltage sags too far on start
- Holds steadyTest: Measure C, S and RExpect: C to R is the lowest, C to S is higher, and S to R equals C to S plus C to R (within a few tenths of an ohm). Residential compressors usually read only a few ohms.
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Any reading to groundCompressor winding shorted to ground
- OL on all threeMechanically stuck or starting against pressure
- C-R and C-S open, S-R readsThe internal overload is open
- One winding openOpen compressor winding
- Near 0 Ω, or doesn’t add upShorted compressor winding
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Trips the breaker or blows fusesTest: Measure C, S and RExpect: C to R is the lowest, C to S is higher, and S to R equals C to S plus C to R (within a few tenths of an ohm). Residential compressors usually read only a few ohms.
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Any reading to groundCompressor winding shorted to ground
- OL on all threeMechanically stuck or starting against pressure
- C-R and C-S open, S-R readsThe internal overload is open
- One winding openOpen compressor winding
- Near 0 Ω, or doesn’t add upShorted compressor winding
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Silent, but the fan runsCarefully touch the top of the compressor shell. How hot is it?
- Too hot to hold your hand onThe internal overload is open
- Warm or coolTest: Measure C, S and RExpect: C to R is the lowest, C to S is higher, and S to R equals C to S plus C to R (within a few tenths of an ohm). Residential compressors usually read only a few ohms.
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Any reading to groundCompressor winding shorted to ground
- OL on all threeMechanically stuck or starting against pressure
- C-R and C-S open, S-R readsThe internal overload is open
- One winding openOpen compressor winding
- Near 0 Ω, or doesn’t add upShorted compressor winding
- Readings add upTest: Test each terminal to groundExpect: OL on every terminal.
- Starts, runs a few minutes, stopsIt starts, then trips on heat or a safety
- Hums, then clicks offTest: Test the HERM side of the capacitorExpect: Within the tolerance on the label, often ±5% or ±6%. A 40 µF section at ±6% should read about 37.6 to 42.4 µF.