Compressor overcurrent interruption (Locked rotor)

What does UF mean?

Compressor overcurrent is detected within 30 seconds of starting, indicating a locked rotor condition. This can be caused by a closed stop valve, low power supply voltage, incorrect compressor wiring, or a defective compressor or outdoor power board. This fault causes an immediate shutdown to prevent damage, but recurring issues can severely shorten compressor lifespan.

• Mechanical seizure of the compressor motor (e.g., bearing failure, internal damage) preventing rotation.
• Refrigerant system restriction (e.g., fully closed suction or discharge service valve, clogged filter drier) creating excessive head pressure.
• Low supply voltage (e.g., below 180VAC for 208V system) at compressor terminals causing high inrush current during startup.
• Incorrect phasing or wiring of the 3-phase compressor motor, leading to motor stall or reverse rotation attempt.
• Defective compressor run capacitor (for single-phase motors) failing to provide proper phase shift for starting torque.

Click steps to track your progress.

1. 1

   Verify all refrigerant service valves (e.g., liquid line, suction line) are fully open as per system requirements.

2. 2

   Measure the supply voltage at the compressor terminals (e.g., T1, T2, T3) during startup to confirm it meets specifications (e.g., 208-230VAC).

3. 3

   With an insulation tester (megohmmeter), check the compressor motor winding insulation resistance to ground (expected >1 MOhm).

4. 4

   Verify continuity and correct phase sequence of the wiring from the inverter drive output (e.g., U, V, W) to the compressor terminals.

5. 5

   If applicable (single phase), test the capacitance of the compressor run capacitor (e.g., 40uF +/- 5%) using a capacitance meter.