Overview
The compressor is the core component responsible for pressurizing refrigerant within the refrigeration cycle. Accurate resistance and insulation testing ensures both the electrical integrity and mechanical health of the compressor motor windings.
This procedure verifies winding balance, insulation condition, and supports fault diagnosis for non-start or trip conditions.
Safety Warning
⚠️ High Voltage and Pressure Hazard
Always isolate power and discharge capacitors before testing.
Never perform resistance or insulation measurements on a live circuit.
Testing Conditions
Power must be OFF.
Use a digital multimeter (resistance mode) and megohmmeter (for insulation).
Perform testing at ambient temperature (20–25°C) for accurate readings.
Procedure
1. Winding Resistance Test
Disconnect compressor terminals.
Identify winding terminals:
U (Red)
V (White)
W (Black)
Measure resistance between each pair of terminals (U–V, V–W, and U–W).
Expected Values:
Resistance between each pair should be equal, typically 0.3 Ω to 2.0 Ω depending on model and temperature.
Example:
AOTG24KMTC: 1.916 Ω (20°C)
AOTH54KRTA: 1.546 Ω (20°C)
AOTH36 Model: 2.164 Ω (25°C)
Note: If readings are 0 Ω (short circuit) or O/L (open circuit), replace the compressor.
2. Insulation Resistance Test
Measure insulation between each terminal and compressor body (earth) using a megohmmeter at 500V DC.
Expected value:
> 1 MΩ minimum (preferably > 10 MΩ).
Low insulation indicates internal moisture or winding breakdown — replacement required.
3. Speed Calculation (for Inverter Compressors)
To calculate approximate compressor speed (rpm):
Compressor Speed (rpm)=Frequency (Hz)Motor Poles/2×60\text{Compressor Speed (rpm)} = \frac{\text{Frequency (Hz)}}{\text{Motor Poles} / 2} \times 60Compressor Speed (rpm)=Motor Poles/2Frequency (Hz)×60
Example:
For a 4-pole compressor operating at 90 Hz:
90(4/2)×60=2700 rpm\frac{90}{(4 / 2)} \times 60 = 2700 \text{ rpm}(4/2)90×60=2700 rpm
Diagnosis and Interpretation
| Observation | Diagnosis | Recommended Action |
|---|---|---|
| Equal winding resistance | Compressor windings normal | No fault detected |
| Open circuit (O/L) | Broken winding | Replace compressor |
| Short circuit (0 Ω) | Internal winding short | Replace compressor |
| Unequal winding values | Partial short or imbalance | Replace compressor |
| Insulation < 1 MΩ | Breakdown or moisture ingress | Replace compressor |
| Normal winding & insulation | Proceed with inverter/IPM checks | Verify system controls |
Technical Notes
Ensure the compressor terminals are clean and dry before testing.
Do not apply voltage to the compressor without confirming correct winding and insulation values.
Compressors used in Fujitsu General systems are primarily Rotary, Twin Rotary, or Scroll type, typically with 4 or 6 poles.
Always confirm expected values with Fujitsu Technical Support for the specific model.
