How do I test inverter compressor motor windings and the IPM driver output using a digital multimeter?

Testing an inverter compressor setup involves isolating the mechanical motor windings (unpowered resistance test) from the Intelligent Power Module (IPM) inverter board output (powered voltage test or unpowered diode test).

Testing compressor winding resistance. Source: Electrical Online 4u

Part 1: Testing Inverter Compressor Motor Windings

Because inverter compressors use a 3-phase BLDC/PMSM motor, all three winding terminals (U,V,W) must have equal resistance.

1

Disconnect Mains Power

Safety Prerequisite

Unplug the refrigerator from the wall socket. Wait at least 3 to 5 minutes to allow high-voltage DC bus capacitors on the inverter PCB to bleed off safely.

2

Disconnect Compressor Wiring Harness

Isolation

Remove the protective terminal cover on the compressor and disconnect the 3-pin wiring harness connecting the inverter board to the compressor pins (U,V,W).

3

Measure Resistance Across Terminals (U-V, V-W, U-W)

Winding Resistance Test

Set your multimeter to the lowest resistance (Ω) scale or auto-range. Measure across each pair of terminal pins:

• U→V
• V→W
• W→U

Pass Result: All three readings must be identical within ±5% tolerance (typically between 6Ω and 20Ω depending on the manufacturer).

Fail Result: If any pair shows 0Ω (shorted winding) or ∞Ω / “OL” (open winding), the compressor motor has failed internally.

4

Check Short to Ground (Frame Isolation)

Ground Fault Test

Set the multimeter to its highest resistance scale (MΩ) or continuity mode. Place one probe on a clean, unpainted copper suction pipe or ground chassis screw, and touch the other probe to each terminal (U, then V, then W).

Pass Result: Must read ∞Ω / “OL” (open circuit/no continuity).

Fail Result: Any resistance reading or beep indicates a grounded motor winding (shorted to shell).

Part 2: Testing the Inverter IPM Board Output

The IPM uses 6 internal IGBT transistors (arranged in a 3-phase bridge) to drive the compressor. You can test it via a Diode Mode Test (unpowered) or an AC Output Test (powered).

Method A: Cold Diode Test on the IPM (Unpowered)

1. Keep the unit unplugged and compressor disconnected.
2. Set your multimeter to Diode Test Mode (→∣←).
3. Locate the DC Bus capacitor terminals or IPM bridge pins: DC+ (Positive) and DC− (Ground).
4. Test Upper IGBTs (DC+ to U,V,W):
   • Place the RED probe on DC+ and touch the BLACK probe to U, then V, then W.
   • You should see a standard diode drop (approx. 0.3V−0.7V) across all three channels.
   • Reverse probes (BLACK on DC+): Should read “OL”.
5. Test Lower IGBTs (DC− to U,V,W):
   • Place the BLACK probe on DC− and touch the RED probe to U, then V, then W.
   • You should see a standard diode drop (approx. 0.3V−0.7V) across all three.
   • Reverse probes (RED on DC−): Should read “OL”.

If any reading shows 0V (shorted) or “OL” in both directions (open), the IPM or gate driver IC is blown.

Method B: Dynamic AC Voltage Test (Powered)

1. Keep the compressor harness disconnected.
2. Set your multimeter to AC Voltage (V∼).
3. Insert probe leads into the unplugged harness socket contacts:
   • Test across U→V, V→W, and U→W.
4. Power on the refrigerator and force a cooling call.
5. Watch the meter during the initial starting burst (the board will output a short pulse before tripping due to no load):
   • Pass Result: The board must output equal AC voltage across all three phase pairs (U−V,V−W,W−U) during startup attempt (typically 50V AC−150V AC depending on speed signal).
   • Fail Result: Zero voltage on one or all phases, or major voltage imbalance between phases, confirms a faulty inverter control board.