Megohm Meter UK: Testing Compressor and Fan Motor Windings

Short answer: A megohm meter (megohmmeter) is what a UK HVAC or refrigeration engineer uses to test a compressor or fan motor winding to earth. Isolate and lock off the unit, disconnect the inverter and control electronics, then test winding-to-earth on the 500V range and compare the megohm reading against the unit's history or a known-good machine. It tells you whether the winding insulation is sound; it does not decide on its own whether to replace the compressor.

What a megohm meter does on HVAC plant

A megohm meter applies a DC test voltage and measures the tiny leakage current through insulation, showing the result in megohms (MΩ). On a stopped chiller, heat pump or condenser fan, that is how you separate an electrical fault — a winding breaking down to earth — from a control or mechanical fault before committing to an expensive compressor swap. A standard multimeter cannot do this; its low-voltage ohms range will not reveal insulation that only breaks down under hundreds of volts.

How to megger a compressor motor winding

1. Isolate and prove dead. Lock off, then prove the terminals dead with an approved voltage indicator before touching anything.
2. Disconnect sensitive electronics first. Inverter drives, control boards and soft-starters can be destroyed by the test voltage — disconnect them so you test the winding, not the electronics.
3. Test winding-to-earth on 500V. 500V DC is the usual range for LV compressor and fan windings; connect between the winding terminals (linked together) and the frame/earth.
4. Read and compare. A healthy winding typically reads well into the hundreds of megohms; a reading collapsing towards single megohms points to moisture or breakdown. Compare against the unit's own history or a known-good machine rather than one absolute number.
5. Discharge before disconnecting. Windings hold a charge — leave the leads on to let it discharge before removing them.

For the general step-by-step method on any circuit or cable, see how to perform an insulation resistance test.

Why temperature and a single reading can mislead

Insulation resistance falls as winding temperature rises, so a warm compressor straight after running reads lower than the same winding cold — always note the temperature with the reading. Surface moisture and dirty terminals also drag readings down. That is why a trend matters more than a snapshot; for building an insulation trend across plant, see the high resistance meter guide.

Choosing a megohm meter for HVAC work

• Test voltages: 250V, 500V and 1000V. 500V covers most LV compressor and fan windings; the Megohmmete Pro offers all three.
• Range: reading up to 2000 MΩ is enough to show a healthy winding sitting well above the danger zone.
• Backlit display: genuinely useful in a dim plant room, on a roof or at the back of a cold aisle.
• Safety: a stated CAT III 600V equivalent design with GS38-compliant leads (confirm the exact detail with the seller).
• Batteries: 6 × 1.5V AA, not included — keep a spare set in the van.

Honest limits

A megohm meter is a diagnostic tool, not a manufacturer sign-off: always follow the equipment maker's procedure, and remember that a single winding-to-earth reading does not test turn-to-turn insulation. The published specification for the Megohmmete Pro does not state a formal accuracy figure, so if you need to hold it against a calibrated reference, confirm the accuracy with the seller rather than assuming — the digital insulation tester guide covers that comparison.