How to Test Compressor Windings and Ground

Why this matters

A compressor condemned on a bad reading is the most expensive mistake a service tech makes in a day, and a compressor passed on a bad reading is the second. Both come from the same three habits: reading a hot compressor as if it were cold, testing with the leads still on so the meter reads the capacitor and contactor along with the motor, and calling an ohmmeter's OL to the shell a ground test. This procedure covers single-phase residential scroll and reciprocating compressors with C, S and R terminals. Three-phase compressors read three equal windings phase to phase, and inverter-driven compressors are tested per the drive maker's procedure with the drive disconnected; neither is covered here.

Start state

You need a lock and tag for the disconnect, a CAT III or better meter you can prove on a known live source, a bleed resistor of roughly 20,000 ohms rated 5 W or more (or a purpose-made capacitor discharge tool), a meter that resolves 0.1 ohm with leads you can zero, a megohmmeter with a 500 V DC range, a surface thermometer, and the compressor maker's winding resistance values. Get those from the model number on the compressor's own label, not the condenser nameplate; the same condenser ships with different compressors.

Before you begin, know what the refrigerant side is doing. If the system has been recovered and pulled into a vacuum, break the vacuum with dry nitrogen or refrigerant before any megohmmeter test: insulation that holds 500 V at atmospheric pressure can flash over in a deep vacuum and damage a good motor. The same rule is why nobody energizes a compressor under vacuum.

The steps

1. Kill it, lock it, prove it dead. Open the disconnect, lock and tag it, then prove absence of voltage at the contactor line side (L1 to L2, L1 to ground, L2 to ground) with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices under 29 CFR 1910.333(b)(2), or 29 CFR 1926.417 on construction work. Why: condensers get fed from mislabeled breakers and from disconnects someone rewired; the test proves the switch you opened is the one feeding this unit. Skip it and the first sign of a second feed is your hand.

2. Discharge the run capacitor. The capacitor holds charge with the power off. Put the bleed resistor across HERM to C, then FAN to C, holding the tool by its insulated handles, then read DC and AC volts across each section and accept only a reading near zero. Do not short it with a screwdriver: the arc pits the terminals and the screwdriver is the thing in your hand when it goes.

3. Inspect the terminals, then pull and label the leads. Remove the terminal cover and look before you touch. Oil on the terminal block, a charred or cracked insulator, or a loose pin means a terminal that can vent oil and refrigerant when disturbed on a pressurized compressor; wear eye protection, keep your face off the line of the pins, and pull each spade straight with a connector puller rather than rocking it. Label each lead by the terminal it came off. Terminal layout varies by maker, so read C, S and R from the cover or the compressor label, never from where they sat on the last unit you serviced. Why pull the leads: with them on, S to R also reads through the run capacitor and C to ground can read through whatever else shares those wires, and the megohmmeter's 500 V lands on parts that were never meant to see it.

4. Read the shell temperature. Copper resistance rises with temperature, about 0.22 percent per degree F near room temperature. The maker's table states the temperature its values were measured at, often 77 F. Correct the expected value with R(hot) = R(table) x (T + 390) / (T(table) + 390), temperatures in F, where 390 is copper's inferred zero-resistance point (234.5 C) expressed as a positive number in F. The shell runs cooler than the windings inside it, so a corrected value is an approximation that will usually sit a few percent under what you read on a compressor that just stopped.

5. Zero the leads and measure the three pairs. Short the probes and note the lead resistance; subtract it from every reading. Then read C to R, C to S and S to R.

 SINGLE-PHASE COMPRESSOR, terminal view (layout varies)

        C (common)
        |
   [internal overload, in the common leg]
        |
        +--------------------+
        |                    |
   run winding          start winding
   (heavy wire,         (fine wire,
    lowest ohms)         more ohms)
        |                    |
        R                    S

 C to R  0.95 ohm   run winding alone
 C to S  2.75 ohm   start winding alone
 S to R  3.70 ohm   both in series
 check: 0.95 + 2.75 = 3.70, so C to R + C to S = S to R

The run winding is heavier wire and reads lowest; the start winding reads higher; S to R reads both in series. If C to R plus C to S does not equal S to R within your meter's resolution, recheck your terminal labels before you believe anything else. A reading lower than the corrected maker's value suggests shorted turns. The maker's data sheet states its tolerance; where it does not, this shop treats more than 10 percent low after temperature correction as suspect and confirms with a second meter before condemning.

6. Allow for an open internal overload. If C to R and C to S both read OL but S to R reads a normal value, the windings are intact and the internal overload in the common leg is open. It opens on heat and current and recloses when it cools, which on a shell that stopped hot can take an hour or more. Leave the disconnect locked off and let it cool. Running water over the shell to speed it up works if the terminal cover is on and no water reaches the terminals or wiring. A compressor that still opens C to S and C to R once the shell is within about 20 F of ambient has an overload or common lead that did not reclose, and that is a condemning finding. An overload that recloses is not a repair: something made the compressor overheat, and the usual suspects are a weak run capacitor, low charge (less suction gas to cool the motor), high head pressure from a dirty condenser, or low voltage.

7. Test each terminal to ground with the megohmmeter. Clip the ground lead to the compressor ground stud or a clean bare spot on the copper, not paint. Apply 500 V DC to each terminal in turn and read at one minute. A dry, healthy hermetic motor commonly reads hundreds of megohms or more. Many makers treat readings under about 1 megohm as failed insulation; check the compressor maker's service bulletin for its own floor. A reading in the low megohms that falls on each retest says insulation is breaking down. A reading in the low tens of megohms on a compressor that sat with liquid in the shell often comes up after the crankcase heater has run several hours, so retest before condemning. Test all three terminals: they connect through the windings, but an open overload isolates C from S and R. Hazard: the megohmmeter puts 500 V on the terminal under test; keep hands off the leads and terminals until the meter reports itself discharged. An ordinary meter reading OL to ground on its highest range is not a ground test, because it applies a few volts, not hundreds.

8. Reconnect, close up, restore and prove. Land each lead on its labeled terminal, seat every spade fully, and put the terminal cover and its gasket back on before any power returns. Never energize a compressor with the terminal cover off. Remove your lock and tag, confirm nobody is at the unit, stand to the side of the condenser rather than over the terminal box, and close the disconnect. Then prove it: the compressor starts on the first call, running amps sit under the RLA on the compressor label, and voltage at the contactor under load is inside the nameplate range.

Worked example: hot shell, dead silence

A 3-ton condenser in July, 92 F outside. Contactor pulled in, condenser fan running, compressor silent. Shell temperature 145 F. Steps 1 to 3 done; leads off and labeled.

The compressor data sheet (illustrative values for this example) lists C to R 0.88 ohm and C to S 2.55 ohm at 77 F, so S to R should be 0.88 + 2.55 = 3.43 ohm at 77 F.

First reading, lead resistance 0.2 ohm subtracted: C to R OL, C to S OL, S to R 3.9 ohm.

Corrected expected S to R at 145 F: 3.43 x (145 + 390) / (77 + 390) = 3.43 x 535 / 467 = 3.43 x 1.146 = 3.93 ohm. The 3.9 reading matches. Both windings are continuous; the only open path runs through C. That is the step 6 pattern, an open internal overload, not a dead compressor.

While it cooled, the tech tested the run capacitor: the HERM section read 31 microfarads against a 45 microfarad label, 31 / 45 = 69 percent of rated, well outside any capacitor tolerance. A compressor starting and running on a weak capacitor draws high current and overheats; that is why the overload opened. See related: Capacitor and Contactor Replacement Standard.

After 70 minutes, shell 98 F: C to R 0.95, C to S 2.75, S to R 3.70. Check: 0.95 + 2.75 = 3.70, closes exactly. Expected at 98 F: ratio (98 + 390) / 467 = 488 / 467 = 1.045, so C to R 0.92 and C to S 2.66. Measured C to R is 0.03 ohm high (0.95 - 0.92, about 3 percent) and C to S 0.09 ohm high (2.75 - 2.66, about 3 percent), consistent with windings still warmer than the shell. Nothing reads low, so no shorted turns.

Megohmmeter, 500 V, one minute: C, S and R each 1,500 megohm to ground. Insulation healthy.

Capacitor replaced, leads landed, cover on, power restored per step 8. The compressor started on the first call; running amps 13.8 A against an RLA of 16.0 A; 236 V at the contactor under load. The ticket recorded every reading above, the shell temperatures they were taken at, and the capacitor value, so the next tech can see the compressor was tested and passed rather than guessed at.

What would have changed the call

• C to ground 0.6 megohm after four hours of crankcase heat. Insulation, not liquid in the shell. Condemn and quote a compressor or a condenser, per the warranty and the system's age.
• S to R OL with the shell at ambient. An open start winding. No amount of cooling recloses that.
• C to R 0.62 ohm against a corrected 0.92. That is about 33 percent low, past the 10 percent suspect line; shorted turns. Confirm with a second meter, then condemn.
• A reading that changes when you wiggle a lead at the terminal block. That is a terminal or connector fault, often repairable, and the burnt spade is your evidence.

How to verify you got it right

Every reading on the ticket carries its temperature. The three resistances close (C to R plus C to S equals S to R). The megohmmeter readings are written per terminal, at 500 V, with the time held. Where an overload had opened, the ticket names the cause found and what was done about it. And the compressor ran under its RLA at nameplate voltage before you left, which is the only proof that what you reconnected is what you tested.

References

• NFPA 70E, Article 120 (120.5 in the 2021 edition, renumbered in 2024), current edition as your safety program adopts it
• 29 CFR 1910.333(b)(2) (general industry) and 29 CFR 1926.417 (construction), electrical lockout and tagging
• Compressor maker's data sheet and service bulletin for winding resistance, temperature basis and insulation floor
• See related: Capacitor and Contactor Replacement Standard