HVAC Technician Academy
Learn how to test single-phase compressor windings correctly and separate a grounded compressor, an open winding, and a compressor that is only temporarily open because its internal thermal protector has tripped.
With power removed and the compressor isolated, measure resistance between the compressor terminals and then from each terminal to a clean compressor shell or ground point. A grounded winding shows an unintended electrical path to the shell. An open winding shows no continuity through part of the winding circuit, but a hot compressor can also appear open temporarily if its internal overload has opened.
One of the easiest compressor misdiagnoses happens when a technician tests a very hot compressor and finds open readings between terminals.
The assumption: the winding is burned open.
But many hermetic compressors contain an internal thermal protector.
If compressor temperature rises high enough, that protector may open the common circuit.
The terminal readings can then look like an open compressor until the compressor cools.
A typical single-phase compressor has three terminals:
With the compressor isolated and at a stable temperature, the resistance relationship is typically:
C to R = Lowest Resistance
C to S = Next Higher Resistance
R to S = Highest Resistance
R-S ≈ C-R + C-S
Exact resistance values vary by compressor model, size, winding temperature, and design.
Measuring R to S places the run and start winding resistance in series through the common point.
That is why the R-S measurement should approximately equal the sum of the other two winding measurements.
If the relationship does not make sense, investigate the compressor, terminal identification, meter connection, and temperature before drawing a conclusion.
Typical relationship: Lowest resistance.
Reason: Run winding resistance.
Typical relationship: Higher resistance.
Reason: Start winding resistance.
Typical relationship: Highest resistance.
Reason: Run + start winding path.
A grounded compressor has an unintended electrical connection between a winding and the compressor shell or ground.
Test from:
A confirmed winding-to-shell path indicates insulation failure and is a serious compressor fault.
A standard digital multimeter can detect many hard shorts to ground.
But insulation can sometimes be deteriorating without producing an obvious low-resistance short on a standard meter.
When manufacturer procedures call for insulation-resistance testing, use the appropriate test instrument and follow compressor-specific guidance.
Do not apply insulation test voltages indiscriminately to connected electronic controls, inverter equipment, or other components that may be damaged by the test.
An open winding means the electrical path through part of the motor winding circuit has been interrupted.
Depending on the failure location, one or more terminal combinations may show no continuity.
Before condemning the winding, verify:
The compressor's internal overload protects the motor from damaging temperature or current conditions.
When it opens, the compressor may stop and winding measurements involving common may appear open.
Possible signs include:
Study: Compressor Thermal Overload Diagnosis
A thermal overload resetting does not mean the repair is complete.
Possible causes of compressor overheating include:
Continue: Compressor Overheating Guide
1. Disconnect power and verify de-energization.
2. Safely discharge applicable capacitors.
3. Isolate the compressor terminals from external wiring.
4. Inspect the compressor temperature. A very hot shell changes the interpretation of open readings.
5. Measure C-R resistance.
6. Measure C-S resistance.
7. Measure R-S resistance.
8. Compare the three readings. Verify that R-S approximately equals the other two added together.
9. Measure each terminal to clean shell or ground.
10. If open while hot, allow the compressor to cool as appropriate and retest.
11. If a fault is confirmed, identify the operating condition that contributed to the compressor failure.
Possible condition: Windings may be electrically intact.
Next step: Continue startup and performance diagnosis.
Possible condition: Grounded winding.
Next step: Confirm the test and follow the appropriate replacement procedure.
Possible condition: Internal thermal overload may be open.
Next step: Allow appropriate cooling and retest.
Possible condition: Failed winding or internal connection.
Next step: Confirm isolation, meter operation, and winding condition.
Possible condition: Starting circuit problem or mechanical lock.
Next step: Check capacitor, loaded voltage, LRA behavior, contactor, and start components.
Technician Case File
Outdoor fan is running.
Compressor is not operating.
Compressor shell is extremely hot.
Winding readings involving common appear open.
Initial conclusion: open compressor.
After the compressor cools, winding continuity returns.
Further diagnosis finds high head pressure caused by a heavily restricted condenser coil.
Lesson: The compressor was on thermal overload. The original open reading did not prove a burned-open winding.
Breaker trips immediately when compressor is energized.
Compressor is isolated.
Terminal-to-terminal resistance is checked.
Terminal-to-shell test shows a definite electrical path from one winding terminal to the compressor shell.
This is fundamentally different from a thermal-overload condition. The motor insulation has failed to ground.
A compressor can pass basic winding resistance and ground checks but still fail mechanically.
Possible examples include:
Winding resistance is one part of compressor diagnosis.
Continue: Compressor Diagnostics Training
If winding tests appear normal but the compressor does not start, verify:
Continue: Bad Capacitor vs Bad Compressor
A winding test should answer a specific electrical question. It does not replace compressor temperature checks, startup diagnosis, amp draw, voltage testing, and refrigeration-performance analysis.
1. Which terminal-to-terminal measurement is normally the highest resistance?
2. What should R-S approximately equal on a typical single-phase compressor?
3. Why can a very hot compressor appear electrically open?
4. What does continuity from a winding terminal to the compressor shell indicate?
5. Do normal winding resistance readings prove that a compressor is mechanically good?
1. R-S is typically the highest resistance.
2. R-S should approximately equal C-R plus C-S.
3. The internal thermal protector may have opened because of excessive compressor temperature.
4. It indicates a possible grounded winding or insulation failure and must be confirmed with proper testing.
5. No. A compressor can have electrically intact windings and still be mechanically locked, weak, or otherwise damaged.
Return to the complete compressor training hub.
Compressor Amp Draw →Interpret RLA, LRA, actual amps, and compressor load.
Locked Rotor vs Hard Start →Separate starting-circuit problems from a mechanically locked compressor.
Capacitor vs Compressor →Verify external starting components before condemning the compressor.
Grounded vs Open Guide →Review the existing technical compressor-winding resource.
Compressor winding testing requires access to high-voltage equipment and capacitors that may retain stored electrical energy. Disconnect and verify power, discharge applicable capacitors safely, isolate compressor wiring, use appropriately rated instruments, and follow manufacturer and lockout procedures. Testing should be performed only by qualified personnel.
Compressor winding readings must be interpreted with compressor temperature, voltage, starting components, amp draw, and system operating conditions before a final diagnosis is made.
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