Compressor motor protection trips

Troubleshooting · Applies to: Direct expansion refrigeration system · Heat pump and air conditioning · CO2 booster (transcritical) · Liquid chiller (water or brine) · Ammonia with pump circulation (R717) · Cold rooms with several evaporators

Motor protection trips: voltage and phase imbalance, overload, an overheated motor, too many starts and winding faults.

Last checked: October 25, 2026 Field mode: large buttons, questions first and the screen kept on.

The motor protection opens when the motor draws too much current or gets too hot. Low or uneven voltage raises the current and can make the internal protector open ^[1]. Measure voltage and current with the compressor running, and let the motor cool down before trying again: an internal protector may need an hour or more to reset.

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Find out which protection trips: the motor protective breaker, the internal thermal protector, an electronic protection module or the fuse. Measure the current on all phases at start and in operation, and the voltage during start. Measure the resistance through the thermal protector (PTC circuit) once the motor has cooled.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1
Flowchart A
Supply fault: low voltage, a lost phase, phase imbalance or a loose connection.
Remedy: Measure the voltage at the compressor during start and operation. Tighten terminals, check the contactor and fuses, and have an electrician fix faults in the supply.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Measure voltage and current under load
Low voltage, phase imbalance or high current make motors overheat and trip. Measure at standstill, during start and in operation and compare with the nameplate. Live measurements in an open panel are for qualified personnel only.
1. Prepare for live measurement. Use a CAT III/IV multimeter with insulated probes and a true-RMS clamp meter with inrush function. Wear safety glasses, no loose clothing, and keep hands clear of fans and belts that can start automatically.
  ✓ Check: Meter and leads are CAT III or CAT IV rated and the meter works on a known source.
2. Measure the supply at standstill. Measure L1–L2, L2–L3 and L3–L1 (and phase to neutral where used) at the incoming terminals with the compressor stopped.
  ✓ Check: All voltages are within the range on the nameplate or data sheet (many compressors allow ±10 % of rated voltage).
3. Measure voltage during start and in operation. Measure the same phases at the compressor contactor while the compressor starts and again after a few minutes of running. A large drop during start points to a weak supply, undersized cables or a loose connection.
  ✓ Check: The voltage stays within the permitted range also during start and in operation.
4. Calculate the voltage imbalance. Imbalance (%) = largest deviation from the average of the three phase-to-phase voltages ÷ average × 100.
  ✓ Check: The imbalance is within the compressor manufacturer's limit (often about 2 %).
5. Measure the current on each phase. Clamp each phase conductor separately: the inrush current at start and the running current after the system has stabilised.
  ✓ Check: The running current does not exceed the rated load amps (RLA) on the nameplate and is about equal on all three phases.
6. Record and act. Write voltages, imbalance and currents in the service report. High voltage drop: tighten and check connections under isolation. Imbalance or high current: trace the supply or the motor before restarting repeatedly.
  ✓ Check: Readings are documented and any fault is traced to supply, connection or motor.
Test a contactor
Worn, welded or loosely connected contactor contacts cause single-phasing, overheating and motor damage. Check the contacts, the coil and the voltage drop, and replace a worn contactor. Electrical work is for qualified personnel only.
1. Isolate and lock off. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all line and load terminals, also the coil supply, which can come from a separate control circuit.
  ✓ Check: 0 V on all poles, the coil terminals and to earth.
2. Inspect the contacts and the mechanism. Remove the arc cover if the design allows. Look for heavily pitted, burnt or welded contacts, and discoloured terminals or melted plastic. Press the armature by hand (power off) and check that it moves freely and springs back.
  ✓ Check: Contacts are without heavy pitting or welds, and the armature moves freely.
3. Check the coil. Disconnect one coil lead and measure the coil resistance. Compare with the manufacturer's data or a new contactor of the same type.
  ✓ Check: The coil is neither open (OL) nor near 0 Ω.
4. Check the coil voltage and pull-in. Reconnect, remove your lock, restore power and use the CAT III/IV meter to measure the voltage across the coil when the controller calls for operation.
  ✓ Check: The coil voltage matches the rating printed on the coil, and the contactor pulls in firmly without humming or chattering.
5. Measure the voltage drop across each pole. With the motor running, measure from line to load terminal across each closed pole.
  ✓ Check: The drop is close to 0 V and about equal on all poles; a pole with clearly higher drop has worn contacts.
6. Tighten or replace. Isolate and lock off again. Tighten all terminals to the manufacturer's torque. Replace a worn contactor with one of the same coil voltage and at least the same motor rating (AC-3), then restart and measure the current on each phase.
  ✓ Check: Terminals are tight, and the motor current is about equal on all phases after restart.
2
Flowchart B
Overload: the compressor works against too high a pressure.
Remedy: Troubleshoot the high pressure. Also check that the motor protection is set to the correct current.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check motor protection setting and trips
Motor protection stops the compressor or fan before the motor burns. A wrong setting causes needless trips or no protection at all, and a protection that is reset again and again without finding the cause destroys the motor.
1. Read the trip and the alarm log. Note which protection has tripped (overload relay, motor protection breaker, electronic module or inverter) and read the alarm log in the controller for time and number of trips.
  ✓ Check: You know which device tripped, when and how often.
2. Check the setting against the nameplate. Compare the current setting of the overload relay or motor protection breaker with the value the compressor or motor manufacturer gives (for example the maximum continuous current on the nameplate or in the data sheet). Never set it higher than that value.
  ✓ Check: The setting agrees with the manufacturer's value.
3. Read the electronic protection module. On compressors with a thermistor protection module (for example Kriwan INT69), read any diagnosis LED or flash code with the module's manual. Check that its supply voltage is correct.
  ✓ Check: The module shows normal, or the code is identified in the manual.
4. Measure current in operation. With the protection reset and the cause not obviously dangerous, start the unit and measure the current on every phase with a clamp meter (CAT III/IV, insulated, no loose clothing). Compare with the nameplate and check that the phases are balanced.
  ✓ Check: The current is below the protection setting and nearly equal on all phases.
5. Count the starts. Count starts per hour from the controller log or by watching. Compare with the maximum starts per hour in the compressor data. Too many starts point to short cycling (setpoint, differential or low pressure switch).
  ✓ Check: The number of starts is within the compressor manufacturer's limit.
6. Find the cause before resetting again. Look for the cause of the trip: high condensing pressure, low or unbalanced voltage, a lost phase, a mechanical fault or a winding fault. Isolate and lock off before opening terminals. Reset only when the cause is found and fixed.
  ✓ Check: The cause is found and corrected, and the unit runs without new trips.
Continue with: System trips on high pressure →
3
Flowchart C
The motor is poorly cooled: too little suction gas, often because of a refrigerant shortage or a valve that underfeeds.
Remedy: Measure superheat and troubleshoot high discharge temperature. Never add refrigerant without leak testing first.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Measure superheat
Superheat shows whether the evaporator is fed with the right amount of refrigerant: too high starves the evaporator, too low risks liquid reaching the compressor. Measure at the evaporator outlet (for the expansion valve) and at the compressor (to protect it).
1. Let the system stabilise. Run at normal load for at least 10–15 minutes, not during or right after defrost. Wear goggles and gloves (cold pipes can cause frostbite); with A2L/A3 refrigerants keep ignition sources away.
  ✓ Check: Suction pressure and pipe temperature no longer drift.
2. Read the suction pressure. Connect the gauge to a port at the evaporator outlet if there is one; a port at the compressor shows a lower pressure because of the pressure drop in the suction line. Select the correct refrigerant.
  ✓ Check: The gauge shows a stable pressure, and you know where it was measured.
3. Measure the pipe temperature. Clamp a pipe thermometer on clean bare pipe next to the expansion valve bulb (evaporator superheat) and on the suction line about 15–30 cm before the compressor (compressor superheat), insulated from the surrounding air.
  ✓ Check: Both temperatures are stable.
4. Calculate superheat. Convert the pressure to the saturation temperature; for zeotropic blends use the dew point. Superheat = measured pipe temperature − saturation temperature.
  ✓ Check: The result is positive and is compared with the manufacturer's target; for a TXV at the evaporator outlet it is often around 5–8 K.
5. Interpret before adjusting. High superheat together with bubbles in the sight glass or low subcooling points to low charge or a restriction, not a valve setting. Very low compressor superheat means a risk of liquid floodback. Check charge, sight glass and filter drier before touching the valve.
  ✓ Check: The cause is supported by at least one other reading.
6. Adjust only when the cause is the valve. Adjust a TXV in small steps (about a quarter turn) and wait 15–20 minutes between steps; on an electronic valve change the setpoint in the controller. Record the values before and after.
  ✓ Check: Superheat settles at the target and stays stable during a full cycle.
Leak test a refrigeration system
Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).
1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.
  ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.
2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.
  ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.
3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.
  ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.
4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.
  ✓ Check: A fluorescent trace marks the leak, or no trace is visible.
5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.
  ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.
6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.
  ✓ Check: The logbook entry is complete and a follow-up check is booked.
Continue with: High discharge temperature or hot compressor →
4
Flowchart D
Too many starts: the motor does not cool down between starts.
Remedy: Troubleshoot short cycling and set minimum run and off times as the manufacturer specifies.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check controller settings and alarm log
Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.
1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.
  ✓ Check: You have a list of alarms with times and know what each code means.
2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.
  ✓ Check: The controller clock shows the correct local date and time.
3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.
  ✓ Check: The service report holds a complete copy of the original settings.
4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.
  ✓ Check: Every deviation is noted with the original and the recommended value.
5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.
  ✓ Check: All safety limits are within the manufacturer's values.
6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.
  ✓ Check: The system runs as expected and the alarm does not return.
Continue with: Compressor short cycling →
5
The compressor starts against high pressure after a short stop: the restart delay or pressure equalisation does not work.
Remedy: Check the restart delay in the controller and, where fitted, the start unloading.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check controller settings and alarm log
Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.
1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.
  ✓ Check: You have a list of alarms with times and know what each code means.
2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.
  ✓ Check: The controller clock shows the correct local date and time.
3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.
  ✓ Check: The service report holds a complete copy of the original settings.
4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.
  ✓ Check: Every deviation is noted with the original and the recommended value.
5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.
  ✓ Check: All safety limits are within the manufacturer's values.
6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.
  ✓ Check: The system runs as expected and the alarm does not return.
Check capacity control
Capacity control makes the plant follow the load. If it does not work, suction pressure swings, compressors short cycle and wear out.
1. Read the controller settings. Note the suction pressure (or temperature) setpoint, neutral zone, number of steps, speed range and the minimum run and off timers. Compare the timers with the compressor maker's limit for starts per hour.
  ✓ Check: Settings are noted and the timers respect the maker's start limit.
2. Check the control sensor. Compare the suction pressure transducer that the controller uses with a calibrated gauge at the same point.
  ✓ Check: The controller reading agrees with the gauge.
3. Follow the control over time. Log suction pressure, capacity steps or speed and compressor starts over at least 30 minutes, or read the controller's trend log.
  ✓ Check: Suction pressure stays within the neutral zone and steps change in order without rapid on-off.
4. Test each step in manual mode. In the controller's manual or test mode, switch each unloader or capacity solenoid. Measure coil voltage with a CAT III/IV meter and watch current and suction pressure. Check in the manufacturer's data whether an energised coil loads or unloads.
  ✓ Check: Each step gives a clear change in current and suction pressure in the expected direction.
5. Check speed and digital control. For an inverter compressor, check that the speed follows the controller signal. For a digital scroll, check the modulation signal and the solenoid.
  ✓ Check: Speed or modulation follows the demand over the whole range.
6. Count starts. Count compressor starts per hour from the log and compare with the manufacturer's maximum.
  ✓ Check: Starts per hour are below the manufacturer's limit.
6
Flowchart E
Fault in the motor protection (or set below the compressor's rated current), the starting equipment or the motor winding, or a mechanical fault that raises the current.
Remedy: Check the protection setting against the nameplate. With the power isolated and locked off, measure winding resistance (equal between phases) and insulation resistance to earth. Replace a faulty protector or start relay; after an earth fault or burnt windings, replace the compressor and clean the system ^[2].
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check motor protection setting and trips
Motor protection stops the compressor or fan before the motor burns. A wrong setting causes needless trips or no protection at all, and a protection that is reset again and again without finding the cause destroys the motor.
1. Read the trip and the alarm log. Note which protection has tripped (overload relay, motor protection breaker, electronic module or inverter) and read the alarm log in the controller for time and number of trips.
  ✓ Check: You know which device tripped, when and how often.
2. Check the setting against the nameplate. Compare the current setting of the overload relay or motor protection breaker with the value the compressor or motor manufacturer gives (for example the maximum continuous current on the nameplate or in the data sheet). Never set it higher than that value.
  ✓ Check: The setting agrees with the manufacturer's value.
3. Read the electronic protection module. On compressors with a thermistor protection module (for example Kriwan INT69), read any diagnosis LED or flash code with the module's manual. Check that its supply voltage is correct.
  ✓ Check: The module shows normal, or the code is identified in the manual.
4. Measure current in operation. With the protection reset and the cause not obviously dangerous, start the unit and measure the current on every phase with a clamp meter (CAT III/IV, insulated, no loose clothing). Compare with the nameplate and check that the phases are balanced.
  ✓ Check: The current is below the protection setting and nearly equal on all phases.
5. Count the starts. Count starts per hour from the controller log or by watching. Compare with the maximum starts per hour in the compressor data. Too many starts point to short cycling (setpoint, differential or low pressure switch).
  ✓ Check: The number of starts is within the compressor manufacturer's limit.
6. Find the cause before resetting again. Look for the cause of the trip: high condensing pressure, low or unbalanced voltage, a lost phase, a mechanical fault or a winding fault. Isolate and lock off before opening terminals. Reset only when the cause is found and fixed.
  ✓ Check: The cause is found and corrected, and the unit runs without new trips.
Measure compressor windings and insulation
Winding resistance shows whether a motor winding is open, shorted or unbalanced, and insulation resistance shows whether it leaks to earth. Together they tell you whether the compressor motor is faulty before you replace it.
1. Isolate, lock off and prove dead. Switch off and lock the isolator, put up a tag and prove dead with a two-pole tester between all phases and between each phase and earth at the compressor terminals.
  ✓ Check: The tester shows 0 V on every combination.
2. Discharge capacitors and disconnect. Discharge any start or run capacitor through a resistor (about 20 kΩ, 5 W, insulated leads), never with a screwdriver. Photograph the wiring, then remove all cables from the compressor terminals. Disconnect any inverter and electronic protection module.
  ✓ Check: The meter shows 0 V DC across the capacitor terminals and the compressor terminals are free.
3. Let a hot compressor cool. If the compressor is hot, an internal protector may be open. Measure the shell temperature; let it cool (it can take an hour or more) before judging the windings.
  ✓ Check: The shell is close to room temperature, or you have noted its temperature.
4. Measure the winding resistance. Use the low-ohm range. Single-phase: measure C–R, C–S and R–S; C–R plus C–S should equal R–S. Three-phase: measure between the three terminal pairs; the values should be nearly equal. Compare with the compressor data sheet at the measured temperature. Open between C and both R and S while R–S reads normal points to an open internal protector.
  ✓ Check: The readings agree with the data sheet and with each other; OL means an open winding or protector.
5. Measure the insulation resistance. Never megger under vacuum or with electronics connected. Connect the insulation tester (500 V DC, or the voltage the manufacturer gives) between each terminal and a clean earth point on the compressor body. Liquid refrigerant in the shell lowers the reading; run the crankcase heater and measure again before condemning the motor.
  ✓ Check: The reading is above the minimum in the compressor manufacturer's data; a reading near zero means the winding is earthed.
6. Reconnect or replace. If the motor is good, reconnect from your photo, refit the cover and test run while measuring the current. If it is faulty, plan a compressor replacement and test the oil for acid.
  ✓ Check: The compressor runs at nameplate current, or the fault is recorded and a replacement planned.
Replace a compressor and clean up after burnout
After a motor burnout the system holds acid, moisture and carbon. A new compressor fitted without a proper clean-up fails again. The work opens the refrigerant circuit and requires F-gas certification.
1. Confirm the fault. Confirm that the compressor is faulty: measure winding and insulation resistance (with the power isolated, locked off and proven dead), and test the oil for acid to find out whether there was a burnout.
  ✓ Check: The fault is confirmed and you know whether the oil is acidic.
2. Recover the refrigerant. Recover the refrigerant into a marked recovery cylinder (gloves and goggles; burnt oil is acidic). For flammable refrigerants (A2L/A3), remove ignition sources and ventilate. Record the quantity.
  ✓ Check: The system is at the final recovery pressure and the quantity is recorded.
3. Remove the old compressor. Get a hot work permit and have an extinguisher at hand. Cut or unbraze the connections with nitrogen flowing. Lift with lifting gear. Keep the old oil for inspection and seal the pipe ends.
  ✓ Check: The compressor is out and the open pipes are sealed.
4. Fit the new compressor and filters. Check that the new compressor has the right model and oil type and the correct oil quantity. Fit it with new mountings, a burnout filter drier in the liquid line and, after a severe burnout, a suction filter. Braze with nitrogen flowing gently.
  ✓ Check: The compressor and filters are fitted and all joints are brazed with nitrogen flowing.
5. Pressure test and evacuate. Pressure test with dry nitrogen, leak test, then evacuate to the target vacuum and do a decay test.
  ✓ Check: The system holds the test pressure and passes the decay test.
6. Charge, start and check. Charge the nameplate quantity by weight. Check the wiring, contactor and motor protection setting before starting. Run the system and check superheat, subcooling and current.
  ✓ Check: Superheat, subcooling and current agree with the design data.
7. Follow up the clean-up. After some operating time, test the oil for acid again and check the pressure drop over the suction filter. Change filter drier and oil until the acid test passes; remove or replace the suction filter as its manufacturer says. Record everything in the logbook.
  ✓ Check: The acid test passes and the logbook shows the work, the quantities and the follow-up.

Checks from the manufacturers’ manuals (32)

Values and checks the manufacturers give for their own products; they apply to the product named.

Kriwan INT69 Y Diagnose: the cold motor PTC chain should total below about 1.8 kΩ; the relay trips at about 4.5 kΩ, resets at about 2.75 kΩ, and below about 30 Ω counts as a short circuit.^[3]
Kriwan INT69 Diagnose: the first flash group gives the area, the second (orange) the detail; red = motor shut down, green = warning only, so 1 red then 1 orange means winding temperature exceeded.^[4]
red ● → orange ● · 0.35 s on, 0.35 s off · 0.8 s between colours · pause 1.5 s
Copeland Stream 4M/6M and CO2 4MTL/4MSL: the two motor PTC chains together should read no more than about 1800 Ω on a cold compressor; never measure them with more than 3 V.^[5]
Danfoss Maneurop MT/MTZ (3-phase): the internal star-point protector cuts all three phases; after a trip it can take up to about 3 hours to cool to about 60 °C and close again.^[6]
Danfoss VLZ scroll with CDS drive: for earth fault alarm A14 or A45, disconnect the motor cable and megger the compressor at 500 V; below about 500 kΩ the compressor is at fault, above it the drive.^[7]
Hanbell RC2/LB screw: a motor thermistor trip points to low suction pressure or flow, a blocked suction filter, high suction superheat (keep under about 10 K) or no liquid injection to the motor; reset by hand.^[8]
Carlyle 06D/06E/06CC cycling on locked rotor: measure voltage on all three legs at the contactor, check for winding short or ground, and on part-winding start the second contactor and time-delay relay.^[9]
Copeland ZF/ZB K5 scrolls with INT69: Kriwan INT69 motor protector switched by a PLC: the module relay contacts M1–M2 need a load current above about 100 mA (max 5 A); with less, the contact resistance rises and gives false compressor trips.^[10]
Copeland ZF/ZB K5 scrolls with INT69: Kriwan INT69 module: measure the voltage across M1–M2. Equal to the control voltage means the contacts are open; below 1 V with the compressor stopped means the fault is outside the module; between 1 V and the control voltage means the module is faulty. After a thermistor trip the module waits 30 minutes before it resets.^[10]
Danfoss DHP heat pumps: Measure the current in all three phases after the soft starter or contactor; the difference between phases should not exceed about 12 %. Also check that the soft starter receives its control signal (A1–A2) and passes all three phases.^[11]
Tecumseh hermetic compressors with external thermal protector: An external line-break overload trips although the compressor is fine because extra loads (condenser fan, pump) are wired through the protector, so it sees their current too. Check the wiring diagram against what is actually connected.^[12]
Danfoss DHP heat pumps: With a soft starter, check that all three phases come out to the compressor when the control signal is present on A1–A2 (single-phase starters: ON–N); one missing phase makes the compressor buzz, run noisily and trip. Measure the trip current with a clamp meter and compare with the protector setting.^[13]
Copeland ZSI scroll units; Danfoss Maneurop MT/MTZ: Measure the voltage at the compressor terminals while it runs: it should be within about ±10 % of the nameplate (Copeland), and the three line-to-line voltages should differ by no more than 2 % from their mean (Danfoss MT/MTZ). A small voltage unbalance gives a much larger current unbalance and overheats one winding.^[14]
Danfoss VLZ scroll with CDS drive: Inverter-driven compressor that stops on 'motor phase missing' or 'mains failure' alarms: usually neither drive nor compressor is faulty. Check the motor cable and terminals, and whether another unit on the same supply pulls the voltage down when it starts.^[15]
Danfoss VLT Refrigeration Drive FC 103: Frequency drive with more than about 3 % current imbalance: move all three input leads one position (A→B, B→C, C→A). If the high phase follows the wire the supply is at fault, if it stays on the same terminal the drive is. Do the same with the motor leads (U→V→W) to separate motor/cable from drive.^[16]
Bitzer SE-B/SE-E/SE-i compressor protection devices: Electronic compressor protection with phase monitoring checks phase sequence and phase failure a few seconds after start. With a soft starter or inverter whose ramp is longer than that delay, the device sees a missing phase and stops the compressor, and locks out if it repeats. Reset by cutting its supply for at least 5 s, after finding the cause.^[17]
Tecumseh hermetic compressors: After a motor burnout, judge how far the contamination went: a clean discharge line and only light deposits in the suction stub mean it stayed in the compressor; dirt or acid in the discharge or suction line means it went round the system. Then fit oversized liquid and suction filter driers and replace the expansion device (and the four-way valve on a heat pump).^[18]
Tecumseh hermetic; Copeland Stream CO2 4MTL/4MSL: Measure the pressure drop across the temporary suction filter drier after about an hour and keep replacing both driers until it stays low. Copeland recommends a 100 % activated alumina suction drier for clean-up, removed after 72 hours, and replacing the suction accumulator, whose oil return hole or screen clogs and starves the new compressor of oil.^[19]
Copeland notes that most in-warranty electrical failures start as mechanical problems (particles or liquid refrigerant in the oil) caused by the system. Unless that cause is found (floodback, migration, poor oil return, contamination), the replacement compressor will fail too.^[20]
Dorin semi-hermetic compressors: Capacity control heads (unloaders) must not be used on a compressor already running on an inverter: at low speed and unloaded the motor gets too little suction gas cooling, and the uneven piston load causes vibration.^[21]
Hussmann plug-in merchandisers: Plug-in cabinets: with the compressor running, the voltage at the cabinet must not be more than 5 % below the rated voltage; lower voltage makes the compressor cycle on its overload and the cabinet runs warm.^[22]
Kold-Draft 60 series ice machines: On single-phase PSC compressors a failed run capacitor overheats the motor so the internal thermal protector opens repeatedly; test the capacitor and measure the voltage at the contactor with the compressor running.^[23]
Temperzone units with Carel Power+ inverter: Repeated inverter faults from the supply: generator power (especially at generator start), new or overloaded grids, lost phases, low or high voltage, and hot joints that add resistance on one phase. Check every termination up to the drive.^[24]
Temperzone units with Carel Power+ inverter: An earth-fault alarm on an inverter compressor often comes from damaged compressor leads rather than the motor itself; inspect and test the leads as well as the compressor windings before condemning the compressor.^[24]
Copeland scrolls with compressor electronics module: Compressors with a motor PTC chain: pull the sensor plug and measure the chain. On Copeland's module it trips above about 4.5 kohm and resets only below 2.75 kohm after 30 minutes; five trips in a row lock the compressor out until power is cycled.^[25]
Actron Air Aires/Advance with VSD; Toshiba/Carrier USX Edge: Inverter drive overheats (IPM protection): loose IPM screws or poor contact with the heat sink, a blocked cooling air path or a failed drive cooling fan, or high condensing pressure from a dirty condenser.^[26]
Fujitsu inverter split and multi-split air conditioners / heat pumps: On Fujitsu inverter splits an IPM protection error (17) is often a symptom, not a PCB fault: a stopped outdoor fan, blocked air inlet or outlet at the outdoor unit, or a wrong refrigerant charge makes the inverter current rise until the power module trips.^[27]
Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): RUA-NP compressor protection: the internal thermostat opens at 130 °C and closes at 108 °C (SEG1 C1/C2 08), and the current transformer trip is set per model at 8.3 A (NP4), 10.6 A (NP5), 13.0 A (NP6), 18.0 A (NP8), 10.6/8.3 A (NP9) and 10.6 A (NP10) (SEG1 C1/C2 39); both reset automatically and cannot be adjusted. Compare the measured running current with these values and check the phase balance before replacing parts.^[28]
Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): RUA-NP fan motor protection: the evaporator fan overcurrent relay is adjustable and must be reset by hand, factory set to 1.2 A (NP4), 1.3 A (NP5), 1.7 A (NP6, NP8) and 3.6 A (NP9, NP10) (code 19); the condenser fan motors have internal thermostats opening at 160 °C and closing at 130 °C (codes F1/F2 57). The control circuit fuse is 2.5 A on all models.^[29]
NTF SM flat flake ice makers (SM500–SM11000): NTF SM flakers: the gear reducer motor that turns the scraper has an internal thermal protector that stops the machine and lets it restart when the motor has cooled. If it trips, check that the gear motor and scraper turn freely; if the gear motor does not run at all, check its capacitor.^[30]
Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II IPM test on the inverter PCB (compressor and ACTPM wires disconnected): resistance from P (yellow) and N (blue) to U, V and W should be several MΩ at all 6 points; a few kΩ or a short means a faulty module. In diode mode all 6 readings should be about 0.3–0.7 V; below 0.1 V is faulty.^[31]
Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II trip detection E.94.1 is set after 5 overcurrent stops in a row after compressor start (the count resets if no new trip comes within 40 s of a restart). It is often not the board: check first for blocked air passages, dirty fins, a faulty outdoor fan motor, other heat sources or recirculated discharge air, then the inverter PCB, the compressor and the SV2 coil.^[32]

📝 Fill in a service report

Calculate it yourself

Electrical calculator →

Terms in this article

Superheat Hermetically sealed equipment Compressor burnout

Sources

ACHR News – Tips for testing and diagnosing faulty compressor motors
Danfoss Ref Tools – Troubleshooter: compressor
Kriwan – INT69 Y Diagnose data sheet, 710.00406.2 (undated; checked 2026-10-28)
Kriwan – INT69 PYF Diagnose data sheet, 710.00721.0 (undated; checked 2026-10-28)
Copeland – Application guidelines Stream CO2 semi-hermetic compressors 4MTL/4MSL, Rev04 (2025)
Danfoss – Application guide Maneurop reciprocating compressors MT and MTZ, AB196386425654en-011901 (2026.07)
Danfoss – Application guidelines scroll compressors VLZ065 with CDS drive, AB444226142054en-000101 (2023.05)
Hanbell – RC2 technical manual RC2-AVI (2021), section 6.2; LB technical manual, section 4.3
Carlyle – Service Guide 06D, 06E and 06CC Semi-Hermetic Reciprocating Compressors, Lit No. 020-611 Rev. F (2/2026)
Copeland – Application bulletin AE4-1383 R15, ZF*K5E/ZB*K5E scrolls (2017), p. 11
Danfoss – Heat Pump Troubleshooting Guide (air and ground source heat pumps), VMIFB202, PDF p. 8 and p. 14
Tecumseh – Electrical Service Parts Guide Book, AF-110 Rev 1 (05/2017) | Tecumseh – Compressor Service Handbook (hermetic compressors), Tecumseh: p. 104–105 | Tecumseh: p. 51–55
Danfoss – DHP-A/AL/C/H service instructions VMGFC302, p. 38–39
Copeland – ZSI scroll condensing units user manual 8504574 | Danfoss – Maneurop MT/MTZ Application Guide AB196386425654en-011901 (2026.07), Copeland: p. 23 | Danfoss: p. 48
Danfoss – VLZ scroll with CDS drive AB444226142054en-000101, p. 45–46
Danfoss – VLT Refrigeration Drive FC 103 Instruction Manual MG16E322 (2014-04-10), p. 46, table 7.5
Bitzer – Technical information CT-120-3, compressor protection devices, p. 49–51 (CT-120-3)
Tecumseh – Compressor Service Handbook (hermetic compressors), p. 45–55
Tecumseh – Compressor Service Handbook (hermetic compressors) | Copeland – Application Guidelines Stream 4MTL/4MSL, AGL_Stream_ST_4MTL_4MSL_EN_Rev04, Tecumseh: p. 84–86 | Copeland: p. 35 (AGL_Stream_ST_4MTL_4MSL_EN_Rev04)
Copeland – Application Guidelines, heat pump scroll XHV variable speed (AGL_HP_VS_XHV_EN_Rev00), p. 30 (AGL_HP_VS_XHV_EN_Rev00)
Dorin – Technical bulletin 1LTZ110_BT_010_03_2025 (inverter operation), sect. 15 (1LTZ110_BT_010_03_2025)
Hussmann – NAV/NAVC Service Deli Merchandisers I&O 2400204_J; Hussmann – GSVM Open Vertical Merchandisers I&O 0545716_E, Troubleshooting guide, p. 5-1/5-2 (GSVM p. 5-3)
Kold-Draft – 60 Series Service Manual rev 04/13/17, Troubleshooting, p. 46–47
Temperzone – Service Training Manual, Issue 08, January 2023, pp. 283–285
Copeland – Bulletin AE8-1384 R9, compressor electronics communications (July 2026), pp. 8 and 11
Actron Air – Troubleshooting Guide Advance / Aires / VCC / Genesis, Doc. 9590-2015-03 Ver. 12 + Toshiba/Carrier – Owner's manual Universal Smart X Edge heat pump chiller, 99NA30KQAISEN421 (2025-06-21), Actron p. 29; USX pp. 79-80
Fujitsu General – Troubleshooting Guide 2019 (manufacturer's service document), US models, Wall Mounted table: code 17 (5 + 2 flashes)
Hitachi – Self-Contained Air Conditioners RUA-NP4ATS…RUA-NP10ATS (R410A), Installation, Operation and Maintenance Instructions Manual 65MP3197-A2 (Bosch Home Comfort Philippines); undated, PDF file created 2025, 6.5 Safety and Control Device Setting, p. 12; 3.3.6 alarm codes 08 and 39, p. 7
Hitachi – Self-Contained Air Conditioners RUA-NP4ATS…RUA-NP10ATS (R410A), Installation, Operation and Maintenance Instructions Manual 65MP3197-A2 (Bosch Home Comfort Philippines); undated, PDF file created 2025, 6.5 Safety and Control Device Setting, p. 12; 3.3.6 alarm codes 19 and 57, p. 7
NTF – Technical Documentation, NTF ice makers families (SL, IC, CM, FRM/IW/DSS/ID, SLF/SLT, CVC/CV, GM/MGT, IFT, SM), 2019 (sections dated 2006–2018), Electric circuit, page 144; Cold flakers series – Gear motor thermal protection, page 148; Gearmotor doesn't run, page 152
Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Service parts information 5, p. 04-88 (undated; copy archived 2026-11-03)
Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 35, p. 04-45 (undated; copy archived 2026-11-03)

Compressor motor protection trips

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Measure voltage and current under load

Test a contactor

Check motor protection setting and trips

Measure superheat

Leak test a refrigeration system

Check controller settings and alarm log

Check controller settings and alarm log

Check capacity control

Check motor protection setting and trips

Measure compressor windings and insulation

Replace a compressor and clean up after burnout

Checks from the manufacturers’ manuals (32)

Service report

Calculate it yourself

Terms in this article

Sources