Compressor is noisy or vibrates

Causes, most likely first

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

1. 1
   Flowchart C

   Mechanical vibration transmitted through pipes and frame.

   Remedy: Tighten mountings, replace worn vibration dampers and make sure pipes do not touch. Fit rubber-lined pipe clamps where pipes vibrate.

   🛠 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.

   Fix vibration and pipe noise

   Vibration from compressors and fans fatigues pipes until they crack and leak. Find the source, fix the mounts and clamps, and leak test afterwards.

   1. Find the source while running. With the plant running, look and listen from a safe distance for where the vibration and noise are worst: compressor, fan or a pipe that touches something. Keep hands away from moving parts.

      ✓ Check: You know which part vibrates and where pipes touch or rub.

   2. Isolate and lock off. Isolate, lock off and prove dead before touching compressor mounts, fans or nearby pipes.

      ✓ Check: The meter shows 0 V and the compressor and fans cannot start.

   3. Check the compressor mounts. Check the rubber mounts for cracks and hardening, and that the transport locks or bolts are loosened or removed as the compressor manufacturer specifies.

      ✓ Check: The compressor rests on intact mounts and is not bolted rigidly.

   4. Check the vibration eliminators. Check braided vibration eliminators for broken wires and twisting. They must be straight, not under torsion, and fitted as the manufacturer specifies.

      ✓ Check: The eliminators are straight, undamaged and correctly fitted.

   5. Fit clamps and supports. Fit rubber-lined clamps and add supports so that pipes do not touch each other, the frame or the wall.

      ✓ Check: No pipe has bare metal-to-metal contact and none moves when the plant starts.

   6. Test run and leak test. Remove your lock, start the plant and check the vibration again. Leak test joints and bends in the vibrating section.

      ✓ Check: The vibration is reduced and no leaks are found.

2. 2

   The noise comes from a fan: bearings, imbalance or a blade touching.

   Remedy: Switch off the fan briefly to confirm. Replace bearings or motor, clean and balance.

   🛠 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 a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

3. 3
   Flowchart A

   Liquid refrigerant reaches the compressor and hits the cylinders or scroll (liquid slugging).

   Remedy: Find out why: an overfeeding expansion valve, an iced evaporator or refrigerant that has migrated to the crankcase during standstill. Check the crankcase heater.

   🛠 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.

   Test the crankcase heater

   The crankcase heater keeps refrigerant from condensing in the compressor oil at standstill. A broken or loose heater leads to foaming oil and liquid slugging at start.

   1. Isolate, lock off and prove dead. Switch off and lock the isolator and prove dead at the heater terminals. The heater and the compressor shell can be hot.

      ✓ Check: The voltage tester shows 0 V on the heater terminals.

   2. Measure the heater resistance. Disconnect one heater lead and measure the resistance. Work out the expected value from the rating: R = V² / P (for example 230 V and 50 W gives about 1060 Ω). Self-regulating (PTC) heaters show a different value; compare with their data sheet.

      ✓ Check: The resistance is close to the calculated or data-sheet value; OL means the heater is broken.

   3. Check the insulation to earth. Measure between the heater leads and the compressor body. A heater with a damaged sheath often leaks to earth and trips the earth-fault breaker.

      ✓ Check: The meter shows no connection (OL) between the leads and earth.

   4. Check the position. Check that the heater sits where the compressor manual says, usually tight around the lower shell near the oil sump, with full contact and the clamp tightened.

      ✓ Check: The heater has full contact with the shell at the position in the manual.

   5. Check that it is on at standstill. Reconnect, restore power and keep the compressor stopped. Measure the voltage at the heater and its current (I = P / V). After a while, check the shell temperature with a contact thermometer.

      ✓ Check: The heater has voltage and draws about its rated current at standstill, and the shell bottom is warmer than the surroundings.

   6. Check the control. On units where the heater switches off while the compressor runs, check that it switches on again when the compressor stops.

      ✓ Check: The heater is on whenever the compressor is stopped.

   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.

   Continue with: Liquid floodback to the compressor →

4. 4
   Flowchart B

   Poor lubrication: lack of oil or oil foaming with refrigerant.

   Remedy: Follow the troubleshooting for low oil level. Never add oil before you know where it has gone ^[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 oil level and oil pressure

   Too little oil, or too little oil pressure, wears out a compressor quickly. Check the level in stable operation, measure the net oil pressure on compressors with an oil pump, and find out where the oil went before topping up.

   1. Read the oil level in stable operation. Let the compressor run stable for at least 15 minutes and read the level in the sight glass. Compare with the marks or the range in the compressor manual. Foaming oil points to refrigerant in the oil.

      ✓ Check: The level is within the range in the manual and the oil is not foaming.

   2. Measure the net oil pressure. On compressors with an oil pump, connect a gauge to the oil pump outlet and one to the crankcase or suction side. Net oil pressure = oil pump pressure − crankcase pressure. Wear goggles and gloves; the pipes can be hot.

      ✓ Check: The net oil pressure agrees with the compressor manufacturer's value.

   3. Check the oil pressure switch. Check that the oil differential pressure switch is set as the compressor manual says and that its time delay is correct. If the procedure in the manual allows, test that it stops the compressor when the net oil pressure is too low.

      ✓ Check: The switch is set as in the manual and stops the compressor after its time delay.

   4. Find the cause of a low level. Before adding oil, look for the cause: oil trapped in the evaporator or suction line, a faulty oil separator return or level regulator, a leak, or frequent short cycles.

      ✓ Check: The cause of the low level is found, or the level was only low after a recent repair.

   5. Top up with the right oil. Use only the oil type on the compressor nameplate (POE, PAG, mineral or PAO) from a sealed container; POE and PAG absorb moisture. Pump it in with an oil pump through the service valve so no air enters, in small amounts.

      ✓ Check: The level is back in the correct range after stable operation, and not above it.

   6. Record. Record the oil type and quantity added and the measured oil pressure in the logbook.

      ✓ Check: The logbook shows what was done.

   Continue with: Low oil level or oil pressure alarm →

5. 5

   A three-phase scroll compressor runs backwards because of wrong phase sequence (after installation or electrical work): loud noise, almost no pressure difference and low current.

   Remedy: Check the phase sequence with a phase rotation meter. With the power isolated, swap two phases at the supply to the system. Do not let it run backwards for long.

   🛠 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 and correct phase sequence

   Scroll and screw compressors and many fans must run in the right direction. A scroll compressor running backwards is noisy, does not pump and is damaged quickly.

   1. Measure the phase sequence. With the unit stopped, connect a phase rotation meter to L1, L2 and L3 at the unit's supply terminals (live measurement: CAT III/IV, insulated probes). If the unit has a phase monitoring relay, read its indicator too.

      ✓ Check: The meter shows right-hand (L1-L2-L3) rotation, or you know that it is reversed.

   2. Decide where to swap. If the whole unit is reversed, swap at the unit's supply terminals after the isolator. If only one motor runs backwards, swap at that motor. For a motor on an inverter, the input sequence does not matter; change direction with a parameter or at the drive output.

      ✓ Check: You know which two conductors to swap and where.

   3. Isolate, lock off and prove dead. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all phases and to earth.

      ✓ Check: The tester shows no voltage on any phase.

   4. Swap two phases. Swap two phase conductors (for example L1 and L2). Never move the earth or neutral conductor. Tighten the terminals to the specified torque.

      ✓ Check: Two phases are swapped and the terminals are tight.

   5. Check the sequence again. Remove your lock, switch on and measure the phase sequence again; check that the phase monitoring relay is satisfied.

      ✓ Check: The meter shows right-hand rotation.

   6. Start and confirm direction. Start the compressor and watch the gauges. Stop at once if it sounds wrong or the pressures do not separate.

      ✓ Check: Within seconds suction pressure falls and discharge pressure rises, the current is normal and the compressor runs quietly.

   Safe isolation and lock-off

   Before you open a panel or touch electrical parts, isolate every supply, lock it off and prove that the circuit is dead. Electrical work is for qualified personnel only.

   1. Identify all supplies. Use the wiring diagram to find every supply: main feed, separate control supply, heaters, alarm and UPS supplies, and inverters that can feed back.

      ✓ Check: You know every isolation point for the equipment you will work on.

   2. Switch off, lock and tag. Switch each isolator to OFF, fit your own padlock and attach a tag with your name and date. Keep the key on you.

      ✓ Check: Every isolator is locked in OFF and cannot be switched on by someone else.

   3. Prove the tester. Check the two-pole voltage tester (EN 61243-3) on a proving unit or a known live source.

      ✓ Check: The tester shows the voltage of the known source.

   4. Prove dead. Measure phase to phase, each phase to neutral and each phase to earth at the point of work, on both line and load side.

      ✓ Check: The tester shows 0 V on every combination.

   5. Prove the tester again. Check the tester on the proving unit once more, to be sure it did not fail during the measurement.

      ✓ Check: The tester again shows the voltage of the known source.

   6. Deal with stored energy. On an inverter, wait the discharge time printed on the drive and measure the DC bus. Discharge start and run capacitors through a resistor of about 20 kΩ, 5 W with insulated leads, never with a screwdriver.

      ✓ Check: 0 V DC on the DC bus and across every capacitor.

6. 6
   Flowchart D

   The compressor is overloaded by excessive pressure.

   Remedy: Troubleshoot the high pressure: condenser, fans, overcharge and air in the system.

   Continue with: System trips on high pressure →

7. 7

   The contactor chatters or hums because of low control voltage or a loose contact.

   Remedy: Measure the control voltage and check the contactor and its connections.

   🛠 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.

   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.

8. 8
   Flowchart E

   Internal wear or damage: bearings, valve plates, crankshaft or scroll set.

   Remedy: Measure the current and compare with the rated data, take an oil sample (colour, acid, metal particles) and contact the manufacturer. Replace the compressor and clean the system after a motor burnout.

   🛠 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 the oil for acid

   Acid and moisture in the oil follow a motor burnout, overheating or a wet system, and they destroy a new compressor quickly. An acid test tells you whether the oil and drier must be changed and when the clean-up is finished.

   1. Choose the right kit. Use an acid test kit made for the oil in the compressor (POE, PAG, mineral or PAO; see the compressor nameplate). Check the expiry date. Wear goggles and gloves: burnt oil is acidic and the reagent is a chemical; follow its safety data sheet.

      ✓ Check: The kit matches the oil type and is within its expiry date.

   2. Take the oil sample. Take the sample from the compressor's oil drain or sampling valve, opening it slowly with low pressure in the crankcase and the sample bottle held firmly. On a hermetic compressor without a valve, the refrigerant must be recovered first and the oil drained from a removed compressor; that work requires F-gas certification (EPA 608 in the USA). Let the first few drops run to waste.

      ✓ Check: The clean bottle holds the amount the kit asks for, without refrigerant foaming over.

   3. Look at and smell the oil. Before testing, judge the oil by eye: clean oil is clear and light. Dark or black oil, a sharp burnt smell, sludge or metal particles point to a burnout or mechanical wear.

      ✓ Check: Colour, smell and any particles are noted.

   4. Run the test. Mix the oil and the reagent exactly as the kit instructions say (amounts, shaking time, waiting time).

      ✓ Check: The mixture has stood for the time stated in the kit instructions.

   5. Read the result against the kit chart. Compare the colour with the chart supplied with the kit. The colours differ between makes, so use only that kit's chart, never colours remembered from another brand.

      ✓ Check: The result is read as 'acceptable' or 'acid' according to the kit's own chart.

   6. Decide and record. If the oil is acidic: change the oil, fit a burnout filter drier (suction and liquid line) and repeat the test after some operating time until it passes. Record the result, the action and the date in the logbook.

      ✓ Check: The logbook shows the test result, the action taken and when the next test is due.

   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.