Air conditioner or heat pump compressor burn-out diagnosis & repair:
This air conditioning repair article discusses the how to diagnose and replace a burned out air conditioner compressor, including evaluation of air conditioner compressor noises, hard starting, lost cooling capacity, and detection of a burned out compressor or A/C compressors at or near end of their life.
Here we catalog and describe the causes of air conditioner or heat pump compressor failures and we suggest compressor motor diagnostic steps.
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When a sealed compressor motor has "burned out" this means that the internal wiring of the motor has become irreparably damaged: the compressor motor windings may be burned and shorted together or shorted to the steel shell of the motor, or the windings may have burned and simply become "open" or disconnected. If the motor has burned out in either of these ways it needs to be replaced.
When an air conditioner / heat pump compressor can no longer develop proper pressures, even if its electric motor has not burned out, the unit will still need to be replaced. Below we give refrigerant pressure details that help make this decision.
Air conditioner failure warning signs: Before an air conditioning compressor fails solid you may notice that the unit is hard-starting, particularly when starting against a head pressure (someone switched the system off while the compressor was running, then tried turning it right back on).
See CAPACITORS for HARD STARTING MOTORS
see MOTOR OVERLOAD RESET SWITCH.
Another sign of a failing air conditioning compressor is noisy motor operation, such as buzzing or clanking sounds coming from the outside unit housing the compressor motor.
See NOISES, COMPRESSOR CONDENSER.
Another sign that might indicate a failing air conditioner compressor motor is an increase in the building's electrical bills even though the air conditioning system "on" time has not changed.
See TIGHT or SEIZED AC COMPRESSORS where we provide additional details about examining and measuring A/C and heat pump compressors to continue compressor condition diagnosis.
Watch out for shock hazards or equipment damage: see DMMs VOMs SAFE USE OF.
The compressor won't start, perhaps just hums, and the motor star/run capacitor(s) are ok or you've tried replacing that part
As soon as the compressor tries to start the circuit breaker trips or fuse blows. The compressor motor has burned out, is internally shorted or gounded. A service technician will use a VOM and typically will find low resistance (low Ohms) between one or more compressor terminals and ground.
A VOM connected to the compressor circuit indicates that no current is being drawn, but the condenser fan is running properly.
Watch out: before assuming that the compressor motor is bad, when the unit won't start at all, check for a loose or disconnected electrical connection or a bad contactor relay. An overheated motor may also leave the compressor motor "off" due to a tripped internal overload switch or a switch that has simply failed. If this is the problem, the motor should start normally once the unit has cooled down. (Some readers describe spraying water on the condenser unit to speed cooling - (don't wet electrical switches & components.)
Watch out for shock hazards or equipment damage: see DMMs VOMs SAFE USE OF.
When an air conditioning compressor has "burned out" by shorting of internal components - it will fail to start at all. This failure is detected by disconnecting all power and wiring from the unit and measuring resistance (ohms) between the motor start/common and run/common terminals.
Zero resistance: If there is zero resistance there is (probably) no break in the wire or circuit being tested. Zero resistance also can mean that wires are shorted together or are shorted to ground.
Watch out: while finding very low or zero resistance would be normal for a simple length of electrical wire, for some circuits and equipment, finding low or no resistance where there should be some can indicate a short circuit. Another problem to watch for is testing electric motor windings; a winding may appear intact, with low resistance, but it may open (showing high or infinite resistance) only when the motor begins to spin.
More details are just below and also at ELECTRIC MOTOR DIAGNOSTIC GUIDE.
Infinite resistance: If you measure the resistance across a compressor winding and your meter's needle is stuck over at infinity, or "OL"/"OVER" on a digital meter, that would indicate the compressor winding is open (burned through). The same effect can be observed from simply connecting the meter to absolutely nothing. Typically if just one widing is "open" you'll see infinite resistance at one check point and in contrast (for the non-open windings) you will read zero resistance ("continuity") between the Common terminal and the Start or between the Common terminal and Run terminal.
Low resistance: If the resistance measured across the air conditioning compressor winding is too close to 0 ohms, it's shorted. The compressor should blow the fuse or trip the circuit breaker when power is turned back on. But watch out: we get field reports of equipment burn ups and even fires when the air conditioning circuit breaker for the compressor happens to be an old FPE Stab-Lok or Zinsco unit.
If there is resistance but not infinite resistance between the motor terminal and the motor casing, the motor has become shorted to ground internally and the unit needs to be replaced. If there is no resistance between the start and run terminals to common, but there is resistance between the start and run terminals, this means that the internal motor overload protection circuit is open. In this last case, allow the motor to cool and re-test it before replacing it.
For details about measuring RLA / FLA, and definitions of RLA, FLA, and LRA, see TIGHT or SEIZED AC COMPRESSORS
2016/09/10 johnteh said:
I had the summer A/C inspection done was told the compressor capacitor was indicating a short life because the compressor mega ohms was 200 which is very low for a unit that is only 2 years old. recommendation was to replace compressor capacitor with a hard start to prolong compressor life. Are compressor capacitors difficult to replace/install? They don't seem to be, since they have only 2 wires.
This question was posted originally at CAPACITORS for HARD STARTING MOTORS
Start capacitors are usually easy to install as they're external to the motor and the wiring is pretty obvious.
Details about choosing and replacing an electric motor or AC compressor motor start/run or hard-start capacitor are at CAPACITORS for HARD STARTING MOTORS
But I'm confused about the compressor diagnosis. Very low resistance across a compressor motor is not a diagnostic I understand. High current draw (measured in amps not ohms) is a well-known indicator that a motor is struggling and may indicate failing or seizing bearings and thus end of life.
Ron Walker, over at hvactrainingsolutions.net gives this nice summary of AC Compressor troubleshooting by measuring resistance across air conditioner or heat pump motor windings from which we excerpt and expand. Mr. Walker notes that these measurements are difficult to obtain as the compressor motor wiring connector can be itself hard to access and he suggests obtaining an old motor wiring harness and connector plug to extend the wires out to where you can reach them - a great idea.
Ohms measurements across the windings terminals of a good electric motor such as is used in an air conditioner or heat pump system will read something like this:
- Pin 1 to Pin 2 – 3 ohms
- Pin 2 to Pin 3 – 6 ohms
- Pin 1 to Pin 3 – 9 ohms
- The lowest electrical resistance reading should be below 10 ohms (this is the [electric motor] run winding).
- The second highest reading should be about 2-4 times higher than the lowest reading (this is the [electric motor] start winding).
- The highest reading should be the sum of the resistance read across the two smaller [electric motor] windings
Mr. Walker adds that:
If any of these readings read infinite or over range on your ohm meter, you have an open winding. A reading of zero ohms indicates a short in a winding.
Very high or infinite resistance across the windings of a motor would typically indicate that a winding wire is broken. What's tricky about this motor widing resistance measurement in ohms is that a winding wire may be damaged but may "open" or "break" only when the motor is trying to spin or only when the motor gets up to full speed.
So you can see that NONE of these air conditioner or heat pump motor ohms test measurements is even close to 200 Ohms and that to me, 200 ohms sounds high, not "very low". Before trusting such a measurement I'd be checking my ohm-meter, probes, quality of contact, and other measurement conditions including of course that power is OFF and disconnected from the motor.
A different air conditioner / heat pump compressor (or fan) motor test that involves measuring resistance is a check for shorted motor windings. Measuring between each motor pin and ground (typically the compressor motor case) should give "infinite" resistance, meaning no current is flowing. If you see some lower resistance with the -to-ground measurement then there's an internal short.
Before changing the start/run capacitor - which may be fixing the "wrong problem" - I suggest that you give the service manager at your AC repair company a call and that you ask for clarification and if needed, ask for a visit from a senior, experienced HVAC technician. Either there has been a misunderstanding or what your tech told you needs some further explanation vis-a-vis what other experts say about AC troubleshooting by measuring ohms across motor windings.
Also see COMPRESSOR / CONDENSER DIAGNOSTICS
Watch out: for a mechanically frozen compressor: a compressor may pass all of these electrical tests and still require replacement. The tests above only test electrical connections and windings. An air conditioner or heat pump or refrigeration compressor that has jammed up mechanically internally will still refuse to start (perhaps will hum) when all of the electrical tests, contactor relay, start capacitor, etc. are tested as perfecliy fine.
A compressor with broken internal parts may also not be frozen, that is its internal electrical motor may start and run, but the compressor fails to produce any refrigerant pressure at its outlet side. In this case internal parts or valves in the unit have broken without jamming the motor itself. In this case, all of the electrical components and tests will look "OK".
A compressor with broken internal motor mounts may make a rattling or clanking sound and needs replacement. It will continue to run but could become shrapnel at any time.
A compressor with bad internal valves will continue to run but is inefficient and should be replaced. The symptom is very quick equalization of high and low side pressure as soon as the motor stops.
Some compressor motor failures are so apparent that there's no question: an electric motor burnout that draws high amps or is internally shorted, for example.
But what about a compressor motor that may be just "worn" ? An HVAC compressor should be able to pull at least 15" of vacuum against 100 psi head pressure or else it is inefficient. Of course to make this test you must be able to isolate the compressor from the rest of the cooling or heat pump system, so this test is not rapidly made in the field.
Bad air conditioner reed valves will be unable to pull pressure down on the low side of the system. A leaky discharge reed valve (on the compressor output side) pulls hot gases back into the compressor cylinder and recompresses them, causing abnormally high head pressures at the compressor motor. And as a result the compressor won't be able to move vapor.
In sum, HVAC compressors do fail and need replacement, but only when you have tested and ruled out the other 80% of the causes of common air conditioning, heat pump, or refrigeration problems (usually electrical in nature) do you go ahead and replace the compressor unit.
Details of compressor/condenser unit fan inspection, diagnosis, and repair are at FAN, COMPRESSOR / CONDENSER UNIT. Basic advice follows:
A failed compressor/condenser fan can cause the air conditioning system to shut down due to an overheating compressor or excessive pressures developed inside the compressor. If your compressor/condenser unit does not include a safety override switch to perform this shutdown and if the condenser unit fan is not working, your compressor motor may be permanently damaged.
at NOISES, COMPRESSOR CONDENSER we include some condenser fan noise problems are traced to the cooling fan motor, bearings, fan blades, obstructions, etc.
Watch out: if your compressor/condenser unit motor is running but the fan itself is not blowing air, the system will not work and may be seriously damaged. The outdoor cooling fan or condenser unit fan is needed not only to cool high pressure, high temperature refrigerant in gas form so that it can condense back to liquid form.
That cooling step also cools down the compressor itself and keeps the compressor internal pressure from becoming too high. Most modern compressor / condenser units include an overpressure sensor that will shut the equipment down if compressor pressures become too high. Some older models and some modern economy air conditioner compressors may lack this function.
See DMMs VOMs SAFE USE OF. Example: testing a blower fan motor winding: referring to the electrical diagram for your equipment, unplug electrical connectors at the fan motor. Measure the resistance between each lead wire with a multimeter or VOM. The multimeter should be set in the X1 range. For accuracy, don't measure when the fan motor is hot, allow it to cool off.
When the resistance between each lead wire are those listed in the specifications for your equipment the fan motor should be normal. Zero resistance or infinite resistance are indicators of a problem. More examples of checking wiring:
see BURNED-OUT COMPRESSOR.
also TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors.
Also see Test a Motor Starting Capacitor
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Also see REFRIGERANT PIPING INSTALLATION FAQs
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