The new inverter compressors ain't like the old skool compressors used in yo momma's beer cooler. Oh, they still do the same basic job-- pump refrigerant vapor. But you have to troubleshoot them differently from the old skool compressors. In their quest to comply with increasingly onerous Energy Star requirements, all the appliance manufacturers are producing refrigerator models that use inverter compressors. Like it or not, inverter compressors are here to stay. Many a fine tech has been befuddled by these new compressor systems but not you! For the Samurai shall reveal the truth unto thee, and the truth shall set thee free.
Conventional Old Skool Compressors
Before we talk about how these inverter compressors work and and how to troubleshoot them, let's quickly review the old skool compressors so it's fresh in our mind when we compare with how the inverter compressors work.
In the conventional compressor technology that's been around for decades, the compressor has a start winding to help kick things off and a main (or run) winding that keeps the compressor going after it's started. A start relay and sometimes a start capacitor are used to momentarily power the start winding and then take it out of the circuit once the compressor is up and running. The compressor runs off standard, single-phase, 60 Hz 120vac.
If the start relay fails, the compressor will sit there trying but failing to start and drawing locked rotor (LR) current. Where the normal operating current draw on a conventional compressor is somewhere between 1 and 2 amps, LR current could be somewhere north of 8 amps. All that current makes heat, lots of heat. As the compressor sits there straining to start, it starts boiling the oil and burning the varnish insulation on the motor windings. If this goes on for very long, the compressor will literally self destruct (internally) from all the heat.
To prevent this unhappy scenario, the engineers added an overload device that's used to kill power to the compressor if something goes wrong, like a bad start relay, open compressor start winding, bad internal bearing, or a seized piston inside the compressor that prevents it from starting correctly. The idea being that if the problem is just a bad start relay (very common), this can be repaired easily and inexpensively compared to replacing the entire compressor (or refrigerator).
If you were to measure the resistance of the start winding and the main (or run) winding, you would find that the start winding has a higher resistance than the run winding. It's also made of thicker wire because the start winding has to handle the extra current flow through it that's needed to get the compressor piston going from a dead stop.
A common troubleshooting technique with these old skool compressors is to rig up a test cord and manually power the compressor to see if it'll run. More details on compressor test cords here: http://appliantology.org/blog/1/entry-669-rigging-and-using-a-compressor-test-cord-to-manually-operate-a-compressor/
Once they're up and running, these old skool, compressors run at the same speed and move the same amount of refrigerant vapor per minute. In other words, their refrigerant capacity and motor RPM is constant the entire time it's running. They can't work "harder," just longer.
So, let's summarize the old skool compressors:
- has two windings, a start and a run winding, which are physically different windings and have different resistances; the start winding has higher resistance than the run winding
- runs off standard 120vac household power
- uses a start relay to initially power the start winding and then take it out of the circuit after the compressor is up and running
- can rig up a test cord to directly power the compressor
- are constant capacity and speed machines
Keep all this in mind as we now look at the new inverter compressors...
Inverter Compressor Systems
Although inverter compressors do the exact same thing as the old skool compressors-- pump refrigerant vapor-- and they physically move the vapor the same way-- through a vapor-compression cycle-- they are powered and controlled very differently.
For one thing, inverter compressors use a special three-phase voltage produced by a special control board called an inverter. Fuggetabout 120 VAC, 60 Hz line voltage. We're not in Kansas anymore, boys and girls! Both the amplitude (amount) and frequency of the input voltage will vary. Typical specs are 80 to 230 VAC with the frequency ranging anywhere from 57 to 104 Hz. The higher the frequency, the faster the inverter compressor will run.
So, inverter compressors, unlike their old skool forebearers, really can work harder. In fact, this is exactly why the manufacturers are using these inverter compressor systems; they can match how hard the compressor needs to work to the actual refrigeration work needed to keep the beer cold. By doing it this way, the compressor draws less power and the manufacturers can meet the Energy Star requirements.
Inverter compressors have three windings, not just two like the old skool units. All three windings should have the exact same resistance. If the resistances vary from each other by as much as a 1 ohm, the compressor will not run correctly. In fact, this is one of the ways of checking an inverter compressor: making sure that all three windings have the exact same resistance. Check the manufacturer's spec for what that exact resistance reading should be. This is different from the old skool compressors with just two windings and the start winding has a much higher resistance than the run winding.
Remember how a common troubleshooting trick with the old skool compressors is to power it directly with a test cord and see if it starts? Don't try that on these inverter compressors because you'll permanently break it. If you're a professional Appliantologist and you do this on a service call, you just bought your customer a new refrigerator!
Let's summarize the inverter compressors:
- have three windings, not just two; all three windings have the exact same resistance
- does not use a start relay/overload device
- runs off a special voltage produced by an inverter board; the voltage varies in both magnitude and frequency: the higher the frequency, the faster the compressor runs
- variable capacity, variable speed
- cannot directly power the compressor (well, you could but you'd regret it)
Troubleshooting Inverter Compressor Systems
If you're working on an inverter compressor system where the compressor isn't running, you can't power an inverter compressor directly to test it. But you can (and should!) check the resistances in all three windings to rule out an open winding. If the compressor windings check good, this is not diagnostically conclusive that the compressor itself is good. But if, OTOH, the winding resistances are imbalanced or one of them is open, this is diagnostically conclusive that the compressor is bad.
Okay, so let's say the compressor windings check good but it's not running. Now what?
Now you have to check the inverter board itself. There are two different tests you can do on the inverter board to see if it's good or not:
1. Check for good input voltages.
An Inverter board will have two different input voltages:
- 120 VAC main power supply
- 4 to 6 VDC control voltage from the main control board (or Muthaboard-- a completely separate circuit board in the refrigerator)
If you're missing one of these voltages, the inverter board can't run the compressor. You'll need to backtrack and find the missing voltage. Could be a bad wire harness connector, bad muthaboard, etc. BTW, make all voltage measurements with everything CONNECTED. Otherwise, you'll get different readings that could be misleading.
OTOH, if you're getting both of these input voltages to the inverter and the compressor isn't running (and you've already checked the compressor winding resistances), then you need to do this next test:
2. Check the current draw on the 120 VAC power supply.
- Disconnect the 120 VAC power supply from the inverter board.
- Connect your amp meter around one of the wires supplying 120 VAC to the inverter board (doesn't matter which one).
- Reconnect the 120 VAC power supply to the inverter board and watch your amp meter.
If the meter stays at 0 amps, the inverter board is toast-- it's not even trying to start the compressor.
If you see the current draw jump to say 4 amps (typical LR current in these inverter compressors) and then drop off, keep watching. Most inverter boards will repeatedly try to power up the compressor. On GE refrigerators, for example, the inverter will try to start the compressor 12 consecutive times. If the compressor fails to start, the inverter will timeout for 8 minutes and then try again. Other manufacturers may have different test schemes but the idea is the same: if the inverter is working properly, you'll see activity on your amp meter as the inverter tries to do its job.
In the video below, I demonstrate troubleshooting an inverter compressor system on a GE refrigerator. The only thing I didn't show in the video is checking the inverter board's current draw.
Here's the replacement inverter board I used to fix this refrigerator: http://www.repairclinic.com/PartDetail/Inverter-Board/WR55X11138/2443233