Inverter Compressor Systems

Although inverter compressors do the exact same thing as the old school 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. Forget120 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 school fore bearers, 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 school 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 school 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 school 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 Appliance Repair Tech 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 Motherboard-- 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 motherboard, 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.