VFD (Variable Frequency Drive) systems have been around long enough now that most of us know the procedure for troubleshooting them. You have three main tests that you perform:
1. Check for the PWM (Pulse Width Modulated) signal from the main control to the inverter. This is a DC square wave data signal that alternates between 0-5 VDC. The inverter has to receive this signal from the main board in order for it to run the motor.
2. Check the 120 VAC input to the inverter board. This has to be present for the inverter to do its job.
3. Check the resistance of the BLDC motor's windings. The exact reading can vary from motor to motor, but the important thing is that the windings' resistances be very close to one another -- within a 10th of an ohm.
If the inverter is receiving both of its inputs and the motor windings are in spec, you can usually safely conclude that you have a failed inverter.
What if you're dealing with a BLDC compressor that has failed mechanically rather than electrically, meaning that the windings measure in spec, but the compressor is mechanically jammed? The inverter will be doing its best to run the compressor, but it simply can't. How do you avoid the false diagnosis of a failed inverter?
The esteemed Brother @EthanRanft has the answer. As he put it:
The last step [in your troubleshooting] would be to check amperage on the 120VAC supply to the inverter. If you disconnect power to the inverter, and then reconnect, a functioning inverter will try to start the compressor and you’ll see a few amps. If you’re seeing amperage on and off and the compressor isn’t starting, you know the compressor is locked. LR amps on an inverter compressor is only about 4A
There you have it -- one simple amp measurement tells you if the inverter is doing its job! That's one more tidbit to add to your mental toolbox...