testing vfd

[rayrayray]

i was wondering how to test the inputs and the outputs of a typically VFD.

 

[DickDV]

If, by inputs and outputs, you mean the AC 3 phase input terminals and the AC three phase motor connection terminals (outputs), then the procedure is quite simple.

Locate the DC bus terminals. Usually they are labeled + and - and are located on the power terminal strip.

With all power and wiring off the drive, you can check the input bridge diodes with an ohmmeter between the + bus and each of the input terminals. You should have near continuity in one direction and an open circuit in the reverse direction.

Repeat the same with the - bus and each of the three inputs. Again, you should see low resistance in one direction on all three and high resistance on all three in the opposite direction.

Except on the largest drives, the diodes will all be in the same block so, if you have one shorted or open, there is no need to go on. The block is bad and will need to be replaced. If the drive is 460V 50hp or smaller, it is probably not worth repairing.

To test the outputs, do the same thing as the input with the ohmmeter in both directions except here you should see an open circuit in both directions in all combinations on all terminals. Again, the output IGBT's are usually all together on one block so, if one is shorted, the whole block must be replaced.

 

[Leadfoot]

If the drive is 460V 50hp or smaller, it is probably not worth repairing.

I will differ with you on the HP size of repairability.

IF you said 5 hp or smaller I would agree.

 

[monkeyhead]

heh... i just sent out a .75hp AB 160C drive to get repaired through a third party (applied industrial). still cheaper than a new one by their pricing scheme.

I always ignored the DC bus, so thanks for the info DickDV... good stuff. My approach has always been: Wires tight and in right place? Settings correct? still doesn't work? then give it the boot.