Diode test result odd?

[ControlsLost]

During a standard diode check to determine the health of some 3 phase Bridge rectifiers and output IGBT module on a 590amp/500hp (cabinet size) VFD. I encountered what i though was probably normal, but odd because I hadn't seen it before.

Diode check in forward bias was normal, 560mv across the board on the Line side to dc bus, then 225mv on the load side IGBT to dc bus.

But when checking the reverse bias for a short. I would see the voltage slowly (took about a minute) rise to the meters maximum of around 3 something volts before going OL like it's supposed to when they are good and not conducting through. and when I would remove the lead for a few seconds then put it back on, it would start at about 3 something volts and climb again to OL.

So it's obviously increasing in the voltage drop slowly before finally giving me a reading with no voltage drop.

I believe it's got something to do with the sheer size of the unit, the diodes in this to run a 500hp 3 phase motor would have to be hefty.

What am I seeing here where the diode voltage drop increases slowly and seems like its varying in resistance when the meter gets connected/disconnected, like its slowly charging and discharging?

Testing done with a fluke 87v.

Thanks!

[bob91343]

Being tested in the circuit gives strange results.  What you are seeing may be the filter capacitor charging.

[m3vuv]

I would say the same as bob as in its charging a cap,pull one leg and retest.

[ControlsLost]

Being tested in the circuit gives strange results.  What you are seeing may be the filter capacitor charging.

appreciate the advice.

Unfortunately the way these are, thats an hours long process to remove the power modules and possibly remove capacitors. These are designed to be able to test in circuit. Like you said though, there is some charging going on, I just wasn't sure how much to expect.


I actually found a good answer in a separate manual for a newer version of these drives. It calls it out explicitly that there will be some charging due to capacitors and such.


troubleshooting visual / 0Drive in question (this unit is about 7 feet tall)




Found the answer here. So that explains it, appreciate it guys