In testing a suspect VFD, I found the diodes in the 3 phase bridge circuit to be giving different readings. Three(3) would read, what I think is proper, 0.4 V, forward biasing and the other 3 would read 1.04 volts when forward biased. The other 6 diodes in the switching circuit also read properly at .36 volts when forward biased. My mates who replaced this machine said they had noticed the DC bus voltages was higher than normal. WOuld the higher forward bias voltage suggest the diodes are depreciating? Would this also have something to do with the excess DC bus voltage?
Thanks in advance

 

Your test is probably influenced by the other components in circuit. All 6 diodes int he bridge will read very similar if your test is valid, ie if you disconnect one leg of each diode so you measure just the diode.

And no it is very unlikely that differences in the diode forward voltages would cause a problem of the main DC bus voltage being high. There is likely to be some control system on the bus that sets it's voltage (and may be defective), or the bus voltage is unregulated and the voltage is high simply because other systems are drawing less current for some reason.

 

I've tested many VFD's and and this is the first one where the forward biasing has given me something different than .3v to .6v, besides either a short or an open. I understand external influences. I'm just following standard power circuit checks on a VFD and I cant find any literature that says to disconnect the diode. Normal checks just call for a diode check from the line side to DC bus.....and the load side to DC bus, to test the bridge and switching diodes. If you have any more info or can direct me to literature on testing a VFD it would be greatly appreciated...especially when it comes to scoping the output on a PWM inverter and detecting issues from the wave form.
I didn't think the excess break over would influence the the DC bus voltage, but thanks for the verification. More research on our particular inverters is needed i guess.
thanks