VFD teco 7300 blown capacitor

Roscoepcoltrane · Oct 24, 2019

Hello everybody,

I have a blown capacitor in my VFD, and now it is dead. I can't see anything else that is obviously grilled. If I replace the capacitor(s), what are my chances it will come back to life? what else could have failed as a result of the capacitor shorting out (I think that's what happened)?
I did a diode test between the positive dc bus terminal and the ac output and input terminals, and that was all ok. However, testing between ground and the ac terminals gives no result. There is a relay behind the capacitors, what does that do? does that make a ground connection maybe?

I hope someone can help out,

....

johansen · Oct 24, 2019

Use the diode check on your meter to verify the input rectifier is still a rectifier and not a short or open. If the diodes blew up then the capacitor would follow.

Jraef · Oct 24, 2019

The relay is undoubtedly part of the "pre-charge circuit". Capacitors, when first energized, will charge themselves up by pulling in current at the Available Fault Current level, to the detriment of themselves and the diodes ahead of them. So to prevent that damage, a resistor is placed in series with the capacitors as a current limiter. But that resistor would then get in the way once its function is complete, so about 1 second after first energizing a VFD, that relay closes and shorts around that resistor. The chances are, the pre-charge resistor failed, which typically results in the loss of capacitors.

Roscoepcoltrane · Oct 25, 2019

johansen said:
Use the diode check on your meter to verify the input rectifier is still a rectifier and not a short or open. If the diodes blew up then the capacitor would follow.

Thank you for you reaction. The input rectifier diodes between ac input and negative dc seem to be blown, I measure open circuit. Since they are encapsulated in the power module, I have declared game over for this vfd.

Roscoepcoltrane · Oct 25, 2019

Thanks for the information. The problem seems to have been blown input rectifier diodes (see reaction above).

Jraef · Oct 28, 2019

Roscoepcoltrane said:
Thanks for the information. The problem seems to have been blown input rectifier diodes (see reaction above).

Again, often a secondary symptom of the pre-charge resistor burning out. No resistor means the pre-charge relay closes after a second and the totally dead caps immediately try to pull so much charging current that the diodes can't handle it, shorting them. This is typically caused by someone putting a motor starter or contactor ahead of the VFD and cycling power to it every time they use it. The little resistor is good for a few thousand cycles, but that can get consumed very quickly when using a switch ahead of the VFD like that.

JST · Oct 28, 2019

Jraef said:
Again, often a secondary symptom of the pre-charge resistor burning out. No resistor means the pre-charge relay closes after a second and the totally dead caps immediately try to pull so much charging current that the diodes can't handle it, shorting them. This is typically caused by someone putting a motor starter or contactor ahead of the VFD and cycling power to it every time they use it. The little resistor is good for a few thousand cycles, but that can get consumed very quickly when using a switch ahead of the VFD like that.

A number of VFDs do not operate on a time delay, but rather operate the relay when the capacitor voltage reaches a certain point, often around 2/3 of full voltage. With those,when the resistor fails, the VFD will not operate at all. Also with that design, a failed rectifier may then kill the capacitors before the resistor fails, because the voltage is never reached with the AC on the capacitors, and the resistor passes it through, heating up toward failure as that happens.

A

mmurray70 · Oct 28, 2019

JST said:
A number of VFDs do not operate on a time delay, but rather operate the relay when the capacitor voltage reaches a certain point, often around 2/3 of full voltage. With those,when the resistor fails, the VFD will not operate at all.

I had a Mitsubishi VFD with failed charge resistor a few years ago. It was this type, relay would not close and it would not operate at all. I changed the resistor and everything is working well again.

The charge circuit is really very simple. If this was an expensive VFD to replace id be tempted to try and make my own rectifier if you cant replace the origional one.

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VFD teco 7300 blown capacitor

Roscoepcoltrane

Plastic

johansen

Stainless

Jraef

Titanium

Roscoepcoltrane

Plastic

Roscoepcoltrane

Plastic

Jraef

Titanium

JST

Diamond

mmurray70

Stainless

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