VFD Under Voltage Fault

[Spudgun]

Hi all,
I've had 2 single phase to 3 phase 2.2kw 10A VFDs for several years now with no issues. One on my lathe, one on a mill.
Recently got an INTERMITTENT error of UU undervoltage (on both VFDs). It can run fine for ages then suddenly trip. Sometimes under load, sometimes not. Tried with different motors, makes no difference. I wired up a new (3rd) vfd of same model to the lathe and works fine. Sometimes before it trips you can hear the relay clicking randomly. I believe this relay controls the input single phase voltage charging the capacitors with rectified DC voltage and shorts the high power resistor to stop the charging when full.
Mains voltage is fine and on 32amp breaker.
Unplugged all other equipment on the same ring. No difference.
Opened the VFD and checked the capacitance of the 2 820uf caps which gives approx. 1476uf for both which is low but within 10%.
Unsoldered relay and this seems fine. Coil resistance is ok I think and switches with 24v appied over the coil.
Unsoldered the rectifier and seems to test ok with multimeter.
Checked the resistor on both VFDs, one about 152ohm and the other 158ohm. I assume this should be 150ohm???
I replaced the resister (158ohm reading) with the (152ohm) resistor and put the VFD back together and it worked flawlessly last night. Tried this morning and tripped again. I think this may be due to the motor struggling because I had the lathe setup with the steady a bit too tight. Now it tries to run the motor and just trips again.

Is this because the resistor is getting burnt out and increasing in resistance when hot preventing the capacitors from charging fully (quick enough)?

Model of the VFD is Ecogoo 9100. Manual attached.

 

[Spudgun]

Please excuse Mrs moaning in the background

 

[JST]

The relay does short the initial charge-up current-limiting resistor. If it is randomly switching, it may be simply due to the DC bus voltage not being high enough.

The 246V is low enough that I'd expect it might be sensed by the VFD as too low. That would likely trigger the resistor to be switched back in-circuit. Too much of that can cause the resistor to burn out, as it is rated only for the turn-on surge, not for continuous duty.

There may be a bad connection to one (or more) of the capacitors, that could cause excess ripple and a low effective DC voltage. The voltage is almost low enough to indicate that.

If one or both of the capacitors has the right capacitance, but has developed an excessive internal resistance, that could also cause the problem.

A bad rectifier or bad connection to it could do something similar, but should cause an even lower voltage. You probably would have fixed that when pulling it for test.

 

[Spudgun]

Thanks JST, the VFD is set to trip at 190v ac input.
I'm not sure what to test/try next?
If it was internal resistance of the caps wouldn't changing the resistor not make a difference?
I've ordered replacement relays and 10w 150ohm resistors.
Here's a few photos if it helps

 

[Spudgun]

 

[Spudgun]

What component reads the dc bus voltage and what reads the input voltage?

 

[JST]

The microprocessor in there is the element that makes the decisions, there will be some resistors that reduce the voltage to something it can measure. From what you showed, it does not seem as if the measurement is wrong, more that the actual voltage is wrong and is being correctly measured.

Capacitor value, capacitor internal resistance, a rectifier problem, or, very likely, a bad connection (solder?) that is intermittent. Hard to be specific, it comes down to on-the-spot troubleshooting.

 

[BT Fabrication]

Id Suspect that mostfet might be dead shorted. Too much to test without having it in your hands.
literally need to break down each system and check if its functioning, sounds like it has a dead short somewhere.

 

[Spudgun]

I picked the silicone away from the rectifier and looked like there could possibly be a bad connection? Its soldered the other side but dont know if its connected through the board?
chucked loads of solder round the joint. Tested with another smaller motor 0.25kw rather than 1kw but ran fine. Voltages seemed fine even when I Fred Flintstone stopped the motor with my shoe.

 

[JST]

Sounds good! If it stays good, you know what the issue was. That would do it. I might have expected the voltage to be lower, but it depends on load.

BTFab:
Shorted MOSFET?

That would not account for it running sometimes but not others. A shorted device would cause it to never run.

It would also have a different fault code, almost certainly an "OC" (overcurrent) or whatever their code for that is.

 

[Spudgun]

I'll give it a try tomorrow but I'm not getting my hopes up.
If it wasn't for the "cost of machining crisis" I would've smashed it to bits with a big hammer by now.

 

[SomeoneSomewhere]

A multimeter (both AC and DC) or ideally oscilloscope connected across the DC bus (with proper precautions) would be interesting. I'd tend to blame the capacitors - cheap, no-name, hot environment, failing after several years.

Increasing ESR will cause increased ripple, which the drive might pick up as undervoltage. The only concern I'd have is that it should only occur at high power, whereas it sounds like you're having it happen at any time.

I'd measure the cap diameter, length, and pin spacing, and pick up some replacements.

 

[Spudgun]

Thanks for the help all.
I'm really starting to get the hump with this thing.
I've wired it up to my mill, changed the parameters to suit and it ran fine for a few days.
I guess I should change the relay, resistor and caps?
Where's best to get 820uf caps from?

 

[Spudgun]

I also tried setting P6.18 to 3. I assume this is to ignore any short voltage dips etc?

 

[BT Fabrication]

I picked the silicone away from the rectifier and looked like there could possibly be a bad connection? Its soldered the other side but dont know if its connected through the board?
chucked loads of solder round the joint. Tested with another smaller motor 0.25kw rather than 1kw but ran fine. Voltages seemed fine even when I Fred Flintstone stopped the motor with my shoe.

Damn, that picture, something got hot on that leg and started arcing and blowing the solder joint apart. Looks like something was pulling too much current, hopefully nothing else is damaged.

Id be inspecting everything down stream of the red line. As that is the + terminal output of the bridge recifier, that goes underneath the plates that I would assume are Mossfet or IGBT etc that do all the switching. If one gets hot and shorts it will pull a ton of current through the bridge rectifier right from line voltage and might blow it up worse then it already did. Unless your output to the motor got shorted.

 

[BT Fabrication]

Where's best to get 820uf caps from?

Usual place is Digikey.

 

[JST]

In the US, DigiKey. I assume they ship to Canada, they are right on the border.

 

[mksj]

I would also check with Mouser Electronics, they have a distribution center in Canada. I assume the capacitors are 820uF 400V or 450V, there are long life versions. What I do not follow is that you had two VFD's with the same issue at about the same time, I would not expect both to have a component failure at the same time. What it suggests to me is that you may have had a voltage surge/spike that might have hit both units at the same time, and this would assume that both were connected to the power line when this occurred. Also if the VFD's capacitors are subjected to very cold temperatures it alters the electrolytic capacitors and they may have been damage if operated at cold temperatures. At some point one needs to look at the cost of repair vs. replacement for smaller VFD's, rather than spend $100 on capaciotrs I would just replace the VFD with a more reliable unit.

https://www.mouser.ca/c/passive-components/capacitors/?q=820uf%20capacitor&capacitance=820%20uF&voltage%20rating%20dc=400%20VDC~~600%20VDC%7C~900%20VDC%7C~2000%20VDC&instock=y&rp=passive-components%2Fcapacitors%7C~Voltage%20Rating%20DC&sort=pricing

 

[BT Fabrication]

In the US, DigiKey. I assume they ship to Canada, they are right on the border.

yep i get things next day usually

 

[Spudgun]

New resistors and relays arrived.