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KB VFD with "SC" fault

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Aluminum
Joined
Oct 12, 2013
Location
San Francisco, CA
Is there a way to reset faults on KB Electronics VFDs? I tried setting parameter 0610 to 0000 (factory setting), to 1010 (default program) and to 1111 (60Hz) - which are probably identical - and none of them removed the SC fault. Is it possible to reset faults on this drive?

Long story but I had to solder in new 1500 uF capacitors on the power board after wiring the power incorrectly on a KBDF-27D VFD. That brought it back to life, but it gives a motor short circuit fault despite never having had a motor attached (the motor is fine, BTW).

Eo5r2so.jpg
 

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A VFD that indicates a fault (other than a "no load" fault) when there is nothing connected to the motor terminals probably has an internal defect of some sort.

A short circuit fault in that condition is often due to a bad IGBT inside the VFD.
 
A VFD that indicates a fault (other than a "no load" fault) when there is nothing connected to the motor terminals probably has an internal defect of some sort.

A short circuit fault in that condition is often due to a bad IGBT inside the VFD.

I would expect it to have some defects! I let the magic smoke out!

How can I test an IGBT with a multimeter? Or do I need some other specialized meter?
 
That depends on what you can get to.

Standard quick test is to measure resistance from each output to the + and - of the power supply, and reverse the leads (total 4 measurements). Unit should be "off", disconnected from power, and have sat for long enough to discharge the power supply (usually 10 or 20 minutes).

You should measure open all ways using the resistance range on a digital meter. If you get any low resistance measurements on the normal digital meter, then there is a problem.

Do NOT use the range that is used to check diodes. That usually has a diode symbol next to the range numbers.

Between the output and the power supply is an IGBT in parallel with a diode. The IGBT should be "open", and the diode only conducts one way. But the low voltage from a digital meter will not make the diode conduct, so it should measure open.

For most VFDs, this requires opening the case, and knowing where the power supply connections are inside.
 
That depends on what you can get to.

Standard quick test is to measure resistance from each output to the + and - of the power supply, and reverse the leads (total 4 measurements). Unit should be "off", disconnected from power, and have sat for long enough to discharge the power supply (usually 10 or 20 minutes).

You should measure open all ways using the resistance range on a digital meter. If you get any low resistance measurements on the normal digital meter, then there is a problem.

Do NOT use the range that is used to check diodes. That usually has a diode symbol next to the range numbers.

Between the output and the power supply is an IGBT in parallel with a diode. The IGBT should be "open", and the diode only conducts one way. But the low voltage from a digital meter will not make the diode conduct, so it should measure open.

For most VFDs, this requires opening the case, and knowing where the power supply connections are inside.

There are four boards in this VFD. The photo in the original post shows the construction.

1) power input/output board
2) ac-to-dc board with a bank of large capacitors, a rectifier and a relay
3) IGBT board with a lot of tiny surface mount components, a few tiny transformers and capacitors
4) logic board with data terminals, display, buttons, etc.

As I mentioned, I already disassembled it and replaced the capacitors. Would hi-res photos help? It's trivial to take apart, and I expected to have to do this a bunch more times.
 
Some higher resolution photos of the assembly:

9IQZOoZ.jpg


K3aIa2p.jpg


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Here's a closeup of the IGBT board. It was splashed with the electrolyte from the capacitor that failed. It wasn't much, but it definitely got on the surface. Could that be the cause of the short?

00rjw3O.jpg
 

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Could be, probably isn't, but it certainly cannot help. Depends on what got splashed.

Did you clean it off? With what, if so?
 
the only way i can imagine you blew up a capacitor is to have incorrectly wired it such that you formed a voltage doubler and the capacitors exploded. in which case, you may need to replace all of the igbts.

another method of blowing up a capacitor is when you connect the ac mains to the motor output leads. one or more igbts will fail shorted due to the uncontrolled inrush current. I'm surprised it turns on.


what you could do is connect a 100 watt lightbulb in series with your nominal 120v mains and complete the circuit through the motor output terminals. try all 3 combinations. if the bulb stays bright and does not dim to nearly nothing as the board powers up, then the igbts are shorted.
 
If just one of each pair of IGBTs is shorted, (or even just one out of the 6) it will turn on, but will fault out with an overcurrent as soon as the output is enabled by trying to drive a motor.

Usually, just one is bad of a pair, and the unit may turn on OK (but not run). Later, it is possible that many tries at running it may blow the other IGBT, and cause a short across the power supply, in which case it will no longer turn on.
 
I originally wired it for 115V, with neutral to N, live to L1 and obviously G for ground. Everything worked flawlessly! Then...

I did not change the input wiring before plugging it in to a step-up transformer. Despite reading the instructions and the input label, I'm still not totally sure how it should have been wired, but my guess is that it should have been live to L2 instead of L1. It did not help that the input and output stickers weren't attached and the adhesive refuses to work on either.

How would you have wired this, through a step-up transformer? There is only one live wire that way, instead of two!

K7Cynkh.jpg
 
the only way i can imagine you blew up a capacitor is to have incorrectly wired it such that you formed a voltage doubler and the capacitors exploded. in which case, you may need to replace all of the igbts.

I simply plugged it into a step-up transformer, which delivered 230V to an input expecting 115. Only one of the capacitors blew, and the rest seemed okay. Their capacities are larger than my meter can measure - 1500uF @200V - so I just replaced them all. The photo below is of the original board:

Xt2DsoZ.jpg


And after fixing it:

K3aIa2p.jpg


I'm obviously looking for guidance on how to make this thing work. What's the next step here?
 
Could be, probably isn't, but it certainly cannot help. Depends on what got splashed.

Did you clean it off? With what, if so?

After soldering in new capacitors, I checked an unpopulated part of the board with ISP first, which did nothing, and then with acetone. Only the acetone seemed to dissolve any residue, and it didn't do anything to the board. I followed by dabbing around with acetone on a Q-Tip to clean up the boards. It wasn't actually that bad, though. The top of the capacitor bulged and some smoke came out, but little else.

Here is the extent of the damage to the only damaged capacitor:

WaNlB7Z.jpg
 
That depends on what you can get to.

Standard quick test is to measure resistance from each output to the + and - of the power supply, and reverse the leads (total 4 measurements). Unit should be "off", disconnected from power, and have sat for long enough to discharge the power supply (usually 10 or 20 minutes).

You should measure open all ways using the resistance range on a digital meter. If you get any low resistance measurements on the normal digital meter, then there is a problem.

Do NOT use the range that is used to check diodes. That usually has a diode symbol next to the range numbers.

Between the output and the power supply is an IGBT in parallel with a diode. The IGBT should be "open", and the diode only conducts one way. But the low voltage from a digital meter will not make the diode conduct, so it should measure open.

For most VFDs, this requires opening the case, and knowing where the power supply connections are inside.

I set my multimeter to resistance, autorange. Here are my measurements:

Red Lead U / Black Lead B+ = 0.652
Red Lead U / Black Lead B- = 3.75

Black Lead U / Red Lead B+ = 3.77
Black Lead U / Red Lead B- = 0.651

Red Lead V / Black Lead B+ = 0.647
Red Lead V / Black Lead B- = 3.79

Black Lead V / Red Lead B+ = 3.83
Black Lead V / Red Lead B- = 0.655

Red Lead W / Black Lead B+ = 0.652
Red Lead W / Black Lead B- = 3.77

Black Lead W / Red Lead B+ = 3.74
Black Lead W / Red Lead B- = 0.643

That all seems pretty consistent to me.

KMfkhrM.jpg


Just for fun I also used the diode setting and I got 0.390 with each of the red leads on the outputs and the black leads on B+, and then again in reverse, with B-. Again, that seemed pretty consistent.
 
so chances are good that the 760 volts that was produced on the dc bus when you powered it up at double the input voltage.. is not enough to blow everything up.

but something failed, and you need to find out what.


the igbts are most likely fine. how much is this thing worth?
 
so chances are good that the 760 volts that was produced on the dc bus when you powered it up at double the input voltage.. is not enough to blow everything up.

but something failed, and you need to find out what.


the igbts are most likely fine. how much is this thing worth?

LOL Currently $0!

All jokes aside, it only cost $230 so it's not a huge loss if it's gone forever. I'm considering this a learning experience and I've already successfully gotten it to light up after replacing the capacitors, so I don't feel like this is a sunken cost fallacy until I spend a good deal more.

I honestly think the whole thing is totally fine, and I just need to clear the SC fault in the logic.

If that's not the issue, what else could be causing this fault?
 
so chances are good that the 760 volts that was produced on the dc bus when you powered it up at double the input voltage.. is not enough to blow everything up.

but something failed, and you need to find out what.


the igbts are most likely fine. how much is this thing worth?

Does this mean you're not going to keep helping me? What could be failing?
 
Does this mean you're not going to keep helping me? What could be failing?


Unless you can find some sort of external thing causing the problem, it is inside.

Fixing VFDs with bad parts is a crapshoot, one that I would not spend much time on, and I have spent a number of years designing them, and debugging prototypes.

If you are experienced with the innards of VFDs, you have a shot at it, but often you run into unobtainable parts, or other issues making the repair a problem. The first issue is identifying the parts and sections of the unit. I have no issues doing that usually, but it's an essential capability if you expect to get anywhere.

Most folks do not have the experience to do that, and it is far more practical to chalk it up as a loss and move on to a new unit. It's advisable to figure out what went wrong, and not repeat that, though.

Does that unit have an ability to run from 120V? If so, then the comments about doublers and high voltage are relevant, in case the unit got hooked to 230 when it was set for 120. That normally messes up things that are hard to fix or replace.

I am not convinced that your issue is from that sort of failure. It sounds more like an accidental hooking of power to the output side. That will "break" many units, but not all types. Some can survive it...I know, I did it once, with no bad effects.

When it does mess something up, it is often a case of shorting the "commuting diode" on one or more of the IGBTs. The result is just what you see, although your measurements do not really show it.

The "3,7" numbers.... what was it measuring? because it looks like it was measuring on a diode range.
 
Unless you can find some sort of external thing causing the problem, it is inside.

Fixing VFDs with bad parts is a crapshoot, one that I would not spend much time on, and I have spent a number of years designing them, and debugging prototypes.

If you are experienced with the innards of VFDs, you have a shot at it, but often you run into unobtainable parts, or other issues making the repair a problem. The first issue is identifying the parts and sections of the unit. I have no issues doing that usually, but it's an essential capability if you expect to get anywhere.

Most folks do not have the experience to do that, and it is far more practical to chalk it up as a loss and move on to a new unit. It's advisable to figure out what went wrong, and not repeat that, though.

Does that unit have an ability to run from 120V? If so, then the comments about doublers and high voltage are relevant, in case the unit got hooked to 230 when it was set for 120. That normally messes up things that are hard to fix or replace.

I am not convinced that your issue is from that sort of failure. It sounds more like an accidental hooking of power to the output side. That will "break" many units, but not all types. Some can survive it...I know, I did it once, with no bad effects.

When it does mess something up, it is often a case of shorting the "commuting diode" on one or more of the IGBTs. The result is just what you see, although your measurements do not really show it.

The "3,7" numbers.... what was it measuring? because it looks like it was measuring on a diode range.

I am somewhat confused by this comment. I very clearly stated what I did to cause this, and what I have done so far to diagnose and repair it so far.

In the US, what I am going to call our “household” power is on two separate lines, and what I would call “domestic” power on regular outlets is only one of those lines plus a neutral and a ground. It’s what we call 115V (or 110V or 120V, depending on location or circumstance). Both lines, plus a ground (and sometimes neutral) is what we call 230V, and is used by ovens, dryers and electric car chargers. Stop me if I’m wrong.

To recap, this VFD accepts 115V and 230V single phase inputs, however there are zero instructions for plugging it into a step-up transformer, which puts all of the current on one lead, like in European houses. I took a gamble that it would work when changing nothing from the input wiring for 115V, and I lost that bet.

I took it completely apart (images above) and replaced the four large capacitors on the power board. This caused the logic board to power up again, and I even have access to the parameters.

I measured the resistance between the B+ and each lead, and the B-. The numbers are listed above. To satisfy my curiosity, I followed that up with the diode setting and each read exactly 3.90. I did not use the diode setting on the other values.

I finally talked to KB Electronics today, and they weren’t prepared to talk about actual repairs. They would only test the boards and scrap what doesn’t work. I would likely have all three boards replaced for near what it would cost new, and since I attempted to fix the power board I probably voided the warranty.

I would prefer to find what’s actually wrong with it and solder in new components, and I would rather do it myself with a little help. I am aware that this is not work for the faint of heart.
 
what i would do at this point is find the 6 wires that turn the igbts on, and disable them. desolder the pin from the board.

then power it up and see if it still says sc fault.

if not, then connect them back in 1 at a time and find out which igbt, or which current sense monitoring circuit is broken.

these drives don't actually measure the output current, they measure the voltage drop across that 13mOhm resistor soldered on the board, that voltage gets filtered. you could try cleaning the board again.
 








 
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