VSD DAMAGED - WHY?

[iGenius]

Hi there! I hope someone will be able to assist me. I work at a factory in South Africa. Recently, we installed a Danfoss VSD to drive an induction motor. About one month after installation, the VSD stopped working. We are now trying to establish the cause for this. During installation, brackets were welded onto the casing of the VSD to mount it against a wall. Some of our engineers now reckon that the VSD stopped working as a result of this welding. I question this however, especially since the VSD worked fine for just over a month. On inspection, I found that the cooling air inlet at the bottom of the VSD was completely blocked. I therefore suspect that that the VSD consistently ran at an above-average temperature during the month, during which the components were gradually damaged until the finally failed after a month. Is this possible? Do you have any other ideas? And most importantly, is it possible that the VSD would be damaged by the fact that we welded the brackets onto its case? Thank you for your assistance.

 

[Skogsgurra]

Hi iGenius,

The welding did not do any harm (if you did not get metal drops inside the unit, but then it wouldn't work from the beginning). Period. There is absolutely nothing to discuss there.

The blocked ventilation should have led to a warning or a fault signal. The VLT 6000 HVAC range, for instance, would have produced alarm 29 (high heat sink temperature). Did anyone see such an alarm during operation?

Then again, inverters do fail for hundreds of reasons. But I think that the blocked air is the culprit.

 

[DickDV]

I tend to see it just about opposite of skogsqurra. Modern drives are well protected against high-temp operation and would have produced a large number of bothersome high-temp faults if the air blockage was problematical.

On the other hand, depending on the type of welding done, damaging currents could easily have been induced into some drive components causing stress damage that only appears over time, such as a month. Many electronic components are vulnerable to this kind of failure mode.

Having said that, all of us are simply speculating. Why not send the drive back to Danfoss and have them analyse it for cause of failure. They would probably want to do this for warranty claim purposes anyway.

Then we can know with greater certainty.

 

[buzzp]

I tend to agree with DickDV on the welding issue. There are concerns with this and they mostly depend on where the ground conductor for the welder was placed. Even so, damage could still be done if the ground was placed right next to place to be welded if the drive ground was hooked up (or anything else for that matter) when they were welding.

I would also think the drive would have tripped out on over temp or a thermal shut down, at least once if the blockage was causing problems. Then again, maybe they do not have provisions for this. Smaller semi-conductors often have a thermal limit where they shut down to avoid damage. Not sure if the drive your using (or any drive for that matter) uses any such components.

 

[panelman]

i wouldn't have thought that danfoss would be that interested in a warranty claim for a drive that had had bits welded onto it...unless you are a good customer

 

[GGOSS]

Hello iGenius,

DickDV & buzzp appear to have covered it well. Component failure resulting from transients that occured some time ago is not uncommon at all. In fact I could list several dozen examples of of this given the time and space.

The comments they have made in relation to nuisance trips arising from the blocked ventillation are also extremely valid.

Regards,
GGOSS

 

[Skogsgurra]

Hello all,

I think that I have to clarify a few points regarding the welding and what can happen:

First. Welding can be resistance, gas, arc etc. We do not yet know what kind of welding took place.

Second. Internal components could not have been affected by currents and voltages that occurred during welding. I think that it is safe to assume that the welding was done before the inverter was installed (brackets needed to install it) so there were no cables attached to the inverter.

Third. Induced voltages? A normal arc weld with a few hundred amperes can not induce any appreciable voltage in the traces on the PCB. There can be a few hundred millivolts induced in the cabling, but these voltages will only hit components that are designed to withstand normal mains voltage. Even a resistance weld with 10 kA (clearly overkill) does not present any problem.

Four. I have designed thyristor controlled resistance welders where the copper bars with 16 kA peak current passed close (50 mm) to the processor and control board and we never had any problems with induced voltage. And, of course, no destruction of components.

Five. The "expose now, failure later" theories are valid when you discuss ESD damage. But it is not possible to envision any such damage given the welding situation.

Six. My opinion is also that it was the thermal blocking that caused the problem. We do not know what Danfoss inverter we are discussing, but the one I mentioned does not trip on high heat sink temperature. It just outputs an alarm. I am not so sure that this alarm was taken seriously. It is possible that it was just ignored.

I said in my first posting: "There is nothing to discuss. Period" And I did say it with the purpose NOT to have this discussion. I failed, obviously.

 

[panelman]

skogsgurra....if you have a 6000 that doesn't shut down on high temp then you have a faulty one. They all shut down on high temp. The early (pre 1999) ones also used to shut down & lock (you had to cycle mains and then reset rather than just reset)

iGenius.....if the controls power up you can look at parameter 604 which will tell you the number of overtemps the unit has seen which might shed some light

I wouldn't take a welder within 10 feet of a new drive and would be amazed if danfoss even look at it as a warrenty claim

 

[DickDV]

That no drive manufacturer would seriously consider warranty replacement of a drive that has been welded on speaks volumes as to the potential for damage to internal components.

And, as far as drives that don't trip on high heat sink temp, I doubt that any responsible supplier would design a unit that way, but, if you happen to have one, note the brand and choose something else next time.

If your drive only had the capacity for three faults, I would think high temp would be one of them.

 

[panelman]

DickDV....you are right, no-one would make a drive that doesn't trip on high temp, as a clue, Danfoss drives have 25 or more faults that will always trip it and maybe twice as many that will give a warning and can be programed to trip it.

Re the warrenty, I don't think danfoss have commented as yet but i would be surprised if they even considered itas a warrenty claim......welding bits on is abuse even if it didn't cause the problem

 

[buzzp]

My take on Skogs replies:
1. You are right we do not know what kind of welding occurred.
2. I agree that cables or grounds may not have been attached during the welding for the sake of the drive, however, a hot and return have to be attached to weld (assuming electric welding). If the ground clamp was placed on the bottom of the drive then, of course, all of the current has to make its way to the ground clamp. The paths it takes to get there are all over the drive. These multiple paths could present problems with components on the board. They can also induce voltages on parts close by to a level they are not designed for (as little as 5-10volts or maybe smaller depending on the processor). This could have weakened components that are normally there to protect in such a case to where when the next incident the component failed. Very easy, to me, to see happening.
4. 16kA flowing within 50mm of a processor is not a good idea. Typical in these types of cases, things appear to work ok but occasionally, something bizarre happens that is not explainable. Attribute this to 16000 amps flowing next to the processor.
I am not saying your statements are entirely wrong but welding can and will damage components. Any welder at an electrical shop can probably tell you cases of welders damaging components.
In any case, I still stand firm that welding could have caused the drive to fail later on.

 

[Skogsgurra]

Hello again,

Sorry that I seem to have upset you. But the fact is that you cannot hurt a modern frequency inverter by welding on its case (if you do not let molten metal into it or overheat it through prolonged welding). The reason is that there is isolation between the power electronics and the chassis. There is also isolation between the electronics and the power electronics. The electronic parts are connected to ground in one point or (if grounding is done around the edges) to a solid piece of plate or aluminium which does not develop voltage gradients as a result of the welding current. That is part of the EMC strategy and no inverter will survive its own EMI if you fail to do that part right. "Connected in one point" means that there is no way for the welding current to enter and leave the active parts - hence no current.

Regarding thermal tripping. My personal experience is only from the 6000 HVAC and in it, parameter 411 decides if the inverter shall tripp on overtemperature or not. If this has been programmed not to trip (and I can very well think of a situation where it was decided to program it not to trip instead of cleaning the air intake) then it simply will not trip.

I am afraid that this question has turned into a "conversation piece" and I do not mind at all. But the main issue was/is to help iGenius with his problem Are we doing that?

 

[panelman]

skogsgurra.....411 does not stop it tripping if it get too hot

Setting 411 to 1 will make the drive reduce frequecy & output power as the temp approches the trip level in an attempt to stop the temp rise. If this fails and the drive hits the trip temp (80 or 90 deg) then it trips and can't be reset until it cools down.

iGenius...got an update for us?

 

[Skogsgurra]

Yes panelman. You are probably right.

I never had that situation. Never ran it that hard. Does it really trip if the frequency and current reduction doesn't help. Manual incomplete?

We need feedback from iGenius.

 

[panelman]

skogsgurra

If reducing the switching frequency and current don't stop the temp hitting 90 deg (maybe 80) then it trips and stays tripped until it cools to i think 60

You are right, entertaining as this is we do need an update...over to you igenius

 

[Farkel]

Let's go back to basics fellows. What components failed? Then we may be able to determine the cause.

 

[Ahr35181]

Lots of interesting replies here.

We used to install a lot of Danfoss drives, Danfoss strongly recommend the fitting of a back plate if the drive is not mounted directly, to ensure all cooling air is dragged passed the cooling fins.

Anyway, why on earth would anyone weld brackets onto a sophisticated piece of equipment??? Channel bar (unistrut) is a wonderful thing which only requires nuts & bolts!!

Alan

 

[iGenius]

Hi to everyone! Sorry for only getting back to you now but I was out of the office for a couple of days.

I appreciate the enormous response I got and must admit that I am somewhat overwhelmed but the varying opinions. We take note of them all and this really helps us to view this problem from all angles.

I regret to inform you that I cannot give you much final feedback at this point in time. You need to keep in mind that I work in South Africa, and in this wonderful country everything happens a little slower than in the rest of the world (we call it "Africa Time"). We are still in the process of conducting our own investigation and Danfoss will assist with this. All of your comments will be used to assist with achieving an accurate outcome.

Some people enquired about the type of welding - we used electric welding. And no cables were connected to the VSD during the welding process.

As soon as we have a final verdict, I will inform you and also give you the reasons for our final conclusions. But as I noted, this might still take some time.

Kind Regards

 

[Skogsgurra]

Thanks iGenius. We are waiting "African way". So take your time.

 

[ejnjkj]

V.F.D's just aren't reliable in any working conditions. They are very complicated electronic devices, and unless you are an electronic wiz they are not servicable. I would suggest switching to a V.S.D. I did. You should check out payback.com. I must sound a little one sided, but this mystery problem happens all the time with v.f.d's.