# Why, oh why. Powerflex 70 bit the dust.



## HARRY304E (Sep 15, 2010)

That will keep you busy...:whistling2:


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## varmit (Apr 19, 2009)

VFDs are sensitive to line side power disturbances. A "bolted short", as the 10 HP motor starter likely was, on the same buss with the VFD, was probably the culprit. A power surge or lightning could have taken out the original 10 HP motor and damaged the drive also.

A line reactor on the line side of a VFD is always a good idea to LESSEN the likelihood of this type of failure.


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## Big John (May 23, 2010)

Not a drive guy, but a capacitor failure says to me that you got a pretty good sized transient overvoltage to the drive. Just a guess: There were no lineside reactors on this VFD? Faults cause a bunch of really irregular waveforms in power systems, and sometimes you do end up with voltage spikes many times higher than nominal. Maybe one of the PQ guys can explain why.

There is something about special about capacitor shorts that makes them fantastically loud. I've heard a couple let go that left my ears ringing.

-John


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## acro (May 3, 2011)

Yea, no lineside reactors on this one. I know VFD's can be sensitive and you don't want to back feed from the load side. But this had to be a pretty big spike, I suppose.


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## MDShunk (Jan 7, 2007)

I've found the PowerFlex drives to be VERY sensitive in that regard. I've had a single drive fail and take out two or three others before. Line reactors solve that. 

This never happened when I used TB Wood's drives. Those things were bulletproof. The A-B drives are pretty weak, but very featureful and uber popular.


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## acro (May 3, 2011)

Any particular concerns about sizing one? How close to fla? Would 85a be too large for a 40hp motor(fla around 50)


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## MDShunk (Jan 7, 2007)

acro said:


> Any particular concerns about sizing one? How close to fla? Would 85a be too large for a 40hp motor(fla around 50)


Not sure what you're asking. A 40hp motor gets at least a 40hp drive.


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## micromind (Aug 11, 2007)

acro said:


> Any particular concerns about sizing one? How close to fla? Would 85a be too large for a 40hp motor(fla around 50)


An 85 amp drive will run a 40 HP motor just fine. 

Some drives have a low limit to set the motor current, but it's way low.


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## varmit (Apr 19, 2009)

MDShunk said:


> I've found the PowerFlex drives to be VERY sensitive in that regard. I've had a single drive fail and take out two or three others before. Line reactors solve that.
> 
> This never happened when I used TB Wood's drives. Those things were bulletproof. The A-B drives are pretty weak, but very featureful and uber popular.


Some other "bombproof" drives are the old Reliance SP-500. I saw one on a bag filler (50 to 80 pound bags) that was powered off and on FOR EVERY BAG. This equipment filled hundreds of bags a day, for years with no problems. I told the plant manager that I could fix this to work "more like it was suppose to". He agreed. I rewired the drive to utilize the start-stop inputs. This drive is about 12 years old now, eight years since I modified the controls, and still running every day. Yes, I realize that I have now jinxed this.


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## Introyble (Jul 10, 2010)

You could of seen this problem coming if you were familiar with the DC links, you just do a capacitor check between the phases.

Common problem of heat exhaustion.

Hint: Find the DC - link

Axial (spot) fan probably bit the dust? ???????


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## JRaef (Mar 23, 2009)

My guess would be this:

When they used the "screwdriver control method" on the contactor, they caused almost all of the damage, the other stuff was colateral; after the fact. When contacts close and current begins to flow, there are strong opposing magnetic forces created by the current flow through the contacts. The coil strength is designed to overcome that. The bigger the contactor, the bigger the coil has to be to prevent those magnetic forces from opening the contacts again and allowing chatter. Whenever someone challenges me on that by saying that those coils don't exert that much force, I challenge back that they should put their fingers in the armature while I close the coil circuit and see if they can hold it back from closing. Noboby has ever accepted that challenge.

On a Size 1 contactor, especially a Buletin 505 like that one where the armature is vertical, there is almost no way a doofus with a screwdriver can exert enough mechanical force to keep the contacts closed against the magnetic force trying to open them. So what happens is, the contacts chatter, violently and rapidly. Every time a set of contacts opens, the arc created raises the voltage above the line voltage level in a sort of a capacitor effect, causing a nasty transient. One opening contact is relatively benign, but a rapid chatter makes things a LOT worse in that regard. That voltage transient will not only smoke the components in that starter, but will exist on the line feeding it. 

Then if you have something else with capacitors on it, like a VFD, that rapid charge / discharge of that voltage transient will act like a tank circuit with the caps, charging them up to a very high voltage, especially if they are not connected to anything on the load side to absorb some of it. So his little screwdriver hack caused a voltage event that exceeded the maximum rating of the caps, causing one to be the weak link and pop. That drive is essentially toast all around now. It's also not something peculiar to PF70s, I've seen it blow lots of different drives. 

Line reactors would have helped a little, as would surge arrestors. But the best thing is to educate the guys who decided to run that starter with a screwdriver.


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## JRaef (Mar 23, 2009)

MDShunk said:


> I've found the PowerFlex drives to be VERY sensitive in that regard. I've had a single drive fail and take out two or three others before. Line reactors solve that.
> 
> This never happened when I used TB Wood's drives. Those things were bulletproof. The A-B drives are pretty weak, but very featureful and uber popular.


 
Re: PF70's being sensitive and "weak", I beg to differ. But some people have had problems because of a particular change in the way A-B drives are hooked up that they may not be aware of. When A-B introduced the PowerFlex family to replace the old 1336 drives, they did so because they went after a wider world market. IEC style drives all come with MOVs in a Y configuration with a reference to ground, because elsewhere in the world, all line voltages are Wye with a ground reference, no Delta, very little HRG used. But here in the US, the MOVs need to be floating with reference to ground if you have a Delta power feed or an HRG system, and those are plentiful. So A-B then put a jumper link in the PowerFlex drives on the ground reference connection for the MOVs, with very clear instructions in the manual as to when and how to remove that link. Other mfrs have the same sort of issue, but they make you clip a wire or replace the MOV board with an ungrounded version. At least A-B made it relatively easy, if you RTFM.

But hardly anyone will ever RTFM, so that link is there almost EVERY time I go out and look at a PF drive in the field and see a Delta feed or an HRG system. In the old 1336 drives (and other US based manufacturers like the old TB Woods and Reliance) they didn't have the ground reference so nobody needed to think about it. The thing is if there is never a voltage transient event you may never know the potential problem exists, then if it happens years later, it's often blamed on the drive being "old" or "weak".


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## JRaef (Mar 23, 2009)

acro said:


> Any particular concerns about sizing one? How close to fla? Would 85a be too large for a 40hp motor(fla around 50)


Wityh PF7 series drives, they come in "Normal Duty" and "Heavy Duty" versions. Normal duty means pumps and fans, 110% rated current for 60 secons, 150% for 3 seconds. Heavy duty means anything that might take some extra "umph" to get moving from a stand still, 150% current for 60 seconds, 200% for 3 seconds. So if you always buy the heavy duty rating, which is essentially one size up from a nromal duty rating, you should be fine for anything. But if it's a pump or fan, you may not need more than the normal duty sizing.


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## acro (May 3, 2011)

MDShunk said:


> Not sure what you're asking. A 40hp motor gets at least a 40hp drive.



Sizing the line reactor is what I was referring to, not the actual drive. Some of the surplus sources just list an amperage, not HP and I image that oversizing a line reactor would decrease it's effectiveness, but wondered how closely matched it should be.

I am likely to get a new one properly sized, but still would like to know.




Introyble said:


> You could of seen this problem coming if you were familiar with the DC links, you just do a capacitor check between the phases.
> 
> Common problem of heat exhaustion.
> 
> ...


Well, that is possible, The environment is very dusty. I did not check them electrically, but all 3 fans do spin freely when I blow on them.

I am not familiar with the test you are referring to.



JRaef said:


> Line reactors would have helped a little, as would surge arrestors. But the best thing is to educate the guys who decided to run that starter with a screwdriver.


Well, this screwdriver "2nd station" has been sop for years. We have had a couple of electricians over the years start to wire in a 2nd set of start/stop/reverse controls in the mcc, but they never seem to get it done. Probably going to be up to me to do it.

You could be right about the chatter, but whenever I have seen it done, they just push the coil in, and then the latch circuit takes over.

And you said the drive is "toast all around now", I had plans of sending it to Global Electronic Services tomorrow to see what they can do with it. Waste of time?


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## acro (May 3, 2011)

I don't think it is related, but I thought I might mention it.

Like I said before, I bypassed the vfd with a standard starter. An old Westinghouse A200.

Anyway, we have nearly all of the controls in that mcc fed from a common breaker in the single phase panel. This has been this way for well over a year. Well, we developed a problem with this starter not pulling in. I was checking the voltages and could not find where the common or ground for the control was bonded to the mcc structure. This was allowing the 120v to drop to about 20v when the start button was pressed.

I had to install a bond to get it to work consistently.


This possibly causing some chatter when the coils pulled in, but it was never evident when I was in the room.


Thanks for the replies.


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## JRaef (Mar 23, 2009)

acro said:


> ...
> You could be right about the chatter, but whenever I have seen it done, they just push the coil in, and then the latch circuit takes over.


Assuming there IS a latch circuit, then that wouldn't be as bad for sure.



> And you said the drive is "toast all around now", I had plans of sending it to Global Electronic Services tomorrow to see what they can do with it. Waste of time?


Waste? Well, you are going to have to remove it either way, so that part's not a waste. The only risk is the freight cost and any fees charged for evaluating it to find out that it's not repairable. Chances are it's not going to be wiorth it, any voltage transient big enough to take out the caps will have stressed or damaged the transistors too, and if they are gone, it will be cheaper to replace it. 

If you have a local A-B distributor, I'd take it to them and have it evaluated and repaired by A-B Services, they do that sort of thing. They charge a $100 evaluation fee that comes off of the repair cost if they deem it repairable.


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## Introyble (Jul 10, 2010)

Since the vfd is out and already blown try this.

Identify the DC (-) link and output phases which well call UVW.

Put your DMM on capacitor test. Put your positive lead to the DC - and your negative lead to U.

Repeat through the phases.

You can also identify the DC + link. When you test from the positive link make sure your negative lead goes to the + link.

Try that out, see if you can see your short.


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## Introyble (Jul 10, 2010)

Not to clog your blog but from my experience every time I seen a capacitor blow in a VFD (the modern ones) 99 times out of a 100 it's from heat exhaustion or from the drive being undersized.


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## Big John (May 23, 2010)

JRaef said:


> ...On a Size 1 contactor, especially a Buletin 505 like that one where the armature is vertical, there is almost no way a doofus with a screwdriver can exert enough mechanical force to keep the contacts closed against the magnetic force trying to open them....


 Yet another good reason why folks shouldn't jam screwdrivers in starters.

I think I might set up a little test with a couple good sized contactors and a scope to see if I can demonstrate that chatter.

Is the opposing force generated from the CEMF in the coil or the current in the poles of the contactor?

-John


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## JRaef (Mar 23, 2009)

Big John said:


> Yet another good reason why folks shouldn't jam screwdrivers in starters.
> 
> I think I might set up a little test with a couple good sized contactors and a scope to see if I can demonstrate that chatter.
> 
> ...


Current through the contacts. If you look at them, there is usually a "horseshoe" shaped path to the current. In your mind's eye, imagine being able to wrap your hand around the two sets of conductors and apply the Right Hand Rule to them. You will see that the magnetic forces will oppose each other, trying to push the contacts apart.


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## acro (May 3, 2011)

Introyble said:


> Not to clog your blog but from my experience every time I seen a capacitor blow in a VFD (the modern ones) 99 times out of a 100 it's from heat exhaustion or from the drive being undersized.



You may be onto something, as I did see an occasional overheat fault, but this failed when the drive was not "on".

I have the drive back together and will be searching for or making a suitable box in the morning. Too many other things going on right now to spend more time poking around on it. I have a new Windows 2008 server to build tomorrow.

But I do need to do some more homework and reading, as the occasional overheats seems to have started after my last problem/change of parameters. Forgive me for not looking it up right now. It will have to be later.


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