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.

 

[Ahr35181]

ejnjkj

"V.F.D's just aren't reliable in any working conditions"

We've been using them with for many years with few problems, particularly modern types.

“They are very complicated electronic devices, and unless you are an electronic wiz they are not serviceable”

The old versions were easy to test & replace individual parts, although I agree modern types do require specialist attention when they fail, although they are very reliable.

In my experience modern electronic devices are much more reliable than older electro mechanical / mechanical devices.

Alan

 

[ScottyUK]

ejnjkj

In Europe VFD and VSD is pretty much used interchangeably. What's the difference in your part of the world?



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If we learn from our mistakes,
I'm getting a great education!

 

[panelman]

looking at payback.com it appears that his idea of a vsd is some sort of electro-mechanical clutch used with a full speed motor

what happened to the original poster? some feedback would be nice

 

[ScottyUK]

The Payback product appears to be an eddy current coupling rather than a mechanical clutch. It must be very lossy in the coupling. Although torque transmission doesn't seem to be a quoted parameter, from the principle of operation I doubt that it will have very good torque characteristics, which would limit its application to fans and similar loads.

It is an interesting idea, but the addition of more bearings into the system plus the control electronics doesn't seem to eliminate the perceived reliability issue, it just moves it from one place to another.



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If we learn from our mistakes,
I'm getting a great education!

 

[ejnjkj]

scottyuk
Yes eddie current drives are for variable torque applications(and in my opinion the only way to do it), and are not used on all constant torque applications because of slip losses. So there are lots of places I would install a VFD and a heavy duty inverter rated motor.But only where a VSD will not perform. This is all way off topic from igenius(sorry). And my first statement might have been little harsh towards the VFD.

 

[DickDV]

Scottyuk must have stock in a network of electric motor rewind shops. Either that or the electricity in his area must be free!

Only use an eddy current clutch on variable torque loads??? Mercy! I haven't heard a comment like that in at least 15 years and I hear a lot of comments!

Scottyuk, I'd be genuinely interested in a rational explanation of why you hold that opinion. I'm not trying to be offensive but I am a bit stunned by your statement and am interested in why you have this view.

 

[panelman]

Sounds like fighting talk to me says he standing well back

 

[DickDV]

I certainly don't want a fight here. I'll just withdraw my comments, scottyuk. Sorry.

I was so surprised at your response that I guess I just went over the edge!

So, again, my apologies.

 

[panelman]

I'm sorry.....I'll have to get one of those smiley things to show what is not meant to be taken seriously

Engineering debate, differences of opinion and exchange of ideas is what this site is all about so carry on

I'd still like some feedback from the original poster...iGenius where are you?

 

[panelman]

:-D got one

 

[ScottyUK]

Hi DickDV,

Apology declined - none needed!

I was only observing how the Payback unit appeared to operate from observation of the sectional drawing on the vendor's website. I wasn't advocating use of it on a variable torque load - my post actually said "It must be very lossy in the coupling". But that appears to be what Payback are using it for in order to get variable speed to the load. The high losses presumably account for the copper lining material that they highlight as a feature of their product.

It sounds like I might have to withdraw my comments and allow you to post a more knowledgable explanation of use of eddy current clutches. They are not somthing I've worked on to any great degree - perhaps half a dozen times - and I'll hopefully learn something new from someone who knows better.

Please keep posting - I enjoy reading your posts. I have no problem at all when someone tells me I'm wrong: it proves they are reading my posts if nothing else!



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If we learn from our mistakes,
I'm getting a great education!

 

[DickDV]

As I look back over the thread, I see that, in addition to being a bit blustery in my disdain for eddy current clutches, I also mis-identified the source of the comments I was responding to. Turns out it wasn't Scottyuk but rather ejnjkj. Guess I was having a really bad day.

Regarding the eddy current clutch, while it is functionally acceptable, the efficiencies are terrible. Even on variable torque loads, the motor is fully magnetized at all loads which, compared to a VFD is wasteful. The clutch itself functions to pass motor torque directly to the load while reducing the speed based on the exciter current level. At maximum speed, the clutch still slips around 3% usually, and as speed is reduced, the losses go up accordingly.

The speed-reduction losses can be calculated bases on the familiar equation hp = torque x rpm/5252 where torque is the load torque in ft-lbs and rpm is the difference between the motor rpm and the output shaft rpm.

It is easy to see that at 50% speed, the losses in the clutch alone are equal to about half the motor hp. Is it any wonder that larger eddy current clutches are water cooled?

By contrast, an electronic inverter will consume only the required load hp plus about 3% for inverter losses plus a little for motor magnetization.

I do a lot of stamping press conversions from eddy current clutch drives to straight AC drives and find that the energy payback is often 12-15 months where energy costs are in the vicinity of $.08/kwhr. If the building happens to be air-conditioned (not common), the energy cost recovery is much faster due to reduced cooling load.

 

[ejnjkj]

If you were to draw out a fan curve comparing VFD to a VSD( I am using specs from the payback drive) with Kilowatts vs load. You can see at 50% the VFD is under 5% more efficient. That’s pretty close.
Having the motor energized and spinning full speed is not as wasteful as you think, because without a load it pulls very little kilowatts. The power consumption throughout a fan curve(variable torque) are really close to a VFD, although 3%-5% less efficient.

On a constant torque apl.(stamp press) I can not argue with you, because there are kilowatts being abused.

Dickdv-This comment was supposed to be for me?
“Scottyuk must have stock in a network of electric motor rewind shops. Either that or the electricity in his area must be free!”
I must disagree. I would love to see a motor harmed in any way by an eddie current drive!! I thought everyone knew about VFD’s causing premature motor failure due to harmonic distortion and heat. And with 4% more energy being used, no one will be going hungry.
One more point that is being overlooked is maintenance. Sure you save a little energy with a VFD. But what if you’re the guy they point to when it burns up or keeps dropping out of service for no reason. It’s nice to know that I can buy the entire electronic controller for a 200 H.P drive for 195 dollars!

 

[Marke]

hello ejnjkj

I am interested in your comments that the eddie current coupling is almost as efficient as the inverter drive system. Is this what you mean, or am I missing something here?
In my understanding, if we look at the inverter drive system, we have the motor losses, plus a small amount due to harmonics etc, and minus an amount due to the reduced fluxing, (assuming a modern drive). We have perhaps 5% losses in the inverter itself, but we have no additional slip losses.
So if we take an application where the load is running at 75% speed and 50% load, then the motor losses would concievable have reduced by a small amount, the slip losses (in the motor only) would be in the order of 2% and the inverter losses would be in the order of 2.5%

With the eddie current coupling, the motor losses are not altered. The eddie current coupling and control has losses that can be as high as the inverter losses (or higher) plus there is the addition of the eddie current slip losses.
In the example above, we have an additional slip loss of 25% of the shaft power which is an additional 12.5% (assuming that the basic losses of the eddie current coupling are of the same order of magnitude to the inverter losses.)

In a constant torque application, operating at half speed, you would have the motor producing full torque at full speed, (full load) the eddie current coupling would be dissipating half the motor rating and the load absorbing the other half.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

ejnjkj, I agree with you that the load kw on a variable torque load as a pump or fan drops off dramatically as the speed slows so the losses in the eddy current clutch drop dramatically too.

However, your statement that no-load kw in an induction motor is very low is not really true. Typical no-load currents in induction motors run between 22 and 35% of full-load current. This represents considerable kw losses simply to magnify the motor before any useful torque is produced.

Also, modern AC drive losses are running about 2% of load kw not 5% as stated. In addition, properly designed, an AC drive and motor need not be a high failure rate item even over 10+ years. After all, even tho you can replace an eddy current clutch exciter circuit for $195, the clutch does occasionally need bearings and, if memory serves me right from my motor shop days, it will take a lot more that $195 to replace bearings even on the smallest clutch. And, in reasonably normal industrial use, the bearings will probably need replacing in 7-8 years if not sooner.

But, anyway, if clutches are what you are comfortable with and you have good sources for them, a variable torque load would be the best possible application for them. Also, applications that don't normally run with much speed reduction are a better choice too. Many pumps run like that.

 

[tmahan]

Back to the original problem of a failed VFD....
There are as noted above many reason for a drive (or any device to fail). It may very well turn out that neither the welding or the blocked fan was responsible.

Having said that though , I am in the camp of the blocked fan. Although most electronic starters have thermal switches designed to trip at a given temperature there are usually (in my experience mounted on the heat sink or the actual switching device (IGBT, SCR...)

My practical experience is that thermal damage usually occurs over time, at lower temperatures and effects the control board rather than the power components. This means that a unit can be damaged due to high temperatures but never trip on "over-temp"

 

[iGenius]

Hi to everyone!

We have finally finished our battle with Danfoss in South Africa and unfortunately we have not received the answer we had hoped for.

First of all, my apologies for the long delay in this process. This is not a common problem that occurs with VSDs, so it took both ourselves and Danfoss a long time to sort it all out.

Below is the final e-mail received from Danfoss SA based on their investigation of the problem:

<Quote>

Hi,

With regards to our investigation about the welding of brackets onto the Danfoss Drive.

This welding onto the chasiss of the drive can be detrimental to the components of the unit.
The first damaging effect can be caused by excessive heat transfer onto the components.
The second problem is the stray currents that flow through the unit while welding.
The effects may not be physically noticable, however some of the components are weakened, and may fail at any time.

I would therefore recommend not welding any material onto the drive at any time, unless however all components are removed prior to the welding being done.


Dave Dyce
Product Manager

Danfoss (Pty) Ltd. - member of the Danfoss Group
Motion Controls
Johannesburg, South Africa

<Unquote>

In a case such as this, it is extremely difficult to determine exactly what caused the failure. Apart from the technical difficulties, there are such a variety of different opinions about this that it cannot be said with authority who is right and who is wrong.

We have learned a very important lesson from this - do not bring welders close to any sensitive electronic equipment.

Thank you very much to everyone who responded and gave their opinion. Personally, I have learned a great deal from each of the responses and I appreciate the assistance that you all gave in this regard.

 

[Skogsgurra]

iGenius,

Given the circumstances. What else had you hoped for? Of course, Danfoss takes the bite that you dangled right in front of them.

I can accept that the heat that resulted from the welding can have done something to the thyristors, even if that seems rather far fetched. But the stray current theme is utter bullshit.

I agree that you shouldn't do such things to inverters. It is not good practice.