...from thread thread 237-66987 [c 1

[jOmega]

...from thread thread 237-66987
Use of VFD output side contacter for E-stop

jOmega said:
"Any contactor that is rated my its manufacturer for ac motor duty, is ok. Suggest NEMA rated as opposed to IEC as an IEC contactor will likely shorten motor life."

ScottyUK asked what the difference was, to which jO replied:

"The IEC contactors have a shorter throw length and studies have shown this to cause motor insulation failures."

afterhrs and GGOSS have requested a additional discussion and it makes sense to start a new thread as afterhrs suggested

Last papers I remember were over 10 years ago ... within a couple of years of the IEC contactors being introduced in USA. Don't recall if article was in trade pub. or in IEEE journal.... Might have a copy somewhere in my archives...
Will look for it. Otherwise I don't have access to the source material.

From what little memory I have left, the gist of the research came about from reports of motor insulation failures when operated with IEC contactors. The investigation led to the fact that the shorter throw length of the IEC contactor promoted more arcing in the process of opening the circuit. This arcing produced voltage stress levels of very high dv/dt that resulted in breakdown of motor insulation.

The NEMA contactors, having a longer throw length, which assures that the arc is extingushed.

Going back to fundamentals where E = L di/dt .... picture the process of opening a set of contacts.....as they begin to separate, arcing occurs.... as the distance between the contacts increases, a point is reached at which the arc is not supported.... and it dies... as a consequence of the distance.... as opposed to the shorter throw length configuration whereby the extinghishing of the arc occurs after some time as a natural diminuation of the decaying energy.

Allen-Bradley is said to have also performed an investigation into this phenomena not long after they introduced their Bul. 19x IEC contactors... and that the investigation substantiated the motor insulation failures and showed the cause to be related to the shorter throw length. That report never saw the light of day and from a marketing aspect, I'm sure you understand why.

That's the recollection....

Now it may be, that IEC contactors have since addressed this issue... and effected product modifications to eliminate this problem.
But not being aware of same, I made the statement about using NEMA instead of IEC contactors.

jOmega

p.s. interesting add to the above comments about Allen-Bradley is this statement in their application note #114 as follows:

"3. Smaller, Iess expensive motors that have less of a design safety factor will be more susceptible to possible damage from any source. (IEC contactors can cause failures in lesser design motors)"

I leave it to you to decide for yourselves what they mean by "lesser design motors"....... non-inverter rated perhaps ?

jO

The app. note can be found at:
Bulletin 1336 Plus Igbt Technology Application Note # 114

http://www.ab.com/support/abdrives/1336s/3114.pdf

 

[GGOSS]

jOmega,

Many thanks for the insight.

Regards,
GGOSS

 

[GusD]

Hi Jomega

I fail to see the connection in bettween Contactor
performance and motor insulation failure.
Of all the probabilities and causes for motor failures
this one was not at the top of the list.
Perhaps we have something new here.




GusD

 

[ScottyUK]

jOmega,

Thanks for the info - not something I have come across before, probably because the IEC contactor manufacturers don't exactly advertise this!

Scotty.

 

[jbartos]

Suggestion: If NEMA contactor dimensions are compared to IEC contactor dimensions, the IEC contactors tend to have much smaller cube dimensions. Therefore, the reduction of the arcs during contact parting has to be accomplished by the contactor contact parting time.
Consider:
E=Ldi/dt..........................................Eq.1
i(t)= (1/L)integral from 0 to t of (E dt) + Constant...Eq.2
v=ds/dt............................................Eq.3
s=distance
v=speed
t=time
If the IEC contactors have reduced the distance s in Eq.3 because of smaller cube dimensions, then under an assumption that v = constant for NEMA contactors as well as for IEC contactors, and it is about the same. Then, the time t has to be reduced in:
s = v x t..........................................Eq.4
This implies that the IEC contactors open faster since t has to be smaller in Eq.4. Now, if I or i(t) in Eq.2 is supposed to be about the same for NEMA and IEC contactors, then the voltage E in Eq.2 has to increase to have Eq. 2 valid. Therefore, this is where the problem with the motor insulation starts. The higher E has a negative impact on the motor insulation. Then, come to think of the contactor selection, the choice is clearly seen.

 

[GGOSS]

Hello all,

Have been looking for more (published)information on this subject without luck. Surely however it would be reasonable to expect the arc chute design in modern IEC contactors is such the the arc is quenched in much the same time/manner as it is in Nema contactors.

Regards,
GGOSS

 

[afterhrs]

GGOSS,
I suggest you go to google and run this search
"IEC vs. NEMA" or "NEMA vs.IEC" and you will find some
articles that way. One of them by A-B.
If anyone wants the real world pros and cons for
each, let me know.

Regards,
Afterhrs

 

[jOmega]

JBartos:

You said:

"Suggestion: If NEMA contactor dimensions are compared to IEC contactor dimensions, the IEC contactors tend to have much smaller cube dimensions. Therefore, the reduction of the arcs during contact parting has to be accomplished by the contactor contact parting time."

Suggestion: review CORONA theory.


jO

 

[afterhrs]

did anyone look up "iec vs. nema" on google or any other search engine

 

[jbartos]

Question to jOmega (Electrical) Aug 18, 2003 marked ///\\JBartos:
You said:
"Suggestion: If NEMA contactor dimensions are compared to IEC contactor dimensions, the IEC contactors tend to have much smaller cube dimensions. Therefore, the reduction of the arcs during contact parting has to be accomplished by the contactor contact parting time."
Suggestion: review CORONA theory.
///For which particular aspect(s)? Radii? High Voltage?\\\

 

[jOmega]

Hi JB,

I think you mis-construed my statement ....

I didn't say anything about the "cube dimension"

I think you'll find that you are the originator of that in your post of Aug. 16, which I quoted back to you on Aug. 18.

Ok ?

So please refer back to my original post of Aug. 13, in which I said...

"The IEC contactors have a shorter throw length and studies have shown this to cause motor insulation failures."

No mention there of cube dimension... just throw length or in other words, the distance between the contacts from closed to fully open state.

And it is that characteristic that is the probem as the shorter throw length of the IEC style contactactor designs supports more arcing time.

Both the NEMA and IEC contactors produce arcing when the conatact poles are opened under load.... BUT... the difference is that with the longer throw length, the NEMA contactor quenches the arc quicker, which over time, is less harmful to the motor insulation than the IEC contactors.

And yes, as you asked, checkout CORONA theory with regard to distance and high voltage arcing.

The reports that I've seen, produced by Allen-Bradley and others, detailed the arcing in their studies of the reduced motor life with IEC contactors.

 

[rbulsara]

Speaking from personal practical experience...I have supervised plants in harsh environment (in 80's) with IEC starters (and contactors)..and do not recall having a problem with motor insulation because of a contactor.

Yes it is true that NEMA contactors for a given HP rating are more sturdier and ruggedly built. While IEC contactors/starters are more 'closely' engineered so to speak. If arcing is a problem, it will show up on the contacts (as pittings) before a motor is damaged. (Motors have burnt for other reasons though)

As long as you monitor the contacts and see no pitting..there will not be a problem. As with any other application be conservative. When in doubt use a next higher size contactor for a given HP. Do not run the normal load current more than 70% of the rating of the contactor.

Also look for correct type of duty on the IEC contactor for correct application.

 

[jbartos]

Suggestion to jOmega: Visit

http://www.schneider-electric.com.tr/ftp/literature/publications/ECT143.pdf

for arcing suppression that does not mention any corona.

 

[jOmega]

JB,

That's a great reference for:

"behaviour of the SF6 - MV circuit-breakers
Fluarc for switching motor starting currents"

.... and if someday, I ever encounter a problem with the SF6 Medium Voltage circuit-breakers, I'll gladly refer to it again.

In the meantime, I am at a loss to see the relevance to this thread.... which....by the way, ... is about IEC contactors..... low voltage (less than 1000 vac) and reduced motor life.

Perhaps you saw something that was relevant that I might have missed, in which case I look forward to your pointing me to it in the Schneider publication on the SF6 Medium Voltage Circuit Breakers...

Thanks

 

[jOmega]

color blue]rbulsara[/color]

Thank you for your comments. You are not alone in your experience... There are many thousands, (I would gestimate) who like you, have never experienced a problem of motor failure due to IEC contactors.

But.. there are also many (number unknown) but of sufficient magnitude to cause various studies of the failure mode to be conducted).

Certainly not every IEC contactor will cause every motor it is connected to .... to fail. But there are some. And the point of this thread was to point out that fact and to state that NEMA contactors do not cause the same failure mode.

There are two concerns with IEC contactors.... device life (as compared to NEMA) ... and motor life (as compared to NEMA).

This thread was ordained as a consequence of previous thread where multiple motor starts/hour were involved.
Because of the reported and documented problems with IEC contactors .... I suggested that the author of the previous thread use NEMA vs IEC contactors for his application.

IEC contactors are not intended to serve the same life span as a NEMA.... the cost has been designed out of them based upon reduced life.

So beit. They serve the market needs for those that put cost-of-purchase ahead of value.

But if you over size to get increased life... then did you really get a cost benefit by going with IEC devices ?


Funny.... I guess downtime costs, maintenance costs and replacement part costs are at $0.00.
Must be so... as they are never factored in to the purchasing decision.

Some old wag.... a long, long time ago said it best.....

"You get what you pay for", and let's not forget that old Latin Saying: Caveat Emptor— Let the buyer beware.

"To Each His Own"

 

[jraef]

JOmega,
I have an anecdote to add credence to your stated issue. Way back in 1978 I left Allen Bradley and went to work for Klockner Moeller (now Moeller Electric). In their effeorts to "deprogram" me about NEMA contactors, they provided me with an internal paper which idirectly confirmed your point. In it they acknowlwedged that NEMA contactors had a longer throw, but KM compensated for it by using a "double-back" stationary contact design which created a manetic field that supposedly aided in opening the contacts faster. I remember they specifically mentioned this as a way to avoid motor insulation damage, a problem inherent in all of their IEC contactor competitors at the time. I too was shocked to hear the indirect reference to IEC contactors being potentially bad for motors, but just as you said of AB, that info never saw the light of day. That was not a paper available for public consumtion but I thought you would like to hear some confirmation of your issue.



Quando Omni Flunkus Moritati

 

[jOmega]

jraef

Blessings upon thee lad.

Thank you for the affirmation.

I walked the hallowed halls of A-B the latter half of the 80's and early 90's ... Which was during the time when they installed the "World Contactor Line" for making IEC devices for their Show & Tell to impress customers with their capabilities. Those on the inside knew that it was more "smoke & mirrors" than reality. Funny.... that line doesn't seem to be around today.

Thanks again for stepping up.

jOmega

Sermo datur cunctis; animi sapientia paucis.

 

[rbulsara]

jomega:

It is still a matter of proper application. As long as what you get what you pay for, you do pay more for NEMA which is not different than buying a properly sized which may be a next stadard size higher contactor.

While bashing IEC contactor may sound like a good passtime fashion, it does not hold water as millions are in service.

If someone is letting a motor burn because of contactor arcing, he/she is not paying attention to contactor maintenance or misaplying it.

This is no different than saying some old heavy duty stuctures (which are in fact were hugely overdesinged) are better build than some modern structures that are closely engineered.

Americal standards are inhererntly very conservative (over sized) as cost has not been a real issue compared to rest of the world, specially the developing countries.

Thanks.

 

[jOmega]

rbulsara:

What you opined might have credence IF the manufacturers of the IEC devices were to be honest with the public. They could well have told them about the problem (which they all know about)...but they did the cowardly thing and copped to the "Emperor's Clothes" syndrome.

So you, the customer... didn't know that there was a chance of shortening the life of your motor by applying an IEC device. See the reason that the IEC Contactor manufacturers all know about the problem.... is because they were all the recipient of customer complaints of short motor life....and they individually or in concert.... investigated the problem. Some attempted to find a solution ... some just ignored it. But none...told the public of the problem.

The fact that you were not aware of it... is certainly evidence of that. Now you may certainly argue that the problem doesn't exist, and that in the face of the fact that the IEC contact manufacturers all know about it. Gosh.... whom should we belive.... ???

Now then, since the IEC contactor manufacturers elected to keep mum about this problem.... I wonder what you consider to be a "properly sized" contactor....? What are the guidelines to use in making a selection that will NOT reduce motor life ?

You can certainly up-size the IEC contactor and extend the useful life of the contactor.... but that's not the issue; motor life is.

No one is "bashing IEC contactor(s)".... we're meerly putting some light on a known problem that the manufacturers have kept hidden from you, the public.

As to the arcing.... well, perhaps you have time to stand around and observe the operation of all your installed IEC contactors to see if any of them ever arc... and how often and with what severity... But I'll venture to say that most people don't have time for that. I'll also venture to say that it doesn't happen every time the contactor is opened.... there are other aspects of the problem that come in to play. Most people probably don't know there's a problem UNLESS some manifestation occurs; i.e., motor fails prematurely, or contactor fails prematurely.... etc.

I first became aware of the problem when a customer told me that he was experiencing shortend motor life after switching from NEMA to IEC contactors on new equipment brought into the plant. You see, this was a point in time when OEM's (and we all know that they are forever finding ways to "cost reduce" their products) ..... began shipping their products with IEC contactors in place of NEMA hardware that they'd previously supplied.

Let's see... I addressed your "IEC bashing" statement, but didn't comment about your claim that the problem as reported herein doesn't hold water because there are millions in use. I would ask of you sir, how many problems in that "millions" do you know of personally... and are you in a position to know of all the problems they have had and continue to have ?

There are some people who thought the Edsel was a great car, as long as they didn't have any problems with it.

And perhaps jraef was just having a bad dream.

One last thought ... If you pay more to up-size an IEC contactor.... instead of using a NEMA device, you still won't get the life out of it that the NEMA contactor will render.

As I said.... Caveat Emptor.