Inverter fed Explosion Proof Motors

[torslum]

Dear colleagues,

I am a little confused regarding ex-certified motors connected to an inverter of any brand. As I live in Europe, I will stick to European/IEC-terms regarding this issue, but I believe that the consequenses should be more or less the same anywhere....

As far as I understand, please do correct me if I am mistaken:

I am concentrating on equipment placed in surface areas outside mines (Group II) and areas with gases (G-group), not dust.

Zone 0 (permanent presence of explosive atmosphere):
N/A

Zone 1 (occasionally, incidental normal duty, presence):
I can use motors certified as Ex d, de, e and p.

Zone 2 (accidental, not normal duty, presence):
I can use motors certified with Ex nA or better, meaning all motors good enough for Zone 1.

So far so good, I believe I have these issues quite clear. However, the problems come when supplying these motors with an inverter. Given the certificates for the different classifications, they say that the supply should not differ from rated, nominal or tested values. So with an inverter, we have an issue....

First of all, I assume that the inverter itself is located in a non-hazardous area or a pressurized panel/cabin. Subsequently the inverter itself does not need any Ex-certification and most brands can be used. But what about the influence on the motor?

If I am correct, Ex d motors are tested with any frequency and an applicable voltage (~constant flux) within the complete range. As far as I understand, an Ex d motor can be inverter fed WITHOUT any certification on the complete package (inverter+motor)- correct??

My question is then; does all other motors (e, p, nA) have to be tested and certified by a notified body for each combination? It would not be good enough to buy any of these motors and combine with any brand of inverters without re-certification? Even if the motor-manufacturer would state that the motor is suitable for inverter-supply?

And what about the Ex de-motor, which basically is a d-motor with an e junction box? Does it have to be certified for each combination as with the other ones or does it go as a d-motor?

Appreciate your opinions on this issue. I believe it is rather tricky to get a complete overview of when a re-certification is needed or not.

Cheers!

t.

 

[ScottyUK]

Somewhere on this site - maybe a couple of years ago - a series of posts were made by someone very knowledgable in this field. I'm sorry I can't remember his handle. Might be worth a search. The Google search isn't always the best one to use - frequently it misses things which the Eng-Tips one finds.


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I don't suffer from insanity. I enjoy it...

 

[ozmosis]

Scotty
It was Iceman and the thread was: thread237-95234

 

[ScottyUK]

Well done sed2 - you are absolutely right. I wonder what happened to him - he was a pretty clued up individual.


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I don't suffer from insanity. I enjoy it...

 

[ozmosis]

Scotty, after we established we both knew each other I dropped him an email to catch up on a few things. I think his ding-dongs with jbartos at the time were a bit wearing. I think he's still alive and well in Scotland

torslum. If you've read the thread, you will see that the new ATEX directive is trying to address some of these issues. Your motor supplier should be upto speed on these issues. I know Siemens and ABB will provide most of the data you are looking for. I guess it is only tricky if the motor companies have not done their homework and also, this is where a motor manufacturer who also manufactures drives has it's benefits.

 

[torslum]

Thanks guys.

I've got a few technical catalogues on the subject, and I believe you are right in assuming that ABB and Siemens are quite up-to-date. However, they tend to favour their own inverters.

A little bit more independent is Loher, which is very well known German company for Ex-motors at least in the North Sea area - they have a very detailed catalogue on this subject.

As far as I understand, with any brand, the Ex d - and I assume also the Ex de - can be run with any inverter without any particular test as it is already taken into the design and certified. However, all other need special certification; Even in the ABB catalogue it is stated for Ex e motors: "... and ABB therefore does not recommend the use of low voltage increased safety motors with VFDs". So that should settle it then, probably way to expensive to perform the test.

However, I am not too sure about the Ex nA - I think that it is usually required to do a test. As far as I understand it is less expensive that the Ex e test, and I've also heard that you can get away without any test for certain applications. Especially the rise time should be focused on, and within some limits it would be possible to run such a motor without any combined test. However, I cannot find this in writing anywhere.....

Enjoy your day!

t.

 

[stardelta]

One of the main issues when running a Ex motor through an inverter is the problem of cooling, or rather the lack of it! The surface temperature must never be allowed to exceed the maximum as specified for its T class. I have found when making similar enquiries with motor manufacturers regarding the use and compatability of inverters that thermistors and a suitable control unit must be used that will disconnect the supply if the motor becomes hot and before it exceeds its T rating.
There is some good information on the Brook Crompton website on this subject.

http://www.brookcrompton.com/