Argon gas and motor windings

[jraef]

I have someone wanting to use an AC motor on a vacuum furnace which uses argon as a quenching gas, the motors are on fans used to evacuate the gas after the cooling cycle. There is an NFPA regulation for these systems, NFPA 86, which dictates that motor windings that are exposed to argon (or other ionizing gasses), must not exceed 260V. Anyone have any idea why? I have always thought of argon as just an inert gas, but what is the reference to being an ionizing gas if there is no arc and/or why would that be a danger at something like 480V? Is it because it will form plasma just from the stronger magnetic fields at those higher voltages even though there is no arc? Will it cause the insulation to break down?

The problem behind this is, he wants to use a 300HP motor, so 300HP at 230V is going to be very difficult plus he needs to use a VFD so his options become extremely limited. I know what his options are, I don't need that, I'm just curious as to the connection between motor winding voltage levels and exposure to argon, mostly just for my edification but he also asked me why the NFPA would impose that limit, and I don't like not having an answer.

"Will work for (the memory of) salami"

 

[Skogsgurra]

I am guessing now. So don't read.

There are two possible reasons. Both are the result of a misunderstanding.

1st misunderstanding: Large rotating Machines were/are sometimes run in H₂ (hydrogen) gas. For two reasons a) to reduce internal windage and b) to get better cooling, H₂ conducts heat a lot better than air. The misunderstanding may be that someone didn't understand the difference between hydrogen, that easily explodes, and argon, that doesn't. Following the better safe than sorry rule, they put some restriction on its use.

2nd misunderstanding: Argon is used in "neon tubes", but rarefied. Someone may have thought about this and, with no deeper knowledge, following the better safe than sorry rule, they put some restriction on its use.

If the argon gas is under vacuum, then I can understand the ionizing thinking. But it isn't? Is it?

3rd misunderstanding: I may have misunderstood it all. You didn't read, didn't you? Good.




Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Compositepro]

I'm guessing somewhat also. "Fans that evacuate gas from a vacuum furnace" must mean turbo-molecular vacuum pumps. So we talking about a motor operating in a vacuum. At low pressure all gases will ionize at much lower voltages than at ambient, not just argon. Even with insulation, when the electric field in a gas reaches a threshold level, there will be a corona discharge.

 

[Skogsgurra]

What I thought also, for a moment. But the motor for such pumps cannot work in the rarefied gas, I think. Mostly because the insulation and the grease will outgas and destroy the vacuum. Or, are we talking soft vacuum? In that case. Yes, definitely ionization problems. Absolutely.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

So the issue might be that this is specific to the fact that this particular NFPA code is only about vacuum furnaces, and the reference is ONLY about the evacuation blower motors, which means they would, be definition, need to be located IN the vacuum and in the presence of an ionizing gas.

Bottom line, this guy needs to possibly consider multiple smaller blowers if he wants to get away from custom motors and drives.

Thanks.

"Will work for (the memory of) salami"

 

[rhatcher]

I'm also guessing. I would think that the corona inception voltage in an ionizing gas would be much, much lower than air. This may be the reasoning behind the requirement to operate at such a low voltage.

That being said, the NFPA requirement sounds pretty firm. A 300HP motor at 230V is certainly possible but it probably will be a custom motor. The same applies for the VFD.

An alternative would be to install multiple smaller motors, VFDs, and fans. However, this may be more costly than a single, larger, custom set-up.

 

[Skogsgurra]

This Wiki article:

http://en.wikipedia.org/wiki/Paschen's_law

shows a set of curves showing that Argon, in fact, ionizes at a much lower voltage than Nitrogen. The difference is more pronounced at 1 Torr than at standard temperature and pressure. Breakdown occurs just above 100 V for Ar and at around 600 V for N.

But this is only true in a 1 Torr vacuum. At STP, the breakdown voltage is in the 10 kV region. So, the presence of Ar is not the problem. The vacuum, combined with Ar atmosphere, is.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

I have not looked but I bet you can find some standard off the shelf 300hp 230/460 motors.... why not use one of your standard 460v drives with it, along with a 2:1 output autotransformer?

Also, if 260v is max, including peaks/spikes, no matter what combo of motor(s) you use, you will have to provide nice sinewave output filter on each.

http://www.KilroyWasHere<dot>com

 

[Skogsgurra]

"must not exceed 260V"

So, a 230 V motor won't do. Corona is a fast thing and reacts to instantaneous voltage - not RMS. The peak voltage at 230 V RMS is 325 V.

I think that you need to find out if the motor will be running in rarefied argon atmosphere or not. If no, I think that you can use any standard voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RPosty]

Sidenote Experience:

I work on Industrial Lasers, CO2 mostly. We had a tank farm for the Laser Gas mix (Helium,N2,CO2) that supplied Gas to our slow flow CO2 lasers. One time someone mixed an Argon Bottle by accident with the Laser Mix bottles, so we had 5 Laser Mix ganged with an Argon (yes, same fittings). The poly gas lines inside the laser caught on fire, melted and made a mess. Granted, The resonator Anodes where the Lines are attached are at 35K Volts DC and are at about 18 torr there.

carry on

"I am stuck on Band-Aid brand, 'cause they are stuck on me"

 

[waross]

On North American systems, 230/240 Volts phase to phase is 133/139 Volts to ground. Will corona be dependent on phase to ground voltage or phase to phase voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Will corona be dependent on phase to ground voltage or phase to phase voltage?

I wouldn't use phase to ground.
Different phases are adjacent to each other in the endwindings and also in some (but not all) slots.
Granted there is supposed to be phase separators paper, but there is also a liner between conductor and ground in the slot.
Most importantly, any system has transient upsets...
So, establishing limits may not be a simple excercize.

The difference is more pronounced at 1 Torr than at standard temperature and pressure.

The wiki link has as a horizontal axis pressure times distance. So we cannot state a minimum as function of pressure without specifying a distance. Without knowing distance (it might be very smnall), it's traditional to assume worst-case distance, which creates the minimum. So we see the same worst-case minimum whether 1 Torr or atmospheric. I have no doubt you know all this and just wrote this sentence quickly. Just keeping the thread on track for benefit of others.


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(2B)+(2B)' ?

 

[electricpete]

...to say it another way, even at stp there is some particular distance where Argon would break down at ~110v


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(2B)+(2B)' ?

 

[mikekilroy]

Quote:
Will corona be dependent on phase to ground voltage or phase to phase voltage?

I wouldn't use phase to ground. Different phases are adjacent to each other in the endwindings

I second that as it is just a voltage difference from point A & point B and so does not have to involve ground.

I have seen multiple motor failures that had in-phase shorts, not even phase to phase, analysis said it was from exceedingly high voltage spikes that ionized surrounding air which then caused corona discharge which left blackened pin hole shorts reducing the number of turns in that coil. It was easy to identify when this happened as result was very loud audible 1-2 khz oscillation in current loop of the servo drive due to changed winding inductance. Once we identified and eliminated to cause of these hi voltage spikes then motors stopped failing.

http://www.KilroyWasHere<dot>com

 

[electricpete]

Some more discussion (to prevent misinterpretation I hope):

The Paschen minimum voltage doesn't change with pressure P (we can always adjust worst-case distance d to make P*d match the minimum). Whether or not that Paschen minimum voltage has relevance to a particular geometry - open to discussion.

On the other hand, for a given geometric (conductor/insulation) configuration, higher pressure is usually (for all but the smallest gaps) better… i.e. tends to move us to the right on the curve. I'm sure no-one disagrees.

There were some comments based on values of P*d = 700, I think. Those values would not have particular significance unless we were considering 1 cm uninsulated airgap.

I don't think many conclusions can be drawn except that the withstand voltage for a given motor is probably lower in air than in Argon (at the same pressure). What voltage derating factor is appropriate probably comes from much more detailed analysis or experience than anything I can come up with.

http://en.wikipedia.org/wiki/Paschen's_law#mediaviewer/File:Paschen_Curves.PNG

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(2B)+(2B)' ?

 

[Compositepro]

"I don't think many conclusions can be drawn except that the withstand voltage for a given motor is probably lower in air than in Argon (at the same pressure)."

Before reading this thread, I would have agreed with that. However, one graph that Gunnar posted says that is not the case. The reason appears to be that an atom of argon has a smaller cross-section than a molecule of nitrogen and therefore has a longer mean-free-path. This allows the argon atom to accelerate to a higher energy than the nitrogen molecule and makes it more prone to cause ionization in a collision, even though the atomic weight of argon is 40 compared to 28 for N2.

This has been very educational and certainly calls for more study when I have the time.

 

[electricpete]

Ah, yes thanks. That was a typo. Definitely lower in Argon than air was my main point.

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(2B)+(2B)' ?