Ventillation fans in a vacuum - fans are failing - windings fault to earth 4

[ryanmech]

Hi

Short question: What type of winding insulation should be used for a fan motor that is within a vacuum pressure?
also posted in the HVAC forum

http://www.eng-tips.com/viewthread.cfm?qid=394983&action=updated

Details:
We have an application where we have ventilation fans in a chamber under a vacuum pressure.
What is the point of a fan in an environment where there is no air you may ask? well there is air it is just 5% of what it would be at atmospheric pressure.
Yes I know that fan motors are able to be designed with their windings quite dense, due to the high cooling efficiency of the fan air running over the motor...And we are running les than 5% of that air over the fan. (FYI - the fan body/hub temp has not ben measured to be over 30deg C during any of the tests below)

The machine is a food vacuum drier.
The environment changes during the process but generally: is initially ~50% humidity and ending at very low humidity. The temp ranges between 5 and 50 deg C. The pressure ranges between atmospheric and -95kPag (or 101kPa to 5kPa absolute)

the fans we are using have squirell cage motors. we are using a VSD and a sinusoidal filter to provide variable speed to the fan depending on the process steps.

There are four fans - three have failed due to initially earth faults (recorded by the VSD) and then eventually they fail by windings short circuit to earth. the earth connection cannot be seen as it is within the innermost of the windings, and the construction is such that the resin has to be baked off to remove the windings and as such the evidence is not available to be sighted.

we are using:
Fans: FC056-VDQ.41.V7
VSD: Allen Bradley PF525 VSD
Filter: Schaffner FN5040 Sine Wave filters, typically they are run at 50Hz

Initially there were no faults in the conditions listed above.

At a period of ~30hrs, the earth ground faults were occurring only when the vacuum was greater than -80Kpag. (the VSD records the earth ground fault due to a measured current difference >25% between any two of the phases/windings).
When the chamber is returned to atmospheric pressure there were no faults. i.e. the insulation at atmospheric pressure is much greater at atmospheric than at a reduced air pressure. insulation test to earth from each of the windings is >1000MΩ at 500V after returning to atmospheric pressure.
Then the earth faults would get gradually more frequent and at pressures closer to atmospheric pressure, eventually the fault is continuous or instantaneous. the insulation resistance is then measured as very low i.e. short circuit.


we have undertaken several tests to try to isolate the equipment.(same environment as above Unless noted otherwise)
1. running without the VSD and filter --> direct online with the fans = failed windings short circuit to earth --> proves the VSD and Filter are not the direct cause.
2. running at atmospheric pressure continuous for 48 hrs --> =no faults -->proves that no degradation of the winding insulation occurs at atmospheric pressure, therefore VSD and filter are not the cause.

we think that the main factor is that the fan motor insulation subject to vacuum degrades over a period and eventually causes a permanent short circuit.

Our client has fan with the same part number that have been operating in the same conditions for 10 years without issue. To be clear - the specifications for the Fan is that it is not specified for anything other than atmospheric pressure environments. The client knows this, we know this and the supplier have clearly stated this.

We think that there had been a change in the insulation of the fans - our clients 10yr old fan has winding insulation better than the current version of the same fan.

that brings us to the short questions:
What type of winding insulation should be used for a fan motor that is within a vacuum pressure?
should the fan windings be totally encased/potted in resin to ensure the best chance of not degrading in the vacuum environment.

We have read a lot of theory about the degradation of winding insulation in vacuum. Practically we don't have the information about what type of winding insulation we should use.


thanks for getting this far down....

regards
ryan







Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[ScottyUK]

It might be worth posting a link to this thread question in forum2 and forum25. If you do so then ask if people could keep the discussion in this thread rather than fragment it.

 

[EdStainless]

The arc at low pressure is surely what is causing the breakdown.
Don't use Teflon for insulation. Over time it develops conductive holes, the air force has learned this the hard way.
For sub pumps running high voltage, tight windings, and high temps we used double wrapped Kapton (with fusible Teflon layer) of for better insulation we went to PEEK.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MikeHalloran]

You might want to avoid Kapton:

http://www.vision.net.au/~apaterson/aviation/kapton_mangold.htm

Mike Halloran
Pembroke Pines, FL, USA

 

[ryanmech]

Hi Scotty

thanks for the suggestion to post on the aircraft and aerospace forums - according to compositepro I would be further outside the posting guidelines.... cross posting etc.

Hi EdStainless and MikeHalloran

I am really looking for a motor manufacturer who is specialised in motors in a vacuum. If any of you can recommend a contact that would be great. Obviously there is cross over for specifications of motors with other environments, HV/ hi temp / altitude / aircraft / and probably most closely aerospace..

ok, that has convinced me that I should be putting a link in the aerospace forum as suggested by Scotty..

thanks for all your suggestions

regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[MikeMay17]

Ryan,

I have a limited amount of experience with motor in vacuum environments, but I can tell you that yes, a fan motor designed for standard atmosphere will fail in vacuum due to the significant decrease in Partial Discharge Inception Voltage (PDIV). We have designed a few specialized motors for vacuum environments, and they have been nothing but trouble, especially on the assembly side.

A bit outside the box, but can you approach the problem from the other side and use lower voltage fan motors, or reduce voltage only at vacuum conditions? If the motor insulation can't handle the voltage at vacuum, then reduce voltage to stay under the PDIV... Motor insulation tends to be the same materials and schemes for low voltage motors, whether they are at the top or bottom of the range.

-Mike

 

[ryanmech]

Thanks Mike

any idea what a suitable voltage would be to avoid partial discharge at 5kPa absolute pressure?



thanks and much appreciated
regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[LionelHutz]

So, it's a 1hp axial fan with a hub motor. These days, I wouldn't put much faith in the insulation of that fan being very durable for use in normal conditions let alone and kind of stressful conditions. I'd guess is that you'll find mush windings with no slot insulation or any kind of coil to coil insulation. My gut feeling is that a good motor shop doing a rewind with care taken to insulate as much as possible would be enough to make it work.

With the vacuum, the fan doesn't have any air to move so I would think you could reduce the voltage quite a bit and keep it spinning. However, I'm not sure how you easily do that with the VFD since that involves changing the V/Hz ratio in the VFD. This is only doable since you have a sinewave filter otherwise the voltage level of the VFD PWM output would be the same regardless of the V/Hz ratio.

 

[ScottyUK]

I have just realised that there's a really obvious question which no one has asked: in a vacuum, what use are fans?

 

[zeusfaber]

The OP asked it himself very early on in his initial post - though there wasn't a lot of detail in the answer.

A,

 

[ScottyUK]

Sorry Ryan, I missed that.

And whoever gave me that pink star - thank you, but certainly not deserved for that question!

 

[ryanmech]

Hi

The fans run fine at normal atmos conditions. Its just a squirrel cage motor with the cage embedded in the fan hub.

We rewound a motor but it failed in the same manner. Earth fault.

It is not a perfect vacuum so there is still air 5%. A fan moves the air to some degree.

All theory points to the insulation needing to be specifically suitable for vacuum. The general motor rewinder has not dealt with vacuum applications.


Thanks for your suggestions
Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[jraef]

https://www.superioressex.com/uploa...h_America/Magnet_Wire_-_Winding_Wire/US_b.pdf

In the semiconductor tooling industry, where vacuum chambers are common, OEMs have been insisting on inverter duty motors being wound with "ISR" (Inverter Spike Resistant) magnet wire. The link above is a commonly referenced version, but there are others. They too are concerned with out gassing of the insulation, albeit for different reasons. But this is a well known phenomenon in that industry. This magnet wire, like others, is rated for up to 3000V to protect against the PD issue. Might be worth a try.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ryanmech]

Jraef - great suggestion thankyou - we will look today

Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[WKTaylor]

ryanmech... an old memory... and related war-story…

In 1997 the Mir Space station was hit by an out-of-control Progress Cargo Ship.

https://www.youtube.com/watch?v=tM7fTLLmgbk

This collision somehow damaged the pressure integrity of one of the modules [Specter? crew quarters?]; which led to the emergency evacuation and sealing off of that module. This included decoupling/removing power-cables from that module's solar panels that were running thru the module port/hatchway, just so they could close/secure the hatch. This cut a huge amount of power to the heart of the station. This incident was almost a catastrophe… like the Russian’s Apollo 13.

Eventually the MIR was recovered and made partially functional and safe to occupy… except of-course, for the depressurized module.

The Ruskies went EVA outside the MIR several times looking for obvious damage caused by the Progress impact [hole, tear or whatever] that led to the depressurization incident, but never found it.

A couple of years later, just before the last crew left MIR (before it's de-orbit (2001)(?), a cosmonaut went EVA into the Specter module looking for any collision damage that might only be visible from the interior. This dangerous internal EVA… in a full space-suit with back-pack and no connection to the station, forced him to go through the connecting node which had pressure hatches to all connected modules. The EVA revealed nothing of value, regarding the mishap.

HOWEVER, the EVA cosmonaut wryly observed, “that the lights were still on and that all of the Russian-made fans were still running smoothly in the hard vacuum of space” ~3-years after the collision [power was still supplied to that module from the solar panels, all that time].

Perhaps what You need is Russian space-grade mechanical fan-specs…


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[ryanmech]

Thanks wktaylor

Yeah - Russians fans could be difficult to procure. They will prob be low voltage dc motors.

For our situation, a fan has been running in the same vacuum environment for 10years. The new fan is not surviving. We think that the winding insulation has changed over the past 10years.

Ideally we use the same new fans but with better insulation.

If we were to use low voltage, and get the same effectiveness then the 24v motors then it would be drawing 40A when at atmospheric pressure.

Regards
Ryan


Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[btrueblood]

Hard vacuum is as easy as 1 atmosphere. It's the middle ground, near Paschen's minimum, where trouble occurs.

see

https://en.wikipedia.org/wiki/Paschen's_law#/media/File:Paschen_curves.svg

which would suggest that, at 5 kPa, you would need to reduce the motor voltage by an order of magnitude (i.e. from 400 to 40 volts) to prevent breakdown.

 

[itsmoked]

btrue; I never knew. Thanks for the lesson!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

Smoked, I think that may be the first time a sparky ever gave me a star in their own forum!

I thought all you ee's learned Paschen's law? Maybe that was only when I was in school, and vacuum tubes were still studied.

Do note, the minimum breakdown voltage is determined by the product of pressure x spark gap, i.e. you could get higher breakdown voltage at 5 kPa by increasing the distance between conductors. Secondary methods, e.g. rounding sharp edges, etc. can gain you a few close orders of magnitude as well (2x to 5x improvement).

(sipping his single malt scotch, and sitting back with a twinkle in his eye)...

...We had this rocket engine, with a heater element in it, see...and when it would hit orbit, and got used the first time, all of the oxides that had formed during ground handling had to burn off the heater...which raised the pressure in the heater cavity just enough that every so often one of them would arc, and destroy the power on that side of the spacecraft...why they put a non-resettable fuse on that particular circuit I will never know...and all the electricians we talked to had this aversion to going up there and fixing it for us. That's why I don't trust sparkies any more...(nods to himself, mumbling, drifting off to sleep again)

 

[itsmoked]

btrue; I've done LOTs of high voltage stuff. I think I've even seen the phenomena, but for some reason I just never have come across Paschen's wonderful analytical statement.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jdunfee5]

Since you don't need a hard vacuum, and you are constantly pumping gas out of the chamber anyway, why not run a shaft from an internal fan, to a motor on the outside of the chamber. That would seem to solve several engineering issues.

And just because I often enjoy speculating on ideas that are probably not practical. Another idea is to keep the fan's electrical portion outside of the vacuum. It would operate in a manner similar to how the magnetic stirrers used for chemistry. In fact, you may be able to use one of those devices, and just substitute a fan you made yourself for the stirring magnet. However, I have no idea if you can get the kind of power you need in this sort of arrangement.

A related ideas might be to somehow pressurize the part of the motor with the windings. You might need to hack your own motor by taking parts from different motors, and substitute a smaller rotor, so that you have space for the pressure chamber. You would surely loose motor efficiency, but I mention it as another brainstorm idea.

-Joe