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]

I wonder if you are seeing degradation due to partial discharge at voltage levels far below those where it occurs at normal atmospheric pressure. It is a recognised problem in aerospace design now that higher voltage applications are starting to appear in aviation. Here's a link from the University of Manchester which might start you off with some information.

 

[ryanmech]

Hi Scotty


yes you are heading down the same path as we expected. we think it is corona discharge causing insulation degradation at vacuum.

we will read thanks



regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[racookpe1978]

To translate that: The insulation appears to be breaking down in the very low air pressures found, but the fan motors themselves are not over-heating (they are less than 30 deg C, right?) due to low air flow?

 

[Compositepro]

Given the process you are describing I think it is probable that you have water condensing on your windings. Check if the dew point of the vapors in your drier approach the temperature of the motor windings. Keep in mind that this maybe a transient situation that occurs only at start-up.

 

[ryanmech]

hi Racookpe1978

yes correct

temp below 30degC on outer hub...
obviously no air flow due to not much air in a vacuum..

regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[ryanmech]

thanks Compositepro

same response to your condensation suggestion in my HVAC post

regards
ryan

"thanks Compositepro

we had inspected the motor and there is no moisture and no condensation on the motor. in any case the dew point of the moisture would not form on the warm windings.


we think it is due to the insulation degradation. there is significant documentation about insulation degradation under a vacuum - corona discharge etc.. We cant find specifically the best insulation to use on the windings.... we are thinking of potting the windings under vacuum with a resin.

any ideas on what type of insulation type to use on a motor under vacuum


anyone know what insulation they use on explosion proof motors?

regards
ryan"


Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[ScottyUK]

Depending on how big your motor is, you might be able to consider using design techniques typically found on high voltage machines. I'm thinking especially of down-hole pumps which often run at a much higher voltage than would normally be expected based on their power output and because they are very compact they don't have space to use the techniques used on big HV machines.

 

[ryanmech]

Hi ScottyUK


yes the uni paper directs us to use Teflon for winding insulation on motors subject to vacuum.
Teflon/PTFE due to its resilience against outgassing, and subsequent insulation degradation, and corona discharge.

we are investigating the use of Teflon sheet for isolating the windings from the laminations slots, and then potting or vacuum impregnating Teflon cast insulation over the whole winding.

I am not sure what the "design techniques" are for high voltage machines.

our fans are 400Vac, 3ph, .8kW, VSD controlled, sinusoidal filters...


regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[Compositepro]

Ryan, this is one of the lesser reasons why eng-tips has a policy against posting the same question in multiple forums. I thought my answer had not posted for some reason.
Your statements do not match how a vacuum drier works. But that is another topic.

 

[ScottyUK]

Hi Ryan,

I'm actually hoping that one of the members who work in motor design will pick up on this thread. Teflon windings are not something I've ever worked with so I'm hoping to learn something myself as this discussion develops.

The HV machines I work with are in the several hundred kW range up to a few hundred MW typically using formed coils, Roebel windings, occasionally water-cooled - none of which would easily adapt to a small machine. When I see partial discharge it's typically at 11kV or above and at atmospheric pressure - I'm aware of the variation of PD inception voltage with pressure but really just as part of general my knowledge about PD.

 

[ryanmech]

Thanks Scotty

I appreciate your time and expertise.
Yes - motor insulation for vacuum seems to be very specialised.
Yes I agree that HV PD and PD at reduced pressure are closely related.....

Lets wait and see if a member has the practical experience in motor winding insulation for protection in a reduced pressure environment.

Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[ryanmech]

Thanks compositepro

Yes - I can see why you posted the same answer! Just aswell I gave you the same
Response!
Apologies for your double response.

Yes - I agree your opinion of how a vacuum drier works is off topic and unrelated. This question is about motor winding insulation for vacuum.

Thanks for your time
Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[zappedagain]

I have a chart in front of me that I probably acquired in 1996; I believe it is from "Reference Data for Radio Engineers". It has a table showing that the sparkgap distance decreases with air pressure, and also decreases as temperature increases, so your 5 kPa, 50C 'corner case' is probably your problem area. This is a handy reference. Google that title and the first hit (ftr_ref_data.pdf) has a similar table on p. 48, as long as you believe atmospheric conditions haven't changed in the last 69 years!

You are probably correct that the motors being bought today are different from those bought 10 years ago. Someone found a less expensive alternative for the wiring that worked fine at 25C.

Z

 

[MikeHalloran]

Just a note to demonstrate my limited understanding of Teflon/PTFE:

You can't 'cast' it. It doesn't come in liquid form. It's actually a condensation polymer, and was discovered when the pressure in a bottle of tetrafluoroethylene gas disappeared overnight, but the bottle weight didn't change. Upon opening the bottle, they found a fine white powder with remarkable properties, one of which is that it can be sintered into simple shapes just as powdered metal is processed, and then machined to a desired shape. The resulting sintered solids are porous to some degree

There exist some related fluoropolymers that behave a little more like a thermoplastic, but none of them behave much like more conventional plastics. You probably need to get some fluoropolymer experts (I am not one) on site.


The other experts who might be of value are getting more rare with each passing year; they managed to get radios and radar sets to work reliably at high altitudes in unpressurized aircraft and early spacecraft.

Mike Halloran
Pembroke Pines, FL, USA

 

[ryanmech]

Thanks Zappedagain
Yes agreed that vacuum and raised temp increases the distance that an arc can form. Or in scottys terms partial discharge will occur at a smaller distance between potentials at reduced pressure and increased temp.
In our case 400Vac between windings -600 to +600V. 1.2kV potential between windings. From zappedagains graph it is a very small gap when the .19 scaling factor is applied for our reduced pressure and increased temp.
This is the reason why we need better insulation.
Regards Ryan

Thanks Mike
Yes agreed - I am no plastics nor electrical nor insulation expert. Some documentation suggests that in a vacuum that ptfe insulation would most be suitable. Whether this means winding wire coated with ptfe insulation, ptfe insulation sheet/paper in the lamination slots and winding bundles out side of slots, and also encapsulating the windings in cast insulation of what ever suitable insulation if required.

Thanks Ryan


The question remains --> does anyone have practical experience in insulating electrical motors for a vacuum environment?

Much appreciated regards ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[itsmoked]

This really seems over-the-top. Why don't you get out of the box and run the fan from an external motor and stop trying to solve an expensive exotic issue?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ryanmech]

Thanks Keith

The application does not lend itself to have the motors outside of the vacuum chamber due to other constraints.

The previous machine has worked with the same fan model for ten years with no issues. We think the fan motor insulation has changed in the past ten years. The fan motor Ziehl FC056 looks different externally and the assumption is that the insulation has changed also.

Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[itsmoked]

Well that sucks.

Have you talked with these people?
[URL unfurl="true"]http://www.empiremagnetics.com/[/url]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ryanmech]

Thanks Keith

Yes we found empiremagnetics people on Thursday...
Indicatively they are very expensive....well lab equipment expensive...

We will contact them next week.

in the mean time we are looking for advice from a engineer with practical experience in insulating motors in a vacuum environment.

Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation