ventillation fans in a vacuum - fans are failing - windings fault to earth

[ryanmech]

Hi

Short question: What type of winding insulation should be used for a fan motor that is within a vacuum pressure?

Note: This question is also posted in the electrical motor section for specific electrical responses.


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

 

[Compositepro]

It seems likely to me that you could be getting condensation in the motor.

 

[ryanmech]

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

 

[chicopee]

Excessive overheating of the windings due to reduced air pressure. You could try to have the motors rewound with slightly heavier gauge wiring to reduce the heat generation within the windings; also talk to the electrical machine shop engineer for a different selection of wiring insulation for the windings.

 

[ryanmech]

thanks Chicopee

The windings are not getting hot, infact we see no heat evidence on the windings at all.


the average motor re-winder does not work in the range of vacuum and as such is not experienced with motors subject to insulation breakdown. we are expecting that the root cause of failure is due to corona discharge at low pressures, eventually degrading the insulation enough to short circuit.

the temp has been measured on the motor/fan hub and it does not get above 29 degC when running the fans in a vacuum at room temp.

regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[chicopee]

I still would talk to an electromechanical shop engineer about your suspicion.

 

[chicopee]

Another idea is to research NASA's references since they do use electric motors in space and NASA's has a ton of material published for the public.

 

[DBronson]

Ryan:

The article at the link discusses this very problem. You might contact the author to get more info on your specific problem.

Link

Regards,

DB

 

[ryanmech]

Hi DB

thanks for the link
yes understood - we are in contact with these people but are yet to hear back from them.

Hi Chicopee

we are in contact with a company which may be able to assist us - yes NASA does have a lot of info - what we are after is people with practical experience solving this issue rather than going through the ordeal/pain ourselves...


thanks for your suggestions
regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

 

[BillPall]

Hi,
I thought of three methods of attack. Two of the methods involved placing the motor outside the chamber. However, as you said there were constraints to prevent placing the motor outside the chamber. I assume that you did not want rotating shafts penetrating the chamber wall. That eliminates method one.

Method two is to mount the motor windings outside the chamber and separated from the rotating parts, inside the chamber, by a flat thin stainless steel diaphragm. Some domestic central heating pumps use this method to eliminate sealing problems.

The third method is to place a modified squirrel cage axial flow fan fully inside the chamber at any location and seal the coils from the chambers environment. The air inside the sealed container, containing the coils, could be controlled with air, at whatever temperature and pressure you desire, pumped from outside the chamber into the sealed container around the coils. This time, very thin stainless steel, in the form of a stationary cylinder, would surround the rotor that has had its diameter very slightly reduced. Ensure the container's pressure will not significantly distort the container.

Regards,
BillPall

 

[LittleInch]

I would try asking and linking to this question in the electric motors forum. Someone there will probably know if insulation materials have changed in 10 years or not.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ScottyUK]

LittleInch,

thread237-395029

 

[ryanmech]

Thanks BillPall
Moving the motor outside of the chamber is difficult due to the chamber being a very tight fit inside a container. so cant put the motor outside.

ideally we want to rewind the motor with winding wire with the correct insulation to survive the vacuum.


regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation