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

4

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.

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

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?

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.

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

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

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.

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

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.

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.

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

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

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

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

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

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

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

Well that sucks.

Have you talked with these people?
http://www.empiremagnetics.com/

Keith Cress
kcress - http://www.flaminsystems.com

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

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

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

You might want to avoid Kapton:

http://www.vision.net.au/~apaterson/aviation/kapto...

Mike Halloran
Pembroke Pines, FL, USA

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

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

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

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.

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

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

A,

Sorry Ryan, I missed that.

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

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

https://www.superioressex.com/uploadedFiles/Magnet...

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

Jraef - great suggestion thankyou - we will look today

Regards
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

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]

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

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%27s_law#/med...

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.

btrue; I never knew. Thanks for the lesson!

Keith Cress
kcress - http://www.flaminsystems.com

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)

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

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

Since EEs like to use vacuum for current interruption and use added insulation at altitude, I think we intuitively know there must be a minimum somewhere even if we never heard of Paschen.

Ryan,

The minimum spark-breakdown voltage (regardless of pressure) for Copper in Air is 250 Volts; if the voltage is less than this, you should be good to go. Just keep in mind that the voltage at your motor terminals (the voltage that matters in this case) may not be the same as the line voltage in to your motor driver... If I were you, I would put a meter on the motor terminals during operation of the fans and see what kind of spikes you are generating with that VFD and those (assumed) long cables; I have seen spikes in excess of 2x line voltage due to standing wave phenomenon and super-position.

-Mike

Have you thoght about a motor driven by something other than electricity. My first thoughts are an air driven motor. Obviously some thought would need to be given to venting the waste air cos it probably won't sit too well with your vacuum.

ryanmech; In view of Paschen's law is there any chance the dryer you're having problems with is running a different 'recipe' or pressure profile, different product? If so it could be why the 'old' motors seemed to last forever.

Keith Cress
kcress - http://www.flaminsystems.com

Any controller changes? Maybe the controller used to keep the fans off during the pressure change (i.e. so voltage wasn't applied when you passed the minimum in the curve) and someone came up with a better plan to run the fans the whole time? Or a relay is stuck closed so the fan run continuously.

Z

Hi I skimmed through the posts. There seems to be a lot of wasted time over insulation and windings when towards the end the engineering design was rightly called into question. I can understand technical interest in the events however the problem needs fixing, fix it. Ok I'll add some thoughts and do the thing to death but the solution is ...fix it. I'm going to try to put this to bed.

Today with almost everything from once good companies, being made in China along with claims of 'meets spec's'I will not buy equipment from there at all however finding genuinely 'made in Sweden, Germany and the like is a mammoth task, so essentially we have become stuck with junk through our own miserable mean-ness (procrastinating euphemism 'competitiveness/level playing field') in buying it on price instead of supporting home industries...it's the lemming rush gripping the world from immorality to foreign policy and living in denial of climate change and sustainability.

In my ruminations there maybe transient spikes in your area which regularly subject them to voltages in excess of rated voltage. There could be lightning strikes on lines doing same thing. That the surface of the windings is dry when you check (you dismantled the fans?...how long did that take each time) doesn't mean there is no residual inside the windings and which may hold acids from the unknown gases and perhaps any 'preservatives' applied to them.

This equipment built by slave labour using 'just gets there' manufacturing spec's may work at some normal, under-stressed situation but doesn't make the grade thereafter. That's an unfortunate result of the change in culture. When one sees equipment from the 20's and 40's still working well having been subjected to overloading and extreme use the answer is obvious. If you tolerate junk you get junk whatever the name on it. With China in the picture the 'good old names' no longer matter.

The Italians by the way I think still locally wind motors.

If I was going to spend money in useless endeavours I'd have the three motors unwound by hand to see where the fault presented itself, looking for a commonality and testing for thickness, purity and types of contamination. I might even have their rep pick them up and let them do it. It's possible that the quality of the copper it self is poor and the problem is coming from inside. It could be that the varnish deteriorates at any change of direction. It could be that the baking process is inadequate. It could be a combination of all. The problem however is that is costing you and the people trying to assist, time and money. Fix the system...isolate the vulnerable equipment from the environment.

Other than that what's the point? They fail. They are not up to the task. Change the system if reasonable and practicable, stop worrying about the fans, take them out of the environment.

The situation itself presented the motors with poor environment and gases of which you have no analysis. TEFC may not help in the chamber as they are still fan cooled...and we are not sure where the problem lies and really, it's already wasted too much time.

Were radiation (fins) or unusual conduction (liquid cooled) the method of cooling in a vacuum that might have a chance but again...easier by far to take it all outside the environment which is more certain BUT if I was going to use the same motors or same type.. I'd be running them close to the vacuum system for some weeks to see whether the problem exists in a more normal environment. Using TEFC's in normal air flow with the extraction changed is better.. or just using an external commercial vacuum pump (or pumps)..get the motor/s out of the picture.

The information given by the 'worried' and in my answer was a bit dragged out, but essentially a vacuum is not going to provide cooling, nor is some uncertain volume of 'gas'. The only cooling will come from radiation and I think there would be hot-spots in the windings.
so put it to bed:
a) external extraction constructing it or using commercial vacuum pumps
b) using ceramic/magnetic motors with wound rotors if determined to keep motors inside
c) try never again to think about the problem...life's too short and it gains you nothing.

'Lump-hammer' Summary:
I'd get on with fixing the system and worry about the motors as a tax dodge.

The suggestion of using external fans is ideal...so long of course as not enveloped by the 'gases'. The principle of the vacuum cleaner can be employed as is done in vacuum pumps.

The suggestion of lowering voltage were fans to stay inside seems theoretically rational ...but not below 5% and really.. would that help?

Personally were I so resistant as to maintain the old system not willing to use my managerial 'nouse' and just fixing he problem I'd be looking for some suitable old 32 Volt motors and running one inside the cabinet just doing nothing and then periodically inspecting it and megger-ing it last thing evening first thing morning and anytime between...recording and ruminating in my lunch times, time -off, meal times, bedtimes and whilst asleep. If it stays good...change to 32 V but again...what a waste of time, health and capital when a more certain, environmentally isolated and a-biotic collectible system is available and practical and not expensive. ...using an external pumping system.

Redesign, fix it using the vacuum cleaner principle (nothing is inside the chamber) forget the dud fans, write them off or claim on insurance if possible and get on with life. Getting bogged down in endless loops is something I have been good-at occasionally and it gets one no where unless you were a 'concerned' scientist or CEO at the motor company but then....I think they'd be told by their CEO..'who gives a toss about failures in poor environments which would be maybe .0000000000000000001% of our sales...forget it...let some other poor mug company worry about it if they are silly enough to care.. we aren't!!'

As we of the Irish mind might say 'd'yer have me there'?...wasting your own time and the time of others on useless endeavours does not fix the problem. Fix the problem by either
i) totally wasted expenditure on scientific evaluation by a consulting engineer and seeing whether he can absolutely guarantee a suitable motor at the end of the cheque writing or
ii) rebuild the system taking into account one sensible response you received but using the vacuum cleaner principle....then sleep at nights.

FYI an update - we are currently doing a test in the original machine with the new fan. We are half way through a week long test. If the new fans do not survive then it indicates that the fans manufacturer has changed the insulation...if they do survive then the issue is probably a combination of several factors on the new machines setup
-Previously the old fans have been running for 10yrs with no issues.
-The new fans with the same part number (but 10 years newer) are not surviving in the new machine...we will see what happens in the old machine with the new fans.

Ideally if it is just a matter of winding insulation then we can rewind the motor with better insulation. The original question was What type of winding insulation should be used for a fan motor that is within a vacuum pressure?
thanks to Edstainless for his suggestion of Kapton insulation - double layer with Teflon fusable layer in between.
thanks itsmoked for your suggestion about using empiremagnetics
thanks to jraef for his suggestion to use supperioressex inverter spike resistant magnet wire.

thanks btrueblood, itsmoked, jdunfee5, stevenal, mikemay17
yes - we understand Paschen's law to a degree....-->better insulation would prevent voltage breakdown..

Thanks mike4472
we had thought of a fan driven by alternative means, but were hoping that the issue could be resolved by better insulation.

thanks itsmoked and Zappedagain
the tests on the new machine were simply at the worst conditions of the previous machine: 5kPa absolute running continuously - the comparative test of the new fan in the old machine will also compare the operational conditions directly.

Thanks Pythagorus
I am not sure your post provides assistance we are looking for - but I do recognise the time and effort you put into your long response. I fully recognise the justification for your post - we just don't have the space for an external motor.


thanks again and I do appreciate all responses - I recognise the efforts and interest that all responders are taking. And I hope to hear from someone who has successfully wound motors used in a vacuum environment, and the insulation specifications used in that motor....

regards
ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

ryanmech said, " we just don't have the space for an external motor. "

If you do not come up with an easier solution using the existing motors, or by re-winding, I think a motor outside of the vacuum is the next best choice.

The "external" motor does not necessarily need to be put outside the area of the chamber. It may be viable to add a can-shaped enclosure that has its open side on the wall of the chamber. If you can have air flow around the motor, then you may be able to use a smaller motor than you use now, and permit it to run hotter. Thus, the motor and its enclosure may end up the same size as the original motor.

I am just now assigned the task of researching the suitability of various materials in a vacuum. So, I am now in the same boat as you. Engineering tables on the properties of materials never include information on vacuum suitability. Does anyone know of a good go-to source for this sort of information?

-Joe

Ryan,

One variable you are forgetting is the VSD; are they the same in both the "old" and "new" setup? If the "old" setup has an older, slower VSD you will not see the same DC voltage spikes at the motor as you would with the "new" setup. If this is the case, the new motor would operate just fine in the old setup, but will still fail in the new setup due to Partial Discharge insulation damage.

-Mike

Have all avenues been exhausted to identify the insulation and winding style of the older fan that seems to have provided good service?
If it aint broke don't reinvent it with exotic insulation.

MikeMay does have some good suggestions. Can you put an o-scope on the motor leads on the new and old motors, and compare (o-scope will need to be able to capture fast DC transients, which is tricky...hope Skogs is reading).

Hi Mikemay and btrueblood and dread
We had looked into the old vsd setup. It is an old pdl xtravert drive with no output filter and 4kHz switching frequency. The new vsd with sinusoidal filter was set to have the same 4kHz switching frequency.
The old machine is in a different city and is in a production environment so a little difficult to get access to for a comparative scope reading. If the new fan lasts in the old system we will have a look at the old fan.

Thanks
Ryan

Ryanmech
Scott Technology Ltd NZ
Machinery automation

Such a nicely detailed post unfortunately prompts a lot of questions. For me,

1) How are the 30°C temperatures measured while the system is near a perfect vacuum? Measurement methods might be suspect without air.

2) Motors require air to cool them. If there is no air, they will not be cooled.

3) Why would ventilating/circulating fans be running to circulate air that does not exist? Can they shut off below a certain pressure?

4) What is the rough amount of air (cfm, lps, etc.) that they need to circulate? There are several vacuum compressor models designed to move/compressor air, all of which have their motors outside the medium and therefore can be cooled by ambient room air.

Sorry if some of this is repeated; just saw there are many responses above and I did not read them all.

Best,
CB

couple ideas to clear up some mis-statements... Radiation is NOT the only way to get heat ou of the motor in a vacuum. In air, it is condvection, conduction, and radiation. In a vacuum it is conduction and radiation. In both cases, radiation only provides about 5% of the total cooling.

When we design motors for vacuum use, we make sure we have the proper amount of metal contact to let the conduction element take the heat away. As one stated, how do you know your 30C is max? I doubt it - I suspect you have hot spots INSIDE the windings, where you admit they are failing, where the temp may be exceeding the rating of the insulation. Something to consider.

When we designed motors for vacuum use in the past, we did NOT change the insulation material - we changed grease in bearings and things that outgassed; once the outgassing is done, it is done I believe.

I would NOT use an induction motor in a vacuum unless it is the only type applicable for some unique reason. You state your motor is just 0.8kw in size. Since you lost the 75% cooling from convection, your left with the 25% conduction, so your motor should be 75% larger than required in air or some such.

You really should be getting rid of that rotor heat that cannot get out thru convection, and change to servo motor (synchronous) motor with magnets to eliminate that internal heat source.

I can kill insulation in a 230v motor w/o inverter duty insulation, running a 460v - even limiting ac fundamental output to 0-230v max - sometimes in less than 5 minutes due to corona discharge

I do not believe you have any business running a 460v (650v bus) motor/drive in a vacuum - and you are doing very well proving it!

It was stated above lower the voltage. You do not like 40a at 40v (many drive and motors available here, but ok), but I would change to 230v system ASAP.


I would contact app engineers at www.kollmorgen.com and explain your issues to them. Their aerospace division - in VA. USA not China - routinely build motors for use in space (vacuum); they may be the source you have been looking for. I will guess your motor built for this app in your size will be around $ 4000ea. If that is too high, then don't bother.

www.KilroyWasHere<dot>com

Hi Chasbean1
notes to your questions below.
1) How are the 30°C temperatures measured while the system is near a perfect vacuum? Measurement methods might be suspect without air.
--> the temps were measured with a infrared temp sensor pointing at the hub in various locations when under a vacuum. The digital display was recorded by video camera when under vacuum. it was a bit "old Skool" a bit like taking a photo of your screen because you cant get the printer to work. When running under vacuum the temp of the hub was never above 32 deg C in various locations. When the system was stopped = vacuum dumped to atmosphere and fans stopped, the temperature creeped up to 34 deg C (this means that there was convection cooling when running in vacuum). In my mind this conclusively confirms that temperature is not the issue. Additionally when the failed windings were inspected there was never any evidence of temperature related failures. i.e. no discolouration even deep within the windings.

2) Motors require air to cool them. If there is no air, they will not be cooled.
--> umm I don't agree. We use plenty of motors under normal atmospheric conditions which are completely sealed with no air circulation fan at all. in our vacuum application there is air, just not a lot - say ~5%. therefore there is convection cooling, just not a lot. But on the other hand there is 95% less air and that means say ~90% less load on the blades = ~90% less work being done = ~90% less heat being generated.....= ~90% less heat being required to be dissipated.

3) Why would ventilating/circulating fans be running to circulate air that does not exist? Can they shut off below a certain pressure?
-->there is some commercial sensitivity which I cant elaborate on further. however there is 5% air if at a 95% vacuum or what ever the vacuum is at.

4) What is the rough amount of air (cfm, lps, etc.) that they need to circulate? There are several vacuum compressor models designed to move/compressor air, all of which have their motors outside the medium and therefore can be cooled by ambient room air.
-->sorry not the right of application for our case.


regards
ryan

Hi mikekilroy

its not a overtemp failing of the windings. we have inspected and found areas of where the windings insulation has been failing. there is no discolouration. there is degradation of insulation coatings. but not temp related.

insulation coatings also outgas...

yes our squirrel cage induction motor ventilation fan is the only type at a suitable cost for our application.

The load on the fan is also far less under a vacuum because it is moving far less air. there is little blade pressure = little load on the motor = little work done = little heat = little heat to be dissipated.

We completely agree that the failures are due to corona discharge.

yes USD4k is too much for a fan.


thanks for your suggestions - we may have to return to KollMorgen if we don't get the long term reliability.


best regards
ryan

Hi All

we had the motors rewound with standard winding wire restored to as new condition. We ran the motors at the same voltage with the same drives under the same environmental conditions. i.e. same, same, same, same...

we made a simple low cost modification which my customer is holding close to their chest with respect to intellectual property and future patents.

Whilst I would like to provide a detailed description of the solution I am prevented from doing so by our customer. I apologise as I would liked to have given back with all the responses and careful thoughts on the posts!

The machine ran for 800hrs without failure and was shipped last week. Hopefully long term testing allows the fans to last ~10 years.

regards
ryan