Radial forces due to uneven air gap 1

[tmoritz]

We have a 100MW pumped-storge unit that is running with an uneven air gap. Nominal air gap is 0.700 inch. Eccentricity is 0.065 inch. The winding is going on 30 years and is showing weakness. We are in the process of procuring a new winding and big question is, should we risk moving the stator?

I'm a mechanical engineer and have forgotten most of my electromagnetics from freshman physics. I have often heard the phrase, "It'll find its electrical center." I know this is true from an axial perspective but is it true radially? Is there a way to estimate the radial forces resulting from an uneven air gap?

So far I have identified the following issues associated with uneven air gap:
1) increased radial forces on the guide bearings
2) increased vibration
3) uneven stator heating resulting in thermal distortion of the stator

What other issues are at hand?

I look forward to reading the responses to this thread.

Thank you,

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[esee135]

edison123: It sounds like there are two schools of the term moving the stator. One being recentering the stator frame and core assembly about the rotor, the other being reshaping the stator core via push and pull methods.

In the latter, we had performed this in our utility on asphalt mica units with okay success. Our results showed that we did make a core reshape of >.100". Basically going from a wipe situation back to a near design air gap in the .400" zone. In my case, yes there is a risk.

For the case of recentering the stator frame assembly, shouldn't be an issue in my mind. I've been involved in sliding earling 1900 units around without failure.

 

[esee135]

...as another note, I can agree with the others, waiting to replace the core once you strip the coils may put you in a compromising position. It would be worth doing a 3 point heat run monitoring all temperatures and infrared to see the core condition. Options such as Elcid could also be considered.

I believe it is possible to do the loop test with the winding intact. If the core is that much in question, then perhaps taking an outage doweling the unit and jacking the stator up to do a loop test in situ may not be a bad option.

I agree with wolf39, new technologies such as the through-bolt core tensioning system are great benefits to the industry that you can tap into with a new frame assembly. I can see the financial restrictions in your energy costing, units of your size could earn upwards of $100,000 per day in my region, making core replacement an easy economic case.

Tom, I can agree with your approach of looking into retaining the core, since manufacturers heavily pressure customers to replace the core as it is much simpler to design your own versus conform to a non-OEM core. With that being said, good engineering can allow you to retain the core, however unstable air gaps, and lamination insulation failure are indicators that a new core may be more beneficial.

There is no question that the older cores were very hearty, having thinner laminations than required, and having excessive steel allowing for low core temperature rises.

 

[tmoritz]

Thank you all for your responses. This is a very interesting discussion.

We will be rewinding this unit. It is currently scheduled three to four years out. We have decided not to risk damaging our fragile winding at this time by moving the stator.

In the mean time I will be monitoring bearing orbits and temps as well as air temps in the generator. Can anybody else recommend data we will want to monitor between now and the rewind? Winding vibration? Stator iron temperature?

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[esee135]

An investment into air gap monitoring would be beneficial to monitor gap stability. There are solutions by the major manufacturers to insert core RTDs in the ventilation ducts as well. The level of core monitoring is more dependent on your interest in keeping the existing core.

Technically with temperatures, you can now afford if necessary to use up your temperature class ratings until rewind.

Major shaping should show it`s ugly head in the neutral voltage. Long term trending versus your operating point would likely tell you if the situation is changing.

As a suggestion, which most utilities are adopting is purchase a Class F winding and operate it at Class B temps. The temperature head room is difficult to gain with already owning a polyester or epoxy-mica winding.

 

[electricpete]

Major shaping should show it`s ugly head in the neutral voltage.

I have a hard time understanding that, considering the small arc subtended by pole phase groups on a slow speed machine, I would think the difference in airgap for adjacent pole phase groups are miniscule... and trying to discern those differences among a sum of several series groups would be futile. I could be wrong, but if so I'd be interested to hear some proof or explanation.

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[esee135]

electricpete: Dang it, I think I'm wrong. I agree with you, the imbalance would be in the form of partial vector sum from the stator pole in the series connection having a higher magnitude. Which would probably be difficult to read in the canceling neutral. Good catch, thanks.