amortisseur cage repair decisions (small generators) 1

[bentov]

We repair/rewind generators (typically 250kw and smaller), generally growl test amortisseur cage windings during evaluation. An engineer at our trade association (EASA) advises that damaged amortisseur windings can be safely removed (not repaired) so long as the unit is not to be parallelled, but I've seen some info about damper function in response to load changes that makes me nervous about that. We have a 75kw in right now in need of a main stator rewind, has several open cage bars (down in the slot, can't just reweld to end rings). It's worth rewinding, but not worth repair if we also fix the damper cage (drill out all the bars, replace and reweld). Does anyone have experience successfully eliminating amortisseur windings for standby (not parallel) duty generators? If so, is it acceptable to simply part (and secure) all bars from the end rings?

 

[jbartos]

Suggestion: Contact electrical generator/motor repair firms that do this routinely, e.g.

http://www.national-electric-coil.com/products.htm

http://www.irscoil.com/customer.shtml

etc.

 

[edison123]

For stand alone generators, damper or amortisseur winding is not required. But you mentioned cracked bars in the damper winding. Was it due to excessive current flow or it was just mechanical failures (due to improper caulking)?

If you machine out the end rings, then I do not see much problem in removing the bars which are not too tight. Once you overcome the initial resistance (due to caulking and the varnishing), you should be able to remove them by tapping them with a non-metal drift and mallet.

 

[bentov]

Thank you for the response, edison123. The end ring in this case is sort of a star shaped aluminum washer (a thin plate that tops the rotor core on each end, continuous except for the shaft hole - this avoids the ring continuation spanning the pole pieces so the fields could be rewound if necessary without parting and then rejoining the end ring material). The bars are aluminum rods, probably 3/16" diameter - we can shine a light down the slot some on the open ones, looks like they've just corroded away over time. The rotating fields meg and drop test OK, so the easiest thing would be to just not worry about the open bars (maybe dip & bake the rotor to seal off ongoing corrosion). It would be a fair amount of work to peel out the end rings (without damaging the field coil end turns) and drive or drill out all those bars. If we just leave it alone, will there be any localized heating of the (non-open) bars or end ring plates (due to unbalanced current flow) that might damage the field windings , or maybe harmonic vibrations associated with unequal electrical distribution of the remaining bars? Does cage eddy current only arise during frequency synchronization of parallel units, or is it also excited by rotor speed changes associated with load fluctuations?

We don't usually see cage problems, but we did repair a 250kw 6 pole unit a month ago where the only problem was the damper cage - customer (a generator service company) observed the unit running, output voltage OK but smoke coming from the interior, peering in saw a glowing trail of light around the rotor winding ends. That unit had non-continuous end rings, so cage current had to flow through the pole iron, accross a small arc of shaft material, and up through the adjacent pole iron to reach the next cage section. All of the end ring plate sections were melted open at the exact center of their width, and most of the bars were melted open from the end plate sections on the outer edges of the pole face arc. There were also arc tracs visible at the edges of the pole piece/shaft joints all the way around. Lot of stuff heating up there! After confirming that the unit was connected in parallel (with the utility, for peak shaving), we replaced the bars and end plates with like material. I guess it's working (no news is good news). The service guy said he watched the owner/user fire things up, doing manual locking throttle adjustment (lot of throttle increase with no speed change as load was added). I'm guessing in that scenario that an abrupt removal of load would cause a sudden generator overspeed and resulting big departure from frequency of the parallel source, and a corresponding rise in amortisseur current flow (proportional to the kva capacity of the parallel source? or just a function of the frequency differential and the hp/momentum of the prime mover, throttle locked at overspeed?)

Thanks for your interest and comments . . .

 

[edison123]

bentov,

I have seen the star (or cross) shaped end rings in 4 pole/6 pole machines. But, first time I am hearing about aluminum bars/end rings. (How did they ever get to do the notorious aluminum weld of the bars with the ring beats me.) Damper bars, by definiton, ar supposed to have a low resistance, ergo copper bars.

Typically, currents are induced in the damper winding when there is a rotor swing (to and fro) over the steady speed. This can happen diesel sets where there is cyclic irregularity of driving torques. That is why a flywheel is fitted to the engine to smooth out the torque pulses.

What is your prime mover ? How is this gennny connected (island mode or in parallel with other geenys) ?

 

[electricpete]

Interesting discussion. The original post said
"...(not parallel) duty..."

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[jbartos]

Comment: The amortisseur winding is rugged and one of those last expected parts to fail in the machine. Bars may have very tight tolerances so that they might have been inserted under high hydraulic pressure. Therefore, drilling them out should not be ruled out.