How old can generator cores get?
How old can generator cores get?
(OP)
We are at the moment looking at a couple of 14MVA ASEA hydro generators from late 1950's. Both stator windings and rotor poles will be replaced but is it completely ridiculous to consider keeping the cores? Of course given that they turn out ok in flux tests. How old can cores really get? All your comments are really appreciated.
Thanks,
jens
Thanks,
jens






RE: How old can generator cores get?
Don't forget to test again AFTER the winding has been burned out of the slots for rewind.
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RE: How old can generator cores get?
RE: How old can generator cores get?
Also look for movement of the stator frames. In certain instances the core laminations may be in a good shape but over the years the frame might have moved out of tolerances, which may lead to restacking of the core.
Just a few thoughts.
Sarg
RE: How old can generator cores get?
I have seen many hydro rewind projects were a new winding has been placed in a much older core.
A good inspection is necessary and minor repairs may take place but in general it is acceptable to keep the old iron. Even a partial restack is very common.
RE: How old can generator cores get?
If your m/c has hot rolled core laminations, then replacing them with cold rolled (CRNO) laminations, which have lower iron losses, may sometimes justify the cost.
In a case of a 40 pole, 50 W generator (which had 4 sections), the customer decided to replace one section of the core which was badly damaged due to winding fault. But when we opened up the other sections just to check, we found entire laminations were rusted right from the teeth to the back iron. So, we replaced about 50 tons of laminations, which was a costly and time consuming affair.
Of course, one advantage was, new core was CRNO (as against original hot rolled) and we eliminated all the section joints / vertical splits.
Generally, core initiated faults are rare and a healthy core is reused since the core plate varnish has a higher temperature capability and hence longer life as compared to the windings.
RE: How old can generator cores get?
I was also reading in a book on magnetism that somebody was able to reduce core losses in a wound core transformer for power frequencies using 2 techniques. The first one was very thin laminations on the order of 0.003 inches thick. The other was to etch very tiny grooves just a few microns deep in the direction of magnatic fields on 5 inch centers. Mechanically scratching the surface did not work because annealing tends to make scratches go away. The groove etching cut the hysterises loss by a large amount and the total losses in half.
The groove etching technique would not work on generator cores because the magnetism needs to turn 90 degrees at the teeth. Hysterises loss will always be higher than for a wound core transformer because in a generator or stacked core transformer some of the magnetism is 90 degrees to the crystal grain direction.
RE: How old can generator cores get?
I do not know of any advances in water turbine design but I do know that there is a lot of room for improvement. I would not be surprised that some hydrodymanics Real Smart Guy has figured out away to double the efficiency of water turbines.
RE: How old can generator cores get?
We will look at the possible efficiency increasement, we are only considering keeping the core if it measures good and the efficiency increasement can not justify the cost of a new core.
mc5w: This surface etching thing is very interesting. The turbine wheels will be replaced.