What happens with Stator Rewinds 4

[CuriousElectron]

Greetings All-
What happens when a generator gets rewound? Typically, is it true that only the insulation gets replaced with new and the winding copper coils are retained?
I wouldn't think the owner would want to throw away the copper and manufacture all new winding coils.

Thanks for the feedback.
EE

 

[davidbeach]

Rewinds that are done on site are certainly all new coils, both copper and insulation. Any planned rewind is probably going to be the full meal deal simply to minimize the down time. An emergency rewind will probably go with whatever is quickest.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

After severe water damage in hurricane Mitch, I had a couple of 600 KW generator ends dipped and re-baked.
The windings had not failed but the insulation resistance was too low to safely run the machines.
This involved removing the ends from the diesel engines and shipping the generators from Central America to Florida where the work was done in a rewind shop.
The windings were contaminated with brackish water.
In the rewind shop, the generators were rinsed clean of any remaining contaminants and then dipped in a thermo-set compound.
The generators were then baked in an oven to set the thermo-set compound.
At least two generators on the island were put into service without being re-insulated and both failed within a few hours, never to be repaired.
However, if the windings have been damaged by burn-out or impact, they may have to have the windings replaced.
One of our dipped and baked machines gave good service.
The other dip and bake generator was dropped while being removed from the ships hold.
This machine was originally wound with flat copper strips rather than wire.
The impact of the fall caused some of the strips to push through the insulation and contact the core.
This was not repairable by dip and bake.
The machine was stripped out and rewound with new, insulated conductors.
Dip and bake about $2000
Complete rewind about $7000. as I recall.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

CuriousElectron

In theory you could re-use the original copper. but getting all the old insulation off would be too many man-hours (too expensive), also you would have to repair any damage to the copper from the fault current.

When I was involved in this some years ago, we often replaced the older type of insulation (class B) with more modern Class F (epoxy etc) insulation. This can be thinner and we made the space up with copper, so we used much the same winding but with larger copper.

Larger copper cross section results in lower resistance and less copper loss and the machine ran cooler - lower temperature means longer insulation life.

The golden rule as waross implies is "keep it clean - keep it dry"

Putting an open ventilate generator in the same room as a diesel engine with oily emissions from the engine gets the interior of the generator contaminated with a sticky deposit of smoke and diesel fuel. Any dust will stick to the sticky deposit and can block the cooling passages and / or attract moisture.

One solution is to 'regularly' (experience will lead you to this) remove the generator, get a repair shop to dismantle, clean, bake (to remove moisture), re dip if necessary, replace the bearings if they are ball/ roller type, retest.

In later days my company ran a fleet of gas and diesel generating sets, and we would overhaul the generators at a minimum of three years, whether they were standby or continuous.

The ultimate engineering solution would be to have generators with a closed air circuit, and water cool the circulating air. High first cost works against this, especially for standby sets.

 

[Shanaya]

Not only is it due to rewinding, it can reduce performance due to wear and tear and long use. Generally, when the motor burns and has to be sent for rewound means, the motor gets older and naturally loses its originality.

This occurs a few times after rewinding when mounting the rotor, the end Shields and the worn-out bearing distance between the rotor and the stator increase causing less speed than the rated speed as the percentage of slip increases.

In this case, we need to replace the new bearings and, if there is still a problem, we may have to change the rotor or fix it by filling the material and turning to the appropriate dimensions.

 

[waross]

I worked in a sawmill in the Pacific North West many years ago when sawmills were noted for extreme abuse of electric motors.
Burn-outs of integral HP motors were common.
We found that rewound motors were better quality than new.
We favoured rewound motors for known problem applications.

This occurs a few times after rewinding when mounting the rotor, the end Shields and the worn-out bearing distance between the rotor and the stator increase causing less speed than the rated speed as the percentage of slip increases.

In this case, we need to replace the new bearings and, if there is still a problem, we may have to change the rotor or fix it by filling the material and turning to the appropriate dimensions.

Interesting. Never seen that done.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Back from a few days off . . .

Bill stated:

When I was involved in this some years ago, we often replaced the older type of insulation (class B) with more modern Class F (epoxy etc) insulation. This can be thinner and we made the space up with copper, so we used much the same winding but with larger copper.

It's been a few years [ 23, actually ], but IIRC when all sixteen ~60 MVA hydraulic generators in the plant I worked in were sequentially rewound as part of a complete plant rehabilitation and unit rebuild, this is very close to if not exactly what happened; brand new windings using the same flat copper conductor configuration but with a larger cross section and with new and improved insulation were installed.

I'd take time now and then during plant inspections to have a look-see at how the rewinds were progressing, and I recall observing the way that once the winding halves were installed and wedged in, the exacting and painstaking work of the installers began. They would patiently tin and solder together the corresponding ends of each winding conductor, apply the insulation, then move to the next pole and do the same thing. By this means the applied epoxy insulation had sufficient time to cure, ensuring that by the time the installer had completed one circuit of the stator, that that joint would not sustain damage when the next set of conductors were tinned and soldered.

I had to admire their patience; I think I would likely have found this to be simply mind-numbing work . . .

Incidentally, the re-built units, not only having re-wound generators but completely redesigned hydraulic turbines, were rated at 65 MW and, again IIRC, at ~75 MVA. This proved to be of great benefit, since more water could be passed during spring freshet conditions without the need to enter spill conditions so as to satisfy the prevailing flow schedule.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

That was Hoxton, not me.
I saw a very old motor that had been upgraded from 150 HP to 300 HP by rewinding with better insulated wire.
The motor was originally a NEMA frame or older. (NEMA frames were updated to NEMA "U" frames in 1952 and upgraded again to NEMA "T" frames in 1964.)
The motor was originally wound with cotton covered wire, bulky and a low temperature rating.
The motor was rewound with modern magnet wire with much thinner insulation and a much higher temperature rating.
The new windings ran more current and ran hotter.
Fortunately those old motors had much more iron in their cores than was needed.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Thanks for pointing that out, Bill; sorry for the mis-accreditation, Hoxton.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[low1]

In our 12 100MW unit plant, four units have been rewound since 2009. The first one was the result of a core clamping finger breaking off and riding around the air gap, destroying everything. Three more units, built with the same materials and methods, have been rewound from 2014 until 2017, with the remaining 8 to be done sequentially starting in 2021/2022. In our case, the entire stator was completely replaced, frame, laminations, coils, etc. Field poles were sent out for refurbishment, and most controls were replaced (new governors/exciters, protection suite, etc). Pretty much the only things that remained in place were the turbine and shaft, rotor, isophase bus and breaker. With the upcoming overhauls, we will be replacing the old ABB DBG2000 air blast breakers with SF6.