3MW, 11kV Motor reconditioning 2

[krisys]

Good day!

I am presently entrusted with a job of reconditioning a 3MW, 11kV Motor. The motor is almost a quarter century old, commissioned in the year 1992. From the beginning this motor had lot of problem, but continues to run all these years with routine maintenance. Now the facility owners (end user) prefers the motor to be re-wound, but as an engineering service personnel, I prefer not to re-wind. Just re-condition the motor and run it for some more time.

Can any one share what are the possible reconditioning works one can undertake for this sized motor.


I would specifically appreciate Muthu's views. Others inputs are certainly appreciated.

 

[ScottyUK]

A bit more detail about the machine would help.
[ul]
[li]Induction or synchronous?[/li]
[li]Bearings are rolling element or Babbitt?[/li]
[li]Cooling is TEFC / TEAAC / TEWAC / something else?[/li]
[li]What problems did it have in the early days?[/li]
[li]Any machine diagnostic information - vibration, PD measurements & trends,etc?[/li]
[/ul]

FWIW I agree with your suggestion not to rewind unless there's an overriding reason to do so.

 

[ScottyUK]

BTW, that orange text is almost impossible to read. What's wrong with black?

 

[electricpete]

possible reconditioning works one can undertake for this sized motor.

Are you looking for a motor refurbishment scope/spec? Here is one from EASA (the trade association for motor repair shops in the U.S.)

https://www.easa.com/sites/files/resource_library_public/EASA_AR100-2015_0815_0.pdf

From the beginning this motor had lot of problem, but continues to run all these years with routine maintenance.

Knowing the specific problems encountered might help to focus the refurbishment scope.

Now the facility owners (end user) prefers the motor to be re-wound, but as an engineering service personnel, I prefer not to re-wind.

There will be a number of things to consider here.
* as-found core loss tests. If the core is trash (showing hotspots on the bore or OD), you probably won't be able to repair that without a rewind.
* visual inspection of the winding. Are there signs of movement. Are there signs of partial discharge. Are there signs the insulation has become dry/brittle?
* electrical tests after steam clean/bake. There are a variety of tests available that will give some insight into the condition of the winding. Some are potentially destructive (dc step voltage hi-pot, surge test) and in that case selecting the test voltage is important. Selection of higher test voltage (within industry standards) has larger chance of uncovering problem(in which case your decision may be made for you if the motor fails). If you have a higher test voltage and the motor passes with no anomalies, you have a better confidence in the condition of the motor going forward than you would if you had tested at lower test voltage (at least for dc step voltage and surge test imo).


=====================================
(2B)+(2B)' ?

 

[edison123]

Hi krisys

Could you post what kinds of problems were faced in this motor and what was the routine maintenance done that kept these problems under control?

A 90's vintage motor winding would be near its end of life and with other problems present, it would be a ripe candidate for a rewind (with a probable insulation and rating upgrade) and for sorting out other OEM issues. This would definitely be cheaper than going for a new motor of this rating.

Of course, I have a rewinder's bias and we are frenemies with OEM's.

Muthu

http://www.edison.co.in

 

[krisys]

• Induction or synchronous? –
Ans: Induction motor

• Bearings are rolling element or Babbitt?
Ans: Sleeve Bearing

• Cooling is TEFC / TEAAC / TEWAC / something else?
Ans: Water cooled motor

• What problems did it have in the early days?
Ans: Only the higher operating temperature. The motor is with class F insulation but, the operating temperature is limited to insulation Class B.

• Any machine diagnostic information - vibration, PD measurements & trends, etc
Ans: Five years back the motor was overhauled at a motor service center. At that time the recommendation was to re-wind.
The problem is that the application of the motor is critical. Spare motor is not available. Now the proposal is to procure a spare motor first and then to release the motor for re-winding.
Now the motor shall be just overhauled.

Due to the power source limitations, this motor was designed to have low starting current. The motor is almost fully loaded, like 95% of the motor name plate rating.
Someone said that during the motor re-winding, there is a possibility that the motor characteristics might change. This is a risk.

If the starting current or the motor loading changes, it will be a problem. This is my concern.

Also, as the motor is very old, the motor core laminations may damage during the removal of old winding.

 

[mike gergen]

This isn't an off the shelf item, it may take months to obtain a replacement.

 

[edison123]

A special design motor will have a long delivery time and it is moot that other OEM's will even agree to do it just for one motor. The client should have gotten a spare motor along with the original purchase if it was such a critical motor.

A higher operating temperature would have accelerated the stator winding insulation aging.

A good rewinder will ensure the motor original design and stator core are not affected by a rewind.

If this is a non-VPI stator winding (which is likely in 90's), the turnaround time is about 4 to 5 weeks for a rewind.

Muthu

http://www.edison.co.in

 

[krisys]

Thanks Muthu

Since the motor is very old, is there a risk of motor characteristics, output etc. getting changed?

If the motor in VPI stator winding, what is the time for rewinding?

 

[edison123]

Hi krisys

The motor original characteristics will not be changed by a rewind as long as the original winding design is maintained.

For VPI machines, it will take additional 4 to 8 weeks (depending on the no. of slots) just to remove the old winding and clean up the slots.

Is there any OEM manual or nameplate that says this is a VPI motor?

Muthu

http://www.edison.co.in

 

[krisys]

As you said, the re-winding shop has asked for 12 weeks time for re-winding.

As the motor is old, I was not able to find the manual.

For the VPI machine, while removing the old winding, is there any risk of core laminations getting shorted or damaged?
If the core laminations are short circuited (may be partially), is there any way to eliminate such shorts?

Also this motor is a low starting current motor, hence it is a special design.
But I think the starting current has more to do with the rotor design. Hence should not have any risk.

 

[edison123]

krisys

Both stator and rotor windings play a role in the starting inrush current.

We do rewind of VPI machines all the time without any damages to the core.

Muthu

http://www.edison.co.in

 

[electricpete]

For the VPI machine, while removing the old winding, is there any risk of core laminations getting shorted or damaged?

A critical aspect will be to control winding burnout over temperature. I suggest your specification limit the oven temperature to 650F and require the shop to provide evidence (chart record) that the temperature was maintained. 650F should be sufficient to limit damage to older C3 core plate. But it is not always enough to burn out the winding, in which case shop will inform you and develop a new plan (either increase the temperature to 675F or 700F or cold strip). In that case, you have have lost a few days starting at 650F, but it's worth trying imo since of course core restack can set you back a LOT more time and money.

We do rewind of VPI machines all the time without any damages to the core.

Knowing your expertise, I'm sure your shop does a great job. I would suggest it also depends on the motor (core insulation type and quality, winding characteristics...how hot do you have to get it for practical removal) and also the procedure specified for core testing (there is some variability among specifications especially on the target flux level and duration).

If the core laminations are short circuited (may be partially), is there any way to eliminate such shorts?

You'd have to ask the shop what they feel comfortable with, given the particular damage situation you end up with. For minor surface shorting they may be able to do some cracking, spreading... possibly with a special solution. See page 3 here for brief overview of some core repairs

http://industrialelectricalco.com/wp-content/uploads/2014/01/Lexseco_WhitePaper.pdf

However there certainly are many cases where a reliable core repair is impossible without restack. At out plant we have two families of motors built in the late 70s by Parsons Peebles. On the motors from this group that have been rewound twice, most of them needed core restack the second time around.


=====================================
(2B)+(2B)' ?

 

[edison123]

pete

We don't use burn-off ovens to protect the core. The core loss test is done at 1.1 to 1.2 T, 50 Hz.

Muthu

http://www.edison.co.in

 

[electricpete]

So you cold strip every single motor that you rewind?

=====================================
(2B)+(2B)' ?

 

[edison123]

Yes, Pete.

Muthu

http://www.edison.co.in