Cracked Rotor End Ring 2

[rockman7892]

I wanted to share some pictures of the rotor we recently has removed from a 1250hp 4.16kV motor. This motor was sent away to a motor shop for scheduled inspection and it was found that it had a cracked rotor end ring. This is a new one to me so I found these pics quite interesing.

Was curious to see if anyone else ever seen this before and what the causes were.

I'm guessing that we should have seen some electrical losses associated with this crack.

 

[electricpete]

Here is a link to an interesting article:

http://tinyurl.com/aurr5l

They talk about 4 types of rotor: copper fab, copper cast, aluminum fab, aluminum cast. They mention aluminum cast might be used up to around 1750HP. We have a 500 hp aluminum fabricated rotor.

How can you tell it is an aluminum conductor rotor? From color of bars where paint worn off, or some other feature?


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[rockman7892]

Oftenlost

I agree with what you said about having the thermal data reviewed. Since I only have the origonal motor datasheet would that be enough to give to a consultant for a review? What other information besides load data would I need to put together to send off? Do I need the thermal data or datasheet for the re-wound motors?

 

[oftenlost]

Rockman
This rotor is a standard design where the bars are extruded and include a sacrifice tab at the bottom of the bar that is milled off during the insertion process to insure a tight fit in the rotor slots. This prevents bar vibration which leads to cracked bars over time. The bars are inserted and a milled end ring is installed over the bar ends and mig welded to attach the bars and fill out the end ring which is then machined to fit. I have personally seen them in motors exceeding 5000 HP but do not know about an upper limit as most of the motors larger than 5000 hp that I have worked on were synchronous. Motors up to 10,000 HP of so of this type are still being manufactured in the GEVISA medium horsepower factory in Brazil and just last year, had bars and endrings on the shelf to rebar a Schenectady designed motor similar to the rotor in the picture. The right consultant can tell much from the model and serial number.

 

[oftenlost]

Pete
The reason that I believe it is aluminum is that 1. Aluminum is standard in this size 2. The bar tops are standard shape for aluminum bars but uncommon in copper bars which are normally rounded or square. 3. The welded end ring imperfections can be seen in the picture. In fact, the weld appears to have holes caused by gaps in the overlay which suggest a possible previous repair.