testing for new motor rotor assembly conducted without stator 3

[electricpete]

We had a scare due to rotor problem on critical motor with long-lead time for rotor replacement.

As a result of this and suspicion of bad rotors waiting to die on the sister units, we are going to buy several more rotors which will go into storage until such time as we overhaul the motors (during next 10 years), at which time they will be installed. The economics of that plan may be a little bit whacky, but that's beside the point.

Now comes the hard part. We will buy the motors in near future but not have opportunity to install them for several years. Also no stator assembly available to test the rotors in. Are there any special tests I should require on the rotor to find problems at time of purchase rather than at time of installation?

The only ones I can think of are the obvious balance and runout checks. I think I will also request a rotor core loop test monitored with infrared. Anything else I should spec?

 

[Addy71]

Pete,

Why not insist on a growler check for rotor bars? Idon't know your motor rating, but if these are aluminium diecast rotors, blowholes are most common.

 

[electricpete]

Thanks. It is 2500hp 13.2kv induction motor rotor. Fabricated copper bar rotor.

Will add a growler check which should be fairly easy.

 

[edison123]

pete,

I can add dye penetrant test on the bar to ring joints to eliminate cracks and assure proper joint welding/brazing.

When you are spending money on so many rotors, why not spend a little bit more to but one stator along with complete bearing etc. also ? ( I assume they are identical motors). That way, you can test the rotor (for no load stator current and vibrations)and also have a standby, ready to use motor. Other wise, I cannot think any particular tests on cage rotor, where everyting is metal and there are no voltage spooks (!)

Another thing, we rarely come across serious failures in cage rotors, which are very sturdy and failure resistant (That was their USP as compared to wound rotors, right?). So, I am surprised by your numbers. What exactly was the problem in these rotors ? Have you identified the cause(s) for such failures ? If so, what instructions have been given to your rotor supplier to avoid such causes?

 

[electricpete]

Hi edison. Did I mention the economics of our plan are a little bit whacky? I was hoping to head off the inevitable questions such as yours which are reasonable but not really productive to explore in detail.

There are a lot of unique circumstances around this set of motors. One is that the motor is in very limited access location where it cannot be removed as a unit... must be disassembled in place, individual pieces rotor, stator, frame rigged out through the plant , sent to a shop for repair, assembled at shop for testing, disassembled so we can rig the pieces back in, and reassembled in place. The motors are approaching 20 years old and we have one round of overhauls planned (motors have never been overhauled before except one motor ... have only this one round of overhauls planned for the life of these machines). During that round of overhauls we want to do everything we can to make sure we never have to pull them out again. The OEM has told us the rotors are horrible design which cannot repaired. That was born out in our recent problem. Complete details of that problem are here:

http://www.reliability-magazine.com/ubb2000/ubb/Forum2/HTML/002392.html

We have a spare wound stator core. What we are missing for testing is a spare stator frame.

There is big support within my organization for buying those spare rotors so when we remove these machines once for overhaul we have better probability it will be the only time. That is based upon recent vivid memory of the pain of removing this machine and fighting a rotor problem. Along with fact that our vendor tells us the other rotors will likely have problems sometime during the next 20-30 years (funny how OEM's can flip-flop on the reliability of their equipment depending upon how it affects them... they will get the contract for new rotors). Spare stator frame does not directly address that concern since all our existing stator frames are healthy and we don't expect any problems there.

There may be justification for buying extra stator frame to support the testing and serve as a spare but I'm not sure I can sell it. It is not all my decision.... more a product of organizational dynamics.

Looking to focus the discussion back towards the original question.
Thanks.

 

[edison123]

pete,

The only tests I can think of for your spare rotors

1. Dynamic balance.
2. Rotor core length and diameter (must match old ones).
3. Axial magnetic centre.
4. Trueness of rotor.
5. Dye penetrant test on ring to bar joints.
6. Loose rotor bars.
7. Cooling fan orientation (if unidirectional).
8. Bearing centre to centre (must match old ones).

 

[UKpete]

overspeed test? (if it's feasible to spin them; and ideally done hot and performing some of edison123s tests before and after)

 

[jbartos]

Comment on edison123 (Electrical) Aug 7, 2003 marked ///\\pete,

The only tests I can think of for your spare rotors

1. Dynamic balance.
///Would be appropriate to test in the stator. So, this one works for having a least one motor stator available.\\2. Rotor core length and diameter (must match old ones).
///This one is o.k. to be tested without stator.\\3. Axial magnetic centre.
///This one appears to be better tested in a stator.\\4. Trueness of rotor.
///Again, it is probably best to be tested in the stator. Alternately, it could be set up in a lathe or carousel. There are not many of these large machines around.\\5. Dye penetrant test on ring to bar joints.
///O.k. without the stator.\\6. Loose rotor bars.
///O.k. without the stator.\\7. Cooling fan orientation (if unidirectional).
///O.k. without the stator.\\8. Bearing centre to centre (must match old ones).
///O.k. without the stator.\\///Now, additional tests might include material tests for cracks, e.g. X-ray test, dimension/tolerance accuracy tests/measurements, etc.\\