aluminum vs copper rotor construction - real world 800 HP experiences

[Tmoose]

We have been getting quotes from various motor suppliers for MV 700, 800 and 900 HP motors. Our application will be almost exclusively continuous service for months at a time. Easy starting via centrifugal clutch .

Some suppliers say (white papers and brochures in addition to sales guy's pitch) fabricated copper is vastly superior so that is all they offer. Others say unless we routinely make multiple starts per hour the durability with pressure cast aluminum will be excellent, and the theoretical efficiency benefit of copper is a tiny fraction of a percent.

Both arguments sound fine to me.

Are there any experiences with modern motors to share?

thanks

Dan T

 

[Laplacian]

It is difficult to quantify the advantages of copper over aluminum. I have seen many aluminum rotor cage failures attibuted to periodic start/stops. Aluminum fatigues faster than copper, and also expands more than copper due to temperature fluctuations.

I have seen copper rotor cages fail due to abuse on start up (exceeding the starts/hour by serveral multiples).

The will both run for many years, but the copper cage has a much higher tolerance for thermal cycling over time.

If this is a very critical application where a very long service life is required, then a case can be made for spending extra on copper. If this is a 5 year project with constant loading when the motor is running, then maybe some cost savings are in order.

 

[electricuwe]

For a motor continously in operation, the savings in energy cost for a copper rotor will compensate the higher initial cost within a quite short periode of time.

 

[cjw81]

Don't go near aluminum as far as danger of fire is concerned. It was tried and pretty much shoved down our throats to save money. After some fatal fires it was finally banned. You will hear a lot about special wire nuts and connectors, I would not trust any Al setup.

 

[davidbeach]

I don't think that 60's-70's aluminum house wiring and the internal conductors of an 800HP motor have a lot to do with each other. Vastly more Al is used than Cu if you take the entire electrical industry as a whole.

 

[Tmoose]

I should have specified the aluminum or copper choice refers to rotor cage/bars/end ring construction.

Sorry,

Dan T

 

[edison123]

GE had an article comparing the copper and aluminum rotors and concluded that copper was better. I will see if I can dig it up from my archives.

Muthu

http://www.edison.co.in

 

[edison123]

Richard Nailen article

http://findarticles.com/p/articles/mi_qa3726/is_200302/ai_n9222174/

Muthu

http://www.edison.co.in

 

[electricpete]

Some more references:

http://www.sea.siemens.com/us/inter... vs. Aluminum Rotors for Induction Motors.pdf

http://www.joliet-equipment.com/pdf/rotorconstruction.pdf

http://www.etimotors.com/copper.htm

We have 30 13.2kv motors and around 70 4kv motors... most were bought in mid-1980's.... is that modern.

Here is one with copper bars where bar looseness/movement within the slot caused thermal bow and very high vibration

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/4151080002?r=4151080002#4151080002

Here is one with copper bars that was picked up on current signature, but never caused any problems. We eventually disassembled and re-soldered

http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/7071076492/p/2

We had one hermetic compressor motor with fabricated aluminum bars that showed up to have opepn bar during shop testing ... don't recall the full details. Since it was a non-critical motor we elected to live with the condition. As I recall the only repair alternative was re-bar and would have been very expensive.


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[Tmoose]

Hi Pete - So of those 100 MV plus motors, only 2 were known to have "issues" and they were fabricated copper construction? Any guess if all of the 100 are copper? WEG is saying they offer cast aluminum as standard in the standard MV lines, but will do copper for 6% more $$. It would seem like a well controlled casting process could result in bars conforming very well and thus more securely locked to the laminations than one relying on old world craftsmanship to swage them individually.

Even API 541 says "Industry experience has demonstrated that aluminum cage designs through 1000 hp are generally acceptable", suggesting manufacturers have got the casting process pretty well figure out.

That Siemens paper you reference is pretty even handed.
Regarding efficiency it says "Normally it would be difficult to justify the expense of the copper bar rotor just for improving efficiency especially on smaller machines."

Dan T

 

[electricpete]

Hi Pete - So of those 100 MV plus motors, only 2 were known to have "issues" and they were fabricated copper construction? Any guess if all of the 100 are copper?

We had 3 problems described above.

Onl the first was a problem. It was the one with high vibration. It was fabricated copper bar.

There 2nd and 3rd I would call findings but non-problems.
The 2nd one described above showed only as db difference on current signature but never caused high vibration. It was fabricated copper bar.
The 3rd one described above was only revealed during shop testing but did not cause us any problems. It was fabricated aluminum bars.

As far as I know all these medium voltage motors have copper bars except a handful of the chiller motors which are fabricated aluminum... maybe 10% of the total We don’t have any cast aluminum among these medium voltage motors. We have lots of cast aluminum bar rotors in 460 volt motors of course and haven’t had any problems there.

I have heard that alumnimum rotor motors of all kinds are less tolerant of short-term abuse (repeated starting in a short time interval) since you can cause melting which kills the motor immediately (there was a case study like that at EPRI LEMUG). In contrast the fabricated bars of both kinds in severe applications can be susceptible to fatigue which accumulates over the long term. For the short-term failure you’re dead in the water. For the long-term fatigue the degradation is gradual and you can see it coming for a long time before it affects the motor function... so there is lots of time to react and it doesn’t particularly jeapordaize overall plant reliability.

I would think if efficiency was a consideration, you would specify efficiency in the spec or require bidders to identify efficiency and roll it into the evaluation on some kind of cost basis.... rather than specifying specific construction details which you think would give high efficiency.


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