Estimating Motor Life time depreciation at SF 5

[gokulkrish2]

Hi Guys,

Recession has hit us. So we are trying to run a 1000HP motor to its SF level. We are going to increase feed into the crusher and by pass the Multilin beyond the overload setting. Now,

We know that we are going to reduce the life time of that motor. Is there a way that i can predict how much life is that motor going to lose?

I read through a lot of stuff in this form about running motor at SF. Was very helpful but in my application where there is frequent spikes in the load pattern, i think i would be wrong to assume the average current and go ahead with my analysis.

Here is a short description of the system.

1. It is a 1000HP motor.
2. We currently run at an average power level of 625KW and the spikes go about 880KW sometimes.
3. We are going to increase the feed into the crusher so that it will be loaded 15% more.
4. The RTD in the stator never get hot. They are around 170-190F maximum. But the motor is a class F motor.
5. My boss says the motor would be good for 2-3 years. I strongly doubt that.
6. I said it depends on the frequency of the spikes.
7. We are thinking to bypass the multilin completely and just go with the RTD protection.
Is this a good idea?
How can i exactly predict how much time that motor is going to run.
Any suggestion of somehow improving the motor life when doign this??
What are your thoughts?

gokul

 

[dpc]

Old-timer's rule of thumb was that every 10 deg C increase in winding temp cuts the insulation life in half.

Bypassing the Multilin is not a good idea. RTDs are much too slow to provide adequate starting/locked rotor/short circuit protection.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[jraef]

I agree that bypassing the Multilin is a bad idea. You can set up the curve to accomodate the SF, but don't turn it off. Depending on which Multilin model you have, the protection can be programmed to allow biasing of the curve by the RTD input, effectively doing what you say without actually turning it off.

dpc's R.O.T. is the same one I have always used; 1/2 the life for each 10 deg. C rise over the rated temperature. But there is no hard rule, it actually depends upon the motor mfr. Some say that there is NO decrease in motor winding insulation life within the Service Factor; some say nothing about it, meaning there probably is. NEMA simply says that if you run into the SF, you can expect that the life will be decreased.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Hoxton]

When i was a student engineer (motors and generators factory), the Chief Insulation Engineer made the same comment - + 10 deg C = half the life, but he added : but you do not know what the life was going to be in the first place.....

 

[Hoxton]

When i was a student engineer (motors and generators factory), the Chief Insulation Engineer made the same comment - + 10 deg C = half the life, but he added : but you do not know what the life was going to be in the first place.....

 

[gokulkrish2]

But this -/+10C rise is above the rated degC or the ambient degC.

One more thing. I am going to predict the increase in degC the following manner. Can someone validate it?

My average current maximum for the current load is 110A
My average temperature maximum from the Stator RTD is 195F

I am assuming Linear relationship between current and temp

So i say for every additional amp we put in we increase the temp by 1.77F(195/110). Is it a rational way to do?

If it is right then i can assume the max life of the motor to be 10 years and then give them an approximate life expectancy of the motor at SF.

Please Guide.

gokul

 

[PeterKsiazek]

Correct me if I'm wrong, shouldn't the temperature rise be proportional to the square of the current change (I squared R losses contribute to heating).

 

[jraef]

"But this -/+10C rise is above the rated degC or the ambient degC. "

Rated.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

Lots of good points made above.

Another comment - The 10C rule is for aging of organic insulation. It is very relevant for oil filled transformers. It may have been relevant for older motor insulation. But I am pretty sure it is irrelevant for materials used in modern insulation systems (daglas enamel strand insulation, mica paper tape, epoxy and polyester resins). They do not have that type of thermal aging process. It is more of a threshhold temperature above which their mechanical properties change.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

Predicting residual life of insulations is a shot in the dark, at best. There are factors other than the current that detemine the residual life - quality of original winding, voltage stress (frequent stops & starts, voltage surges in the system), mechanical vibrations, winding contamination etc.

For a class F machine, operating below 120 deg C is "supposed" to ensure a 20 to 25 year life.

 

[starkopete]

For what it's worth.

The chart on the attached is one attempt at quantifying the effect of temperature on the "life" of a motor. One caveat. Although the article was written in 2003, the IEEE standards referenced, 117 & 101, were published in 1974 & 1987 respectively.

 

[electricpete]

It's a good point starkopete. There is still a lot of accelerated life thermal aging testing that goes on, and that approach is based on some assumed or measured relationship between aging at lower temperatures and higher temperatures (not consistent with a temperature threshhold type behavior that I described). Each part of the insulation system has its own behavior. I may have oversimplified or worse incorrectly characterized discussion of thermal aging of modern insulating materials as a group. My comments were repeating what I remember reading somewhere in EPRI 5036 Volume 16 - I will see if I can dig it up and refresh my memory.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[starkopete]

electripete,
I agree with you. Interestingly, IEEE 117 (Standard Test Procedure for Evaluation of Systems of Insulating Materials for Random-Wound AC Electric Machinery) has actually been withdrawn.

 

[LionelHutz]

Personally, I'd be inclined to think that as long as you don't approach the limits of the insulation rating then you won't be causing a rapid depreciation of the motor life. I really don't believe the insulation life is a straight line. I'd expect the life line is more than likely fairly flat up to the limits of the insulation then it would have a knee in the curve and quickly roll off to a very reduced life.

Class F insulation is 311F rated. You are measuring 195F at the stator RTD's. I personally would give myself a 20% safety margin, partly because the stator RTD's are likely not measuring the hottest spot in the stator. So, if the motor doesn't go over about 250F then it's life should not be reduced very much.

One thing to note. You will lose your re-start capability if you push the motor temperature to it's limits. So, plan on setting the proper re-start delay timers if you do this.

 

[gokulkrish2]

One more question guys, what happen if we load it beyond the service factor. Will the motor run? If it runs will it run with the same speed? What factors would be impacted if we run more than the SF?

 

[starkopete]

Gokulkrish,
You might find this interesting.

 

[edison123]

starkopete

how do you keep finding these good articles ?

 

[gokulkrish2]

Thank you so much. That was so helpful

 

[electricpete]

Here are a bunch of good papers from the same source:

http://www2.sea.siemens.com/Products/Electric-Motors/Motors-White-Papers.htm

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[starkopete]

Edison 123,
I’ve been involved with the repair of industrial electric machines for over 30 years, but because I lack any formal education I have become adept at finding/storing/using a myriad of resources, a person can collect a lot of information over that many years. One of the best resources that I have found is this forum. I am amazed at the level of both technical and practical knowledge that the regular posters to this site possess. I’m learning a lot from you guys, thanks.