Motor running above max temp for decades?
Motor running above max temp for decades?
(OP)
Now this:
A compressor motor was installed in 1972. A 1600 kW 3 kV asynchronous motor. It is fed from its own transformer, 20/3 kV 2 MVA and 7 percent uk.
The motors have always been equipped with Pt100 temperature transducers but these have been connected to a not-so-reliable temperature indicator and no-one really cared what they said. The temperatures seemed to be in the 90 - 95 centigrades region most of the time.
Now, the indicator system has been replaced with a new one and everything has been checked out. The temperature is now rising to 140 C within one hour and still increasing. The isolation class is B (yes, old machine) so the people closed the machine down and yelled for help.
Checked cooling, heat exchanger, motor voltage, symmetry, distortion, slip, current consumption - I think everything. All OK.
The conclusion is that the motor probably has been running quite hot before the change of temperature indicators - but no-one knew. My question: Is it at all possible to run a class B motor for years at temperatures close to or slightly above 150 C?
(Yes, I know about the halfing for each 8 - 10 C increase in temperature. It is not that part of the problem I am trying to grasp. The question is more like this: are there any "magic limits" that makes a vintage class B insulation break down very quickly or do we still follow the "2exp(-T/10)" law?
In other words: Is it probable that the motor has been running 20 - 30 C above rated temperature for many years?
A compressor motor was installed in 1972. A 1600 kW 3 kV asynchronous motor. It is fed from its own transformer, 20/3 kV 2 MVA and 7 percent uk.
The motors have always been equipped with Pt100 temperature transducers but these have been connected to a not-so-reliable temperature indicator and no-one really cared what they said. The temperatures seemed to be in the 90 - 95 centigrades region most of the time.
Now, the indicator system has been replaced with a new one and everything has been checked out. The temperature is now rising to 140 C within one hour and still increasing. The isolation class is B (yes, old machine) so the people closed the machine down and yelled for help.
Checked cooling, heat exchanger, motor voltage, symmetry, distortion, slip, current consumption - I think everything. All OK.
The conclusion is that the motor probably has been running quite hot before the change of temperature indicators - but no-one knew. My question: Is it at all possible to run a class B motor for years at temperatures close to or slightly above 150 C?
(Yes, I know about the halfing for each 8 - 10 C increase in temperature. It is not that part of the problem I am trying to grasp. The question is more like this: are there any "magic limits" that makes a vintage class B insulation break down very quickly or do we still follow the "2exp(-T/10)" law?
In other words: Is it probable that the motor has been running 20 - 30 C above rated temperature for many years?





RE: Motor running above max temp for decades?
Early 1970's vintage would possibly be asphaltic based insulation. If heated I believe it will become puffy, so inspection may tell.
Temperature rating for class B hotspot (which should be RTD) is in fact 130C (140C for 1.15 SF).
So when you were at 140C that was the SF temperature. How much higher we don't know. Clearly continuous loading at this temperature is beyond the design or normal practice. But sometimes equipment does tolerate abuse pretty well.
Bottom line after all that rambling - who knows! It's certainly possible.
One question - did they double check the rtd measurment setup - proper for 3-wire RTD if applicable. If you don't compensate for lead length temperature can read much hotter than actual.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor running above max temp for decades?
Yes, we also measured the Pt100s directly at the junction box at the motor and got similar results.
Appreciate your views on this. I tried to argue along your lines - but needed more support. A few more opinions and I will go to the next meeting better armed.
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
I haven't been able to find the tempco for Ni100. Will have a look in CRC handbook.
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
However, Pete makes a good point about the ashphalt insulation. That stuff tends to soften up significantly when heated. Seems at that temp it would be oozing out the casing.
RE: Motor running above max temp for decades?
Great tip by Laplacian!! . I think he deserves at least a star.
RE: Motor running above max temp for decades?
"Venditori de oleum-vipera non vigere excordis populi"
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
If you are doubtful about the accuracy of the temperature monitoring, is it practical to do a resistance shut-down test?
Also 1972 doesn't sound that old to me, I don't think materials and practices have changed much in that time.
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
Vanstoja, your data support the thought that the motors have been running quite hot for many years and that the problem was detected only when new temperature indicators were installed. The local repair shop has cleaned and "painted" the motors several times during the last twenty years. "Painting" means dipping and hardening. It is possible that important cooling channels have been narrowed over the years.
No answers from the manufacturer so far.
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
Result from winding resistance test: The windings were not room temperature but lower. The motors have forced ventilation and air/water heat exchanger. The cooling was active also at standstill. That's why we measured 108 ohms at room temperature. It should have been around 112 ohms if the windings were at room temperature.
All in all: You never know what your next mistake will be. Thanks to all - and especially Laplacian.
RE: Motor running above max temp for decades?
Thanks for sharing your findings.
RE: Motor running above max temp for decades?
RE: Motor running above max temp for decades?
A while back we added a Multilin 469 to a 2500HP motor starter and decided to use the RTD's to monitor temperature. I found some poor quality original documentation on the motor and saw that the RTD's were 100 ohm. I "assumed" platinum since I couldn't make out the type from the documentation.
About a week into service, operators were screaming that the machine was running hot and about to trip up operations. We monitored that air temp coming out of the enclosure while motor was running and it did not correlate with reported RTD temperature.
I went back into the Multilin and was able to easily change the RTD type to 100 ohm nickel. That was the first time I had seen 100 ohm nickel RTD's used in a motor.
I'm glad I was able to share my learning on this forum.
RE: Motor running above max temp for decades?