OVERHEATING IN A SYNCH MOTOR 1

[petronila]

Good Day,

What could cause overheating in a Synch Motor?? The name plate is 400 HP, 720 RPM, 100.3 Amps, Exc: 230 V-22.5 Amps.

The motor is running but is overheated, the run parameters
are: 101.6 amps,exc: 245 V-16.6 Amps. No Insulation Problems, No Misaligment, no high level vibrations.

Any Help be appreciated.

Petronila

 

[ScottyUK]

winding temperature is 65 Degrees C

What is the basis for thinking there is overheating? Winding temp of 65[°]C (or a rise of 60[°]C) is remarkably low for a motor of that size running at full load.

What cooling method does the motor use? Is the motor's own fan and air circuit in good shape? What method has been used to verify this?


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I don't suffer from insanity. I enjoy it...

 

[edison123]

I agree. 65 deg C ain't high temperature at all.


* The shin is the device to find your furnitures in the dark *

 

[waross]

We may be misunderstanding you, petronila.
Is the 65 deg. temperature the actual temperature, or the temperature rise above ambient?
Is the temperature shown by the tc (70 deg.) the original temperature or the temperature with the external fans running?
As I understand the data, the tc is indicating a frame temperature of 70 deg. actual and the windings have been measured at 65 deg actual.
Replace the thermo couple controller or relay.
Has the setting on the thermo couple controller inadvertently been changed from 60 deg. rise to 60 deg. actual?
It's possible that someone with more dedication than information saw a setting of 90 deg or 95 deg on the thermocouple controller. This would be a normal setting for over temperature. 30 deg ambient plus 60 deg rise. If the controller was mastakenly reset to 60 deg or 65 deg it would be in alarm under normal conditions.
If we are misunderstanding the information, then I think you have a serious problem with the fields.
With the actual current so close to the rated current, you would not normally have overheating.
However, a serious field unbalance problem can cause unequal division of voltage in the coil groups which can result in unequal division of current in parallel paths.
Example.
Current 100 amps.
Assumed coil resistance .1 ohm.
Two coils in parallel, 50 amps each.
Watts loss in each coil = 250 watts per winding. Total loss 500 Watts.
Now suppose that due to field problems the instantaneous current divides 25A-75A.
The loss in the first coil is 62.5 Watts.
The loss in the second coil is 562.5 Watts.
Total loss 625 watts.

Bottom line. Either you don't actually have an overheating problem or you may have a serious field problem.

A motor with a rated temp rise of 60 deg. only has to be 10 deg overheated to be able to percolate coffee.
I'm not sure about tea. In a pinch can you brew an acceptable cup of tea with 90 deg water?
yours

 

[electricpete]

I don't work much with sync machines. I can't comment on the rotor or possible field problem.

Focusing on the stator and trying to figure out what's going on:

The nameplate says the temperature rise is 60C. That seems quite unusual.

As you know the rated temperature rise (winding avarage temp above ambient) by insulation class is A - 60C, B - 80C, F - 105C, H - 135C

Class A hasn't been used for motors since the 1960's I think. What is the vintage?

Maybe the motor was specified with inherent margin by using class B insulation specified at a class A temperature rise. That has not been common practice any time in the recent past (often specify class F insulation with a class B rise). But this might be a reasonable explanation again if the motor is very very old.

Maybe the temperature rise is intended to refer to the rise at the point of measurement on the frame which might be 20C below average winding temperature? This would be an unusual practice to my knowledge (of course I have never heard of permanent temperature monitoring on the frame either). Is there any documentation of the expected difference

The fact that someone established an alarm at 70C gives you some idea that someone considered 70C at the measurement point to be excessive but there are still a lot of questions to understand where it came from.

One possible way to follow up on the difference between your installed temperature indicator and your actual average winding temp is to get some resistance measurements immediately after shutdown, and then later at ambient temperature. The simpler procedure assumes the measurements immediately after shutdown are the same as during operation, and just use thermal coeffcient of resistance for copper (~0.4%/C) to figure out how much higher the operating temperature must have been than the later ambient temperature to cause that difference in resistance measurement. The more complex procedure to account for the fact that you can't get in immediately and there is some cooling, would be to plot resistance vs time after shutdown. Use a log/log scale and extrapolate back as a straight line to the moment of shutdown. I can provide more details if you're interested but I suspect you are already familiar with these methods.

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