OVERHEATING IN A SYNCH MOTOR
OVERHEATING IN A SYNCH MOTOR
(OP)
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
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





RE: OVERHEATING IN A SYNCH MOTOR
The running data you mention indicate a much higher resistance; 245/16.6 which is close to 150 ohms.
Could it be that parts of the excitation winding is not carrying any current? Seems that one third or one fourth is missing.
Gunnar Englund
www.gke.org
RE: OVERHEATING IN A SYNCH MOTOR
The first cause of overheating is high current, check the nameplate Power factor (PF) and the operational. The currents you are reporting are not out of the “name plate figures” double check that those are real and accurate.
When the excitation is too high, the rotor current increases, the motor returns reactive power to the line, the motor power factor is lower than unit, the core losses increase too due to and increase of the magnetic field strength.
When the excitation is low, the field strength is reduced, the PF drops and the motor demands reactive power from the line. The stator current increases as much as the PF drops.
Another possible source of heating is Line Overvoltage, the currents drop due to the higher voltage but that increases the magnetic flux densities increasing the core losses and the rotor temperature.
Reverse rotation of Fans and clogging of filters and/or air vents could be another cause of overheating, now due to poor cooling.
Damaged bearings, loose salient poles or misalignment could cause the rotor dragging against the stator and generating heat by friction, this is a very destructive condition for the machine.
RE: OVERHEATING IN A SYNCH MOTOR
Some questions to think about:
1 - Is the indicated temperature higher than expected for the load?
Related questions: What is motor nameplate rise and ambient temperature. Is there an overexcited condition? Is this a change from historical behavior?
2 - Are you confident in the temperature measurement?
Related questions:
I assume you are using RTD measurements. Are the readings similar among RTD's or vary among RTD's.
What type of RTD and How/where did you measure?
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RE: OVERHEATING IN A SYNCH MOTOR
The motor is overexcited, the costumer have to take out the shield covers and add external fans. Historically is a high temperature, the temperature measure is registred by a termocouple installed in the stator´s frame.
petronila
RE: OVERHEATING IN A SYNCH MOTOR
RE: OVERHEATING IN A SYNCH MOTOR
And is it a thermocouple, RTD or a PTC resistor? There is a difference, and I have not heard of using thermocouples in large motors like that because of their susceptability to induced voltages causing false readings. The sensor equipment needed to use a thrmocouple is more expensive than a PTC or RTD system.
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RE: OVERHEATING IN A SYNCH MOTOR
I think you may have missed a decimal, but that doesn't change the fact that the field resistance appears to have increased almost 45%.
It could be one or more of several parallel coils open in the field.
The field power disipation is down about 20%. I agree the field doesn't look right. 720 RPM, that's 10 poles isn't it?
Probably some bad field coils out of the circuit. With one or more poles missing excitation, it would almost certainly cause overheating.
petronila
Can you give us the rated temperature rise and the indicated temperature please?
jraef Take a look at these devices.
http://ww
RE: OVERHEATING IN A SYNCH MOTOR
Gunnar Englund
www.gke.org
RE: OVERHEATING IN A SYNCH MOTOR
The complete name plate is : Westinghouse Synchronous Motor,Type and Frame: GF/A 4A45, 400 HP, 720 RPM, 2300 V-100.3 Amp-Exc: 230 V/22.5 Amps, SF : 1.15, Temp RISE: 60 degrees C, 60 Hz,%load: 100-24 Hr, Temp Mess Method: THERMO.
Thanks
Petronila
RE: OVERHEATING IN A SYNCH MOTOR
Sorry I didn't make this clear the first time. I know how a TC relay works, the problem I was mentioning is in using them on AC motors, because a TC relay works on a variable voltage level and embedding the TC in an inductive field like that is just asking for trouble if not properly filtered. My point was that the reason why TCs are not typically found in AC motors is that process of making a TC work reliably in an AC motor is more difficult than just using a resistive device such as a PTC or RTD. So that's why I would suspect that either he has something other than a TC in his motor (hopefully), or someone was trying to use one and THAT might be the cause of his reading "excess temperature" (which is still as of yet undefined).
All that said, I would tend to agree that if this is a true temperature problem and not a measurement or perceptual error, the direction you and Gunnar are heading him in is a likely cause. I believe we had a thread on a similar subject a few years back, and that is what turned out to be the cause.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: OVERHEATING IN A SYNCH MOTOR
thread237-91361
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: OVERHEATING IN A SYNCH MOTOR
Brushgear problems could account for it too, but the increase in resistance seems a bit too great for that to be a likely cause (and the power dissipated in the brushgear would cause it to glow).
Are you able to determine if the source of the additional heat is from the rotor or the stator? Rotor temperature measurement is tricky, but abnormal shaft temperature could be one way to see it. Are your voltages balanced at the machine terminals? NPS currents cause the amazing levels of rotor heating from seemingly insignificant voltage differences.
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I don't suffer from insanity. I enjoy it...
RE: OVERHEATING IN A SYNCH MOTOR
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RE: OVERHEATING IN A SYNCH MOTOR
I'm just guessing.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: OVERHEATING IN A SYNCH MOTOR
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I don't suffer from insanity. I enjoy it...
RE: OVERHEATING IN A SYNCH MOTOR
<Slaps forehead>
RE: OVERHEATING IN A SYNCH MOTOR
You say the motor is overexcited - yet the field current is around 75% of the design value. The stator current is close to the nameplate, which suggests the motor is underexcited (lagging power factor). From the nameplate data it looks like the rated power factor is 0.8. is the excitatiuon controller manual or automatic? As Scotty asked, is the machine brushless? What is the kilowatt load or the power factor now?
Sorry, have lots of questions but the machine is notacting normally if it is overheating in the conditions you desribe. As other contributors have said, could be missing poles, could be instrumentation error, or unbalance. But without some more information it is difficult to do be sure
Regards
RE: OVERHEATING IN A SYNCH MOTOR
As jraef asked, how high is your "high" temperature ?
You also said the temperature is measured from TC from the stator frame. Are there no winding RTD's ? At 2300 V, the stator coils must be form wound and should have embedded RTD's.
We are missing some vital info here.
* The shin is the device to find your furnitures in the dark *
RE: OVERHEATING IN A SYNCH MOTOR
This is an old Synch Motor and don´t have windings RTD´s only a Thermocouple J Type installed direct on the stator frame, this device is shooting alarm for high temperature and is setted at 70 Degrees C.
With the end shield(end belts) covers out and with an external fan blowing air to the stator windings, the winding temperature is 65 Degrees C.
The Rise is 60 Degrees C.
Any comments are wellcome.
Thanks
Petronila
RE: OVERHEATING IN A SYNCH MOTOR
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...
RE: OVERHEATING IN A SYNCH MOTOR
* The shin is the device to find your furnitures in the dark *
RE: OVERHEATING IN A SYNCH MOTOR
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
RE: OVERHEATING IN A SYNCH MOTOR
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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