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

 

[Skogsgurra]

The field coils seem to have a nominal resistance = 230/22.5, which is close to 100 ohms.

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

http://www.gke.org

 

[aolalde]

Hello Petronila.
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.

 

[electricpete]

You are running at full load as far as stator current goes so you shouldn't be surprised to see somewhere close to nameplate temperature rise.

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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[petronila]

Thanks Pete,

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

 

[thewellguy]

You mentioned that the current was at 101 amps. Is that on all legs? Is the current balanced? I have seen unbalanced current cause heating problems

 

[jraef]

You still haven't defined "high" temperature as compared to the motor temperature rise rating and ambient.

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.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello skogsgurra
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://www.omega.com/ppt/pptsc.asp?ref=CNI_Series&Nav=temw01Hello

 

[Skogsgurra]

Yes, waross. My mental calculation was off about 900 percent. But the important thing is that the real resistance is about 50 percent higher than nominal and that points to a excitation problem.

Gunnar Englund

http://www.gke.org

 

[petronila]

Dear Friends,
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

 

[jraef]

Waross,
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

 

[jraef]

This is the thread I was thinking of. The comment from edison123 is probably most applicable here.
thread237-91361

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

Is this a directly excited motor with brushgear or a more modern design carrying a shaft mounted rectifier? The apparent higher resistance of the field would be accounted for by loss of part of the shaft mounted rectifier causing a effective drop in gain for the exciter.

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...

 

[electricpete]

What does NPS mean?

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[jraef]

?? Negative Pole Sequence maybe??
I'm just guessing.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

Sorry, guilty of the dreaded TLA! Yes, jraef is correct, NPS = Negative Phase Sequence.


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

 

[jraef]

I can't believe I didn't think "Phase".
<Slaps forehead>

 

[motorspert]

Petronila
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

 

[edison123]

petronila,

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 *

 

[petronila]

Edison and Jraef,

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