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Can power factor capacitors cause additional heating in motor? 1

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rockman7892

rockman7892

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Apr 7, 2008
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We have a 2300hp 4.16kV motor that has recently been tripping out on high stator RTD values that are set to 135deg C per the motor datasheet. We seem to see these high stator temps when the motor is only running about 280A out of 312A full load. I have been trying to determine what could be causing this additioanl heating. Motor is in 40deg ambient temp and has a rated temp rise of 85deg C. This issue has just started happening recently.

We have consulted with the motor manufacturer and examined the motor thorughly for possible reasons that motor cooling may not be working. The motor is an open drip proof motor with baffels and rotor fins to circulate cooling air. Everything in motor seems to be o.k. including all rtd's and their connections. I should also note that there is a bout a 3% current imbalance on the motor when running however this imbalance has always been there.

This motor is connected to a 450kvar (2 250kvar caps in parallel)which is connected on the line side of the CT's in the protective relay which is providing current reading and therefore CT's are seeing total current of line and capacitor contribution. We have also verified CT ratios and they are correct. The caps in the pfc are connected in a delta configuration.

I wanted to know if it was possible that there could be an issue with the pfc that could be causing additional heating in the motor. If one of the caps in the pfc were to fail what would happen? Would we see unbalanced currents at the CT's downstream of the caps or would the line current for the lines connected to the failed caps just produce this extra reactive current for these phases thus not showing any changes on the downstream CT's?

I was going to hook a fluke power meter to the secondary of the CT's as well as PT's which are just upstream of the pfc. Will I be able to tell anything from this power meter. For instance will I see differnt power factors on the lines if one of the caps is bad? I would think not becuase as I mentioned the line current will just compensate for the extra vars showing no change on the downstream caps. Will there be any kind of voltage or current phase shift that can be seen?

A couple of other thought that pop into my head:

Can there be a possible harmonics issue that can cause this heating the we will be able to see?

Could these be a resonance situation between the cap and motor causing additional circulating current that we can see?

Thanks for the help.
 
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With 415 KVAR on a 2300 HP motor, resonance can be ruled out. You would need closer to 800 KVAR for a resonance. And I don't see the capacitors heating your motor. In all the probability, the motor is not rated for 2300 HP or the cooling circuit is improperly designed or fitted.

Muthu
 
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Yes, cooling may be the more likely culprit.

450kvar (2 250kvar caps in parallel)which is connected on the line side of the CT's in the protective relay which is providing current reading and therefore CT's are seeing total current of line and capacitor contribution.
Click to expand...
.

One of the halves of the above statement is incorrect. If the caps are on line side of the CTs, the CTs will only see the motor's current, not corrected by the PFC.

If the CT is seeing total of the PFC and the motor current (i.e. PFC are on load side of the CTS), you may be overloading the motor. So confirm which is correct.

Nonetheless effect of PFC should be easy to check by disconnecting the caps. Although I do not think PFC causes motor overheating.

Current imbalance may be a factor, although you seem to be ruling it out. Cooling is the biggest culprit. Check for broken or loose fan, choked/ restricted cooling pipes etc. What is the ambient? Even the motor winding may be dirty.

Could the sensor be faulty? Is the rise consistent in all other sensor?

And do not rule out an actual problem in the motor that may be causing localized heating.

Rafiq Bulsara
 
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rbulsara

I'm not understanding your statement above regarding the located of the pfc in relation to the CT's. The pfc is located upstream of the CT's (CT's located between pfc and motor)so the CT's should be seeing the contributions from the pfc and therefore be seeing the total current seen by the motor (both contributions from caps and line). Is this correct.

To answer your other questions:

During operation we see current unbalance of about 3-4%. I dont think this will cause that much additional heating.

Ambient appears to be about 35deg that I have recoreded when the Mill is not running. I'm not sure what ambient increases to when entire mill is running.

The rise is consistent in all rtd's. I took actual resistance measurments of rtd's and compared measured values to corrosponding temperatures publised in relay manual and the values all seem to be correct.

I will continue to investigate motor itself.

Just out of curiousity what would happen if one of the caps in the pfc were to open as I mentioned abvoe? What would happen with the line currents? Would the utility source simply have to make up for the reactive current that was being supplied by the caps? Would this change only be noticed upstream of the pfc with everything downstream staying the same?

Thanks
 
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The PFC only "corrects" the current upstream of its connection. In your case the utility meter will see the reduction in the current, the motor (and its CTs) sees the original and uncorrected motor current.

Wait for other motor experts for comments on effect of current (and voltage) imbalance.

Rafiq Bulsara
 
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rbulsara

Am I correct by setting the overload relay to the FLA of the motor since the CT's are after the pfc and are seeing the total current seen by the motor?

If the pfc was located after the CT's then I assume this would be a case where you would need to reduce the FLA value in the relay?

 
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I'd have to go back to look at trends for motor currents. The pfc caps have been installed for several years now, but we have other pfc's in the plant that loose a cap from time to time so thats why I was curious what effect this would have.

I'm going to go back and trend some currents. From what I am told the motor is usually running at between the 260A - 280A range.

From what I am told this problem started showing up after shortly after we sent the motor out for routine maintnance. We have contacted motor repair shop to also help provide an explanation and look at what may have changed on the motor. We are going to run the motor at no load and have them verify adequate cooling air flow. Most likely as mentioned it is related to motor cooling.

I just want to be certain however that their isnt something on our supply side that could be causing this additional heating.
 
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I agree that you need to have a check-up of the ventilation system of this motor. Especially the motor's been overheated after an overhaul.
Take notice:
-Is there something missing? For example an internal cooling fan or any shield?
-Is there something on the wind-path being blocked? For example cooling pipes, or cooling gap between stator core stacks, being blocked by varnish after impregnation...
If you have the assembly drawing of this motor, please post for us to see.
 
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NEMA derating for 3% current unbalance: 0.91
NEMA derating for 4% current unbalance: 0.85
where unbalance is [max deviation from average] / average
Considering this and the fact that the 280A measurement does not include the full reactive component of motor current, the motor may be somewhat overloaded.

=====================================
(2B)+(2B)' ?
 
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Whoops. That was derating for voltage unbalance, not current unbalance. Sorry, it is early in the morning... need coffee!

=====================================
(2B)+(2B)' ?
 
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electricpete

With the CT's located between the pfc and motor why would the 280A measurement not include the reactive component seen by the motor? I believe the CT's would see both the real and reactive current giving the toal current seen by the motor.
 
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My mistake, you said line side of CT's, which means the CT's are seeing the motor current. Your original comment "therefore CT's are seeing total current of line and capacitor contribution." throws me off. I agree with Rafiq's comments on that terminology. But caps on line side of CTs says it all... CT's won't interfere with the measurement.

=====================================
(2B)+(2B)' ?
 
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Going back to your original question.

First I will say as others have said look for the simple explanations of overheating: loading, cooling, dirty motor, loose windings, degraded core (particularly if the motor was just rewound... there would be winding strip required... core loss should have been checked before/after). As was mentioned possible sensor problem, especially when determinated and reterminated after recent maintenance. Is the lead resistance compenstaion properly connected. Troubleshooting can involve local measurements of all available RTD's (does it match the computer... do the different RTD's differ). Sometimes motors just run hotter than factory data. You imply there has been some change but it's not clear if you have closely compared the loading before and after.

I have never heard anyone suggest power factor caps can cause heating of a motor in any way, so I am pretty sure as a practical matter it is not a concern.

However we can also do a little bit of theoretical thinking about the question.

1 - First if the caps change the terminal voltage, that can have an effect as you know. In this case it seems you're undercorrected, so really the caps might just reduce the voltage drop in the supply upstream of the caps.

2 - Second what about harmonics. Two separate cases:
2A - If the harmonics come from the voltage supply, then the existence of caps tends to filter those harmonics and prevent them from reaching the motor. Would tend to reduce harmonic current in the motor that would occur in the presence of suply harmonics.
2B - If the harmonics originate in the motor, then the existence of caps tends to create a low-impedance path for harmonic current flow outside the motor which can increase the harmonic content of current flowing in the motor. My guess is it would normally be a very small effect.

=====================================
(2B)+(2B)' ?
 
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