normal current imbalance levels
normal current imbalance levels
(OP)
What levels of current imbalance are considered normal.
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normal current imbalance levels
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normal current imbalance levelsnormal current imbalance levels(OP)
What levels of current imbalance are considered normal.
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RE: normal current imbalance levels
I don’t think you’ll find hard numbers in any standard. ANSI Standard C84.1-1995 [..Voltage Ratings, 60Hz] suggests a maximum if 2½%, but that’s just a figure that electric-utility personnel and appliance/equipment manufacturers agree will give reasonably acceptable performance.
Do you have access to the IAS transactions? I know it’s been covered there; not sure how recently.
From a standards approach, it looks like IEEE is conducting major work on defining the problem. Have you ever heard of: ‘phase-angle jump?’
IEEE draft P1346 Working Group -- Electric Power System Compatibility with Electronic Process Equipment http://grouper.ieee.org/groups/sag/
http://grouper.ieee.org/groups/sag/IEEEP1564_00_12.ppt Good/Quick overview.
http://grouper.ieee.org/groups/sag/IEEEP1564_00_11.doc ‘Meaty’ review.
IEEE Std. 1159 Working Group -- Monitoring Electric Power Quality http://grouper.ieee.org/groups/1159/
‘PQ’ Magazine articles: http://industryclick.com/magazinearticle.asp?releaseid=9474&magazinearticleid=133526&siteid=30&magazineid=286
http://industryclick.com/magazinearticle.asp?releaseid=5682&magazinearticleid=67767&siteid=30&magazineid=286
Utility POV http://www.pge.com/002_biz_svc/002c1c_understanding.shtml#balance
RE: normal current imbalance levels
I should clarify that I'm not as interested in judging the long-term performance/health of the motor (it is not fully loaded), but more interested in knowing what typical maximum observed current imbalance values during measurments of a healthy motor are ASSUMING the voltage is balanced.
It's kind of a strange question I realize. I'm in the middle of troubleshooting a motor with high 7200cpm (2*line frequency vib). We've looked very closely at all the usual suspects (distorted motor feet, misalignment, base resonance etc). Checked the phase currents and saw 96A, 98A, 104A. The sister unit was running around 96A, 96A, 98A. This is second motor in the same position to have high 7200cpm (we've swapped the motor as a last-ditch effot after exhausting other possibilities).
I realize it could be possibly a problem in the contactor etc affecting both motors. I realize I could swap the leads to check. But I want to be sure that the relatively small current imbalance does truly represent an anomaly before I send out the troops troubleshooting this aspect (we have already been chasing around quite a bit on this one and don't want to get too carried away chasing other rabbit trails).
My second real question of interest in this situation is:
Has anyone ever seen current imbalance correlated with 2 times line frequency vibrations?
RE: normal current imbalance levels
Any chance there is a ‘mechanical’ database like Doble’s for electrical equipment? Vibration analysis seeems like a useful diagnostic, but electrical problems/demands/brushfires have taken most of my time. [Sorry for the rambling.]
RE: normal current imbalance levels
Once load is applied to the motor, the imbalance in current reduces to hopefully a small imbalance at rated load. In terms of problems with the motor, the imbalance at no load is not a problem, but the tolerance to current imbalance reduces as the load increases. Figures that I have seen suggest that the imbalance should be less than 2 - 3% at full load. If you have an imbalance problem, and a vibration problem, there could be problems with the laminations in the stator causing localised saturation. This can happen during rewinds. I would expect to see a higher open shaft current and possibly a higher imbalance as well. Check the open shaft powerfactor.
Mark Empson
http://www.lmphotonics.com
RE: normal current imbalance levels
I have not heard of any correlation between 2 times line freq vibrations and current imbalance, though.
RE: normal current imbalance levels
Buzz - Just assume for the moment that we knew the source voltage at the load center was perfectly balanced. Would the readings surprise you in that case?
By the way this is a Siemens 100hp 460V horizontal motor with FLA=112, SF=1.15, frame 365TS
RE: normal current imbalance levels
Your current imbalance (calculated from the average current) is about 3.4%. This would correspond to a a voltage imbalance of only 0.56% to 0.34%.
You would probably have to measure voltage directly at the motor terminals to get readings within that accuracy.
I wouldn't think the current imbalance you are seeing would be a cause for concern, in and of itself.
Could the problem be in the bearings of the driven equipment?
RE: normal current imbalance levels
RE: normal current imbalance levels
So, as a general statement, even if the motor is not fully loaded, it is possible to damage the motor depending on the degree of unbalance and also on the protection system.
RE: normal current imbalance levels
RE: normal current imbalance levels
When the dertain curve of Figure 14-1 is applied for operation on unbalanced voltages, the selection and setting of the overload device should take into account the combination of the derating factor applied to the motor and the increase in current resulting from the unalanced voltages. This is a complex problem.. [stuff ignored]..it is recommended that overlod devices be selected so as to be responsive to Imax vs Iaverage.
RE: normal current imbalance levels
Voltages tend to bounce around slightly while taking measurements. So to calculate a voltage imbalance with the kind of resolution we need to say it has a 0.3% imbalance is difficult with standard equipment. You need to sample all three simultaneously and do this for some period of time. Here is a way to determine if it is from the utility or from the service entrance down (motoror other): make note of the phases of the power coming in (A,B,C- I use ABC for nomenclature) and the current draw on each. Roll the leads one spot to the right and make note of the current draw on the same phases. Remember the phases labeled ABC will not move as they are from the service entrance. Do this three times. If the high leg of current follows the utility phases labeled ABC, then the imbalance is likely due to the utility or if it follows the same motor leg, then it is in the downstream wiring or the motor. This rule has been around longer than me. When your done with this you can calculate the current unbalance for each of the three scenarios and set it up with the lowest current imbalance. This is what I would do if I encountered something like your suggesting. Hope this helps. Buzzp
RE: normal current imbalance levels