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
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?
peter-- I don’t recall the correlation being discussed in IAS transactions in the last several years, though there may have been a paper in one of the IAS conference records. Gut level, PCIC seems to be most likely at having their ear to the ground on this type of problem, but I'm not close to the engineering libraries these days. {Aging} C37 and C57 ‘phone books’ are my primary source of information lately.
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.]
As no supply is perfectly balanced (magnitude and angle), and no motor is perfect, it is common to find that there is a significant imbalance in the magnetising current on an open shaft motor. We find that rotating the phases will find a combination where the imbalance is minimised. Part of the problem is the grain orientation of the steel in the laminations when the motor is fabricated. If the die is rotated relative to the coil of steel as the laminations are pressed, then you will get a much better balance within the motor, but this is not normal.
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
I don't know if those readings are 'normal', but I would try to get them balanced better. I would not be shocked at all if someone gave me those numbers on a tech call or something. My recommendation for them would be to roll the leads (shift all to right, take readings, make note of high leg,etc). As Mark pointed out, one of the three configurations will give you a better imbalance than the other two. If the high leg of current stays with the same utility leg, then the problem could likely be traced back to the utility and not your motor. I am sure you have heard of this before electricpete. I would try this before your hounds going sniffing down the rabbits trail. I certainly would not be alarmed at seeing a 4.7% CUB. Myself, I would roll the leads and set wiring according to lowest CUB. If your not running into SFA then you should be fine, although you may be using more power than you need too.
I have not heard of any correlation between 2 times line freq vibrations and current imbalance, though.
Whoops, the readings were actually 96A, 98A, 102A (not 96A, 98A, 104A).
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
One thing that always surprises me is how little voltage imbalance it would take to cause this level of current imbalance. NEMA says that current imbalance will be 6 to 10 times the voltage imbalance.
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?
Let’s say the motor had phase currents: 90A, 93A, 104A. Hypothetical (or not) question-- The motor nameplate lists Full-Load Current at 106 amperes. Does the stator risk damage from overheating, as in the case where currents were; e.g., 118/118/118? If phase currents do not exceed nameplate, but are unbalanced, will windings be damaged over the long haul?
Motor heating (stator and rotor) is caused by the I2T heating of both positive and negative sequence currents. This heating is compared with the manufacturer's thermal damage curves.
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.
busbar - I agree with coolfish. NEMA MG1 makes it clear that you take into account BOTH #1 - the maximum current and #2 - the derating factor. So the maximum of the three phase currents should fall below the derated current limit.
busbar - After thinking about it, maybe I have not portrayed NEMA 100% accurately. Let me list a portion of the NEMA standard so you can judge for yourself.
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.
The readings would still not surprise me too much. I would make attempts to get them better by rolling the leads as suggested. The negative phase sequence currents will heat up the motor as well as coolfish pointed out. So just looking at the high current in one leg is not suggested. This is why several manufacturers embed RTD's within the motors windings. They have something called thermal capacity which is what really is the damaging affect of an overload condition or an imbalance. Most electronic overloads have a thermal type of model built in to estimate the remaining capacity. When the capacity is gone the motor is tripped. If it is started in x minutes then the time to trip will be shorter because the thermal capacity of the motor is reduced due to the previous trip.
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
These methods/tricks must have probably been around at least 75 years—back when stator coils had double-silk-coated magnet wire. They are mentioned in a lot of utility-service handbooks. Discussed at thread237-15890 also.
