Unbalanced current 2

[controltech]

Killing time on the internet I found this site. I have a 150 hp vertical 480 volt 3 phase motor. It is connected to a irrigation well. I have had @ 4 failures at this site. I just installed the motor after being rewound. The motor megged good and all of the lubrication points were checked. I checked the unloaded voltage and it was witin range. Started the motor and checked the loaded voltage on the load side of the contactor. The voltage did sustain a drop but it was witin range. The problem I have been experiencing is unbalanced current. I have even tried to move the phases around to determine if it is a line issue. The full load current for this motor is 173 amps. Phases A & B run at 165 but C phase is running at 197. I checked my PM records from the previous year and C phase was running at 185 amps. Would anyone know what else I could check. I believe the incoming line is a grounded or maybe a ungrounded delta.

 

[jbartos]

Suggestion: Systematic troubleshooting of the motor upstream power distribution is needed. There may be various causes, e.g. two line conductors may have smaller gage, causing voltage drops, lower currents, and unbalanced current. Usually, a licensed electrical contractor corrects this type of irregularities.

 

[busbar]

If it’s a single-motor/single-transformer arrangement, “rolling” might help. See:

http://www.pge.com/002_biz_svc/pdf/054619g.pdf

§10. Service to Three-Phase Pumps

Also, search electrical forums at this site for phase “unbalance” or “imbalance”.

Now, about the failures, is the utility-provided electric service ungrounded delta, as opposed to wye- or corner-grounded?

 

[DanDel]

controltech, what are the running voltages?

 

[SteveKW]

Here is another good link.

http://www.franklinelect.com/Manual/AIM_29.htm

 

[peebee]

Re: "Phases A & B run at 165 but C phase is running at 197."

Please verify you mean phase C on the panel, not on the motor.

 

[d23]

controltech:

Being an irrigation system I would guess this is a remote location. Next time you go to the location watch the power distribution line. I had one years ago where the power company had about 15 miles of three phase line where all the houses in a rural area came off the same phase.

When you checked the voltage it should have indicated if one corner of the delta was grounded. If one phase to ground reading was 0 volts it is grounded somewhere. I would check it with the pump running and no-load. This could locate a down hole short in the cable or motor.

Is the delta connection a three transformer connection or did they just use two transformers in an open delta connection? An open delta connection will give you an unbalanced current. The only way to fix an open delta is use a three transformer system or oversize the motor.

Good Luck!

 

[shree1]

hi controltech;
Is it a star/delta motor ?
Are you sure the motor is rewound correctly?because if the start and finish ends of the windings are interchanged then you might face such problem.
Please check with the rewinding agency if the motor is rewound properly and ask specifically if he has checked for the above point.

 

[stardelta]

If a reverse phase existed in a motor of this size you would know about it, it would not even begin to turn and the current demand be be off the scale!!! Contact the rewind company and get a copy of the final figures, this will indicate the no-load currents and give you something to compare the load currents to. Its not uncommon for current demand to change slightly between phases following a rewind, particularly on larger 2 pole machines as different size coils may be have been used in each phase of the winding. However on this occasion the differnce seems a bit extreme. If possible check the phase resistances with an accurate ductor (they will be very low). If they differ by more than 2.5% take it up with the rewind company.

 

[controltech]

Thanks for all the feedback guys. I put in a call to the service provider and they are coming out and put a Rustrac Recorder on the service. Here are the numbers taken from the load side of the contactor except the unloaded numbers.
Unloaded Voltage
L1-L2 482
L1-L3 483
L2-L3 487
Loaded Voltage
L1-L2 456
L1-L3 456
L2-L3 470
Current
L1 162
L2 164
L3 195
The motor has a nameplate rating at 173 FLA and a service factor at 1.15. Standard stuff with this type of motor.
The service is a grounded delta.

 

[busbar]

Again, if this is a single transformer/single motor installation, suggest the triple phase-rolling procedure linked here while carefully logging currents with a known-accurate meter and CT.

The important issue is—do the currents follow the line connections [L1, L2, L3] or the load connections [T1, T2, T3]? Also, do any of the reconnections produce better-balanced loaded ø-ø voltages?

 

[DanDel]

controltech, you have approximately 1% voltage inbalance at no load, and a 3% inbalance at full load. Typically, the current inbalance will be up to ten times the voltage inbalance for a 3-phase motor, or 30%. Your current imbalance is 20%.

Your problem is due to the voltage inbalance in the system, which must be corrected. How far away from the utility connection is the motor? Possibly, you have a bad connection or another load causing a voltage drop. The differences between your no-load and full-load voltages lead me to believe that the source is far away or very weak.

What do you mean by 'grounded delta'? Is one phase grounded, or is the center tap of one phase grounded?

 

[GGOSS]

Well done DanDel

For every 1% line voltage error you can expect anywhere between 6 and 10% line current error. This information is readilly available from most motor manufacturers.

Phase imbalance is said to contribute to 27% of all motor failures. Let's not overcomplicate the issue!

Regards,
GGOSS

 

[buzzp]

That is a significant drop in voltage between loaded and unloaded indicating to me that the equipment sizes feeding your motor are border line as far as capacity. Any fluctuations in down stream power demands will affect this voltage significantly. I bet you have a utility side problem causing your failures. Just because you measure one time and everything appears ok does not mean it remains that way when you leave. I belive the recorder being put on will reveal the real problem, incoming power problems. I suggest you use some protection for unbalanced currents as well as your overload protection. This will kill the motor when the current unbalance (imbalance, whatever you prefer to call it) falls out of a range you dial in (usually 5%). Good luck

 

[jraef]

In line with the previous 3 posts and D23's post regarding the remote location issue I think we have hit the nail squarely on the head. I too have experienced wide swings in voltage balance thoughout the day in rural / agricultural areas because the utilities tend to get lax about voltage balancing. It is not necessarrily their fault either. Many farms only get 1 phase power delivery because of the cost of transmission and the relatively low usage. This means that when they do turn on their occasional loads, it has a severe effect on voltage balance in the entire grid. Couple that with marginal equipment sizing as buzzp suggested and I think you have it. Please post the results of the recorder proceedure, and tell us what the utility says they plan to do about it! Quando Omni Flunkus Moritati

 

[nrupalakolkar]

your problem is that u have not consulted the proper person during recommissioning of your motor. i find it a problem of thermal overload. as such also the current in c phase is 196 that is 110% of your motor full load current approx. so the problem is overloading of your motor, that means that the motor is of under capacity or your have to buyback it in lieu of a higher kw rating motor.
also the unbalance in current may be due to improper supply voltage i.e. it is having unbalanced input. check out the problem if so. if not then mail me for further details of analysis.

 

[d23]

All:

One more question:

Assuming this is a remote location and there is a weak power supply would a set of PF correcting caps help stabilize the voltage? I know this is not the conventional reason to install capacitors, but could they help this type problem?

 

[controltech]

Well guys, as we all suspected it was definitely the power providers problem. They changed one of the transformers on the bank and everything looks good. There is minimal voltage drop and 1 amp difference in the current on the phases. I really appreciate all the input, I hope I can help someone else in the future. Once again, thanks guys.

 

[electrosys]

we are talking of current deviations between phases, which can seriously damage the motor if the deviation is beyond the limit.
A. To know the exact origin of the problem is first reverse the two leads of the motor from the power supply. If the phase C current reading did not change(consider phase C is registering with higher drawn current)then you can readily pinpoint that problem is originating from the following:
1. Magnetic starter by measuring the voltage drop across load side of phase B and C. note the voltage deviations between phases; or
2. At the feeder itself as there maybe loose connection causing the voltage drop. Note the voltage deviations between phases; or
3. At the main power supply or transformer.

Or otherwise after reversing the leads and if the drawn current reading shifted from phase C to either phase A or B then the problem is in the windings of the motor.

goodluck!

 

[buzzp]

Its nice to know the origin of the problem.
One quick note on last post, If you only swap two leads the motor will turn in the opposite direction. I belive electrosys realizes this, just was not clear.