Unbalanced current
Unbalanced current
(OP)
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.





RE: Unbalanced current
RE: Unbalanced current
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?
RE: Unbalanced current
RE: Unbalanced current
http://www.franklinelect.com/Manual/AIM_29.htm
RE: Unbalanced current
Please verify you mean phase C on the panel, not on the motor.
RE: Unbalanced current
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!
RE: Unbalanced current
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.
RE: Unbalanced current
RE: Unbalanced current
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.
RE: Unbalanced current
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?
RE: Unbalanced current
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?
RE: Unbalanced current
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
RE: Unbalanced current
RE: Unbalanced current
Quando Omni Flunkus Moritati
RE: Unbalanced current
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.
RE: Unbalanced current
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?
RE: Unbalanced current
RE: Unbalanced current
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!
RE: Unbalanced current
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.