Motor question 480/3/60hz

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I have two new submersible pump installs along with a control panel. The power system is a 480/277 wye. The voltage is balanced within 1% or so across the three legs and from leg to ground. When retrieving a amp draw on the pumps, the middle leg amps about 10 amps higher on both pumps than on the other two legs. I am getting this amp info from the bottom of the motor starter. Anybody give me some direction on why this is?
 
Could be I guess. The pumps are currently running and have been since May of this year. When I see a higher amprage on one leg, I am curious as to why. I have seen this before on start ups I have done.
 
Could be I guess. The pumps are currently running and have been since May of this year. When I see a higher amprage on one leg, I am curious as to why. I have seen this before on start ups I have done.

The long and skinny stator and rotor makes it very difficult to create three identical magnetic fields so this type windings are often unbalanced. (Ran across this when I was designing immersing concrete vibrators where the entire motor with the eccentric weights on the shaft ends were in a 70mm head.)

BTW it is more meaningful to talk about the % of difference. You could adjust the individual overloads accordingly if you want to be picky.:smile:
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Usually when a find a big percentage difference, I'll shift all the T leads one phase over on the starter to see if it stays on the same phase or follows the same motor lead.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Usually when a find a big percentage difference, I'll shift all the T leads one phase over on the starter to see if it stays on the same phase or follows the same motor lead.
I was gonna suggest the same thing.


Honest! ;)
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Usually when a find a big percentage difference, I'll shift all the T leads one phase over on the starter to see if it stays on the same phase or follows the same motor lead.
Yes, this is called "rolling" the conductors and is usually recommended by submersible pump manufacturers before trying anything else. As posted earlier, it is very difficult for them to make those long skinny motor windings perfectly balanced (electrically), so "rolling" the conductors at installation takes advantage of finding other minor impedance and/or voltage mismatches in the circuit and using them to your advantage to help balance the currents.

And by the way, I also agree that it's better to speak of percentages. 10A imbalance on a 250A motor is a lot different than 10A imbalance on a 20A motor!
 
Thanks Guys...good info. The motor FLA is 42.2 amps. I was getting about 52 on the middle leg and 41 to 42 on the other two.
 
Yes, this is called "rolling" the conductors and is usually recommended by submersible pump manufacturers before trying anything else. As posted earlier, it is very difficult for them to make those long skinny motor windings perfectly balanced (electrically), so "rolling" the conductors at installation takes advantage of finding other minor impedance and/or voltage mismatches in the circuit and using them to your advantage to help balance the currents.

And by the way, I also agree that it's better to speak of percentages. 10A imbalance on a 250A motor is a lot different than 10A imbalance on a 20A motor!

The 1% imbalance threw the rolling out for me. Unless of course it is a 250A load, in which case:

  • the difference is negligible
  • I really wanna see a well pump that draws 250A( never see one that big before...):smile:
 

Besoeker

Senior Member
Location
UK
The 1% imbalance threw the rolling out for me. Unless of course it is a 250A load, in which case:

  • the difference is negligible
  • I really wanna see a well pump that draws 250A( never see one that big before...):smile:
The last submersible we did was 225kW. In round figures, 400A at 400V 3-phase.
 
If I do the "rolling" that you guys suggest, and the higher amps follow over to the third leg, could I say that the problem is in the motor? If it stays high on the second leg, would that mean the power company may have issues?
 
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