Wye/Delta current
Wye/Delta current
(OP)
We have a 350HP motor operating on 480 volts. FLA = 380. Starter is a Wye/Delta.
Question has do with amp readings:
L1=386, L2=382, L3=388
T1=199, T2=194, T3=286, T4=201, T5=193, T6=287
What would cause the unbalance in the load side current readings?
The motor was just recently rewound.
Thanks in advance for your help!
Question has do with amp readings:
L1=386, L2=382, L3=388
T1=199, T2=194, T3=286, T4=201, T5=193, T6=287
What would cause the unbalance in the load side current readings?
The motor was just recently rewound.
Thanks in advance for your help!





RE: Wye/Delta current
If anythig I would worry about overloading the motor a little above its rated current.
RE: Wye/Delta current
At less than 1% current imbalance, it's almost perfect.
A rule of thumb for induction motors is each 1% voltage imbalance produces roughly 6% current imbalance.
RE: Wye/Delta current
Something does not look right to me.
The pahse currents that you quote look fine, but the winding currents are a long way out. I would expect the phase current to be unbalanced with the winding current that you quote.
I would be concerned about the imbalance in the winding currents, but not the phase currents.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Wye/Delta current
Alternately, the measurement of currents might have been inaccurate for some reason or other
or
some load is powered from those terminals
RE: Wye/Delta current
Marke, thanks for noting the correction. I got in a hurry looking at the posted numbers. One would expect to see winding currents all close to 220 amperes { 380A/√3 }
RE: Wye/Delta current
Are T4, T5, T6 the same currents as T1, T2, T3 respectively (measured at other end of the same legs)?
If both answers yes, I have a hard time understanding that there can be an unbalance in the leg currents but not in the line currents.
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RE: Wye/Delta current
RE: Wye/Delta current
I agree that something is wrong with the winding currents. This motor was just rewound. We just reinstalled it and took the readings that you see.
The winding currents are within an amp or two of each other in the respective winding: T1=199 and T4=201, T2=194 and T5=193, T3=286 and T6=287. But compared to the phase currents, T1,T4 and T2,T5 should be higher and T3,T6 should be lower. What do you think the problem is?
RE: Wye/Delta current
RE: Wye/Delta current
I think that you should be able to test this hypothosis by simply measuring resistance across the three loops (T1-T4, T2-T5, T3-T6). I wouldn't take any action on this though until someone with a lot more motor experience comments on this suggestion.
RE: Wye/Delta current
It1 = -It4 +/- 200
It2= -It5 +/- 193.5
It3= -It6 +/-286.5
But the vectorial add of phase currents do not match the line currents.
It1+It6 = IL1 ( I calculated 423 amps) you reported 386 amps.
It2+It4 = IL2 ( I calculated 342 amps) you reported 382 amps.
It3+It5 = IL3 ( I calculated 417 amps) you reported 388 amps.
Please verify the current measurements and report the line voltages. It seems like a wrong winding, but it does not match with the line currents almost balanced.
Did you measure the motor phase currents at the motor terminal box and line currents at the incoming power to the starter?
RE: Wye/Delta current
I agree that something is wrong with the winding currents. This motor was just rewound.
///The motor may have improperly been rewound. If this is the case, it will have to be rewound again.\\\
We just reinstalled it and took the readings that you see.
RE: Wye/Delta current
RE: Wye/Delta current