Wye/Delta current 1

[SACNJ]

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!

 

[rbulsara]

I would not categorize this as imbalance. Looks perfectly normal readings.

If anythig I would worry about overloading the motor a little above its rated current.

 

[busbar]

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.

 

[Marke]

Hello SACNJ

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

 

[jbartos]

Suggestion: If the motor is somewhat old, interturn shorts have probably materialized.
Alternately, the measurement of currents might have been inaccurate for some reason or other
or
some load is powered from those terminals

 

[busbar]

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 }

 

[electricpete]

Is T1, T2, T3 reading the currents in 3 legs of the delta?
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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jbartos]

Comment: I concur with Busbar posting that the measured currents at T terminals are consistently off, namely, the consitent current reading at a good quality delta/star motor should follow: Iline=sqrt3 x Idelta,leg

 

[SACNJ]

Thank you all for your responses.
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?

 

[jbartos]

Comment on the previous posting: The cause may eventually become known once the motor is disassembled in the motor repair shop and tested.

 

[tmahan]

I'm just guessing, but it looks to me like the coils that make up the T3-T6 loop were put into parallel rather than series. That would explain why the currents on that winding loop are so much higher. The various phase currents are simply taking the path of least reistance and that happen to be T3-T6. (This is very simplistic, but then I'm a mechanical engineer by eductaion)

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.

 

[aolalde]

The reported currents in the motor phases match.

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?

 

[jbartos]

Suggestion to SACNJ (Electrical) May 9, 2004 marked ///\\
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.

 

[aolalde]

Check the shaft speed (rpm) and operating temperature of the motor . Are them into nameplate specification?

 

[jbartos]

Comment: I concur with the previous posting regarding the motor nameplate parameters testing.