Max % of allowable currrent unbalance in lighting transformer

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Carultch

Senior Member
Location
Massachusetts
GoldDigger said:
The current circulating among the delta coils does NOT appear on any of the lines going into the transformer. That is what circulating means.
I do not think that Phil meant what you though he meant. He assumed unity PF.

If they do not add up to zero, it is a measurement error.

Don't forget that the phase angle for each winding current is independent since you have different line to neutral loads, so you cannot just use 0, 120, 240 for the vector angles.

Measure the currents with a scope and add the vectors. You should get zero.
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I did a Pspice simulation to get a better understanding. Sure enough, the delta side current amplitudes are what they are calculated to be, and the unbalanced currents do add up to zero on the DETLA primary. I fed it with three 277VAC sources wired in a WYE manner. One issue is that you need to add a tiny milli/micro-ohm resistor between each pair of primary coils, because the program sees it as a short circuit if there is no resistance in any given loop.

I tried adjusting the load resistors for a more extreme imbalance example, and it is still the case that currents add up to zero.

What ends up happening, is that even though there is a pure resistive load, there ends up being a different phase shift for each line, to compensate for the imbalance.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Carultch said:
What ends up happening, is that even though there is a pure resistive load, there ends up being a different phase shift for each line, to compensate for the imbalance.
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The reason is simple: IF sequence is ABC, AND secondary voltage angles are 0, -120, +120, AND sec'y load PF is unity, THEN sec'y load-current angles are ALSO, 0, -120, +120!

To all, if you would like a copy of my approach contact me! However, there will be cost, i.e., you must give up your anonymity and reveal your true name and location!

Phil Corso
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
Phil Corso said:
No! The phase-shift is constant! What's different is that the load-angle changes when PF is other than unity!

Phil
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The phase shift resulting from the delta-wye transformer configuration is constant (±30°), but Carultch is correct regarding their being an additional shift in primary line currents resulting from the imbalance of load current magnitudes.
 
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