Delta to wye conversion of sources

Discussion in 'Homework Help' started by elecidiot, Sep 11, 2013.

1. elecidiot Thread Starter New Member

Jun 29, 2010
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Hi all, we know the conversion of delta connected sources into wye connected sources ie we have to reduce the delta voltage by a magnitude of √3 and shift the angle by -30°.
So far so good.
This works only when there is no internal impedance of the voltage sources. But if the each of the delta source has an internal impedance, then how to convert this system into a wye connected system of sources with internal impedance s ?? So confusing!! Any help on this topic will be much appreciated.

2. WBahn Moderator

Mar 31, 2012
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6,941
You are trying to apply a memorized approach that only applies to a specific special case to a more general case for which is does not apply (or at least not very well).

Take a step back and go through the general transformation of how to convert any thing things connected in delta to an equivalent set of three things connected in wye.

Show your work and then we can discuss.

Sep 15, 2013
6
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hmm.. the first thing that comes into my mind is that, well, a delta connected source WITH internal resistance or impedance can be simplified to wye connections by simply treating the original connection as two separate, one delta connected sources only and one delta connected impedances.. correct me if I am wrong, but this is just more like an intuition.. gonna solve or verify this later...

4. WBahn Moderator

Mar 31, 2012
23,089
6,941
I'm correcting you because you are wrong.

The internal impedances are in series with each generator. You are trying to treat them as if they are in parallel.

Sep 15, 2013
6
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yeah right... Well, the phase voltage (Vab) is not just equal to the source voltage in that "leg" but rather the voltage of the source plus the voltage of the impedance (iab * Z).... is that it?.. I am a little confused on this one.... Sorry, this is also a new topic to me (just introduced days ago about balanced three phase)...

6. WBahn Moderator

Mar 31, 2012
23,089
6,941
Don't let the fancy words trip you up. You've been working with AC circuits for some time and have developed some level of comfort with analyzing them, right? Even if there was more than a single source in the circuit, right? Well, a three phase system is nothing more than an AC circuit that just happens to have three AC sources in it. When all else fails, treat a balanced three phase system for what it is -- an AC circuit that happens to have multiple sources.

But speaking of terminology, the "phase voltage" is the voltage from one of the lines to the neutral (or what would be the neutral if it were wye-connected). Normally you talk about Va, Vb, and Vc as being the phase voltages and Vab, Vbc, and Vca as being the line voltages. The term "line voltage" implies "line-to-line voltage".

Similarly, the "phase current" refers to the total current in a line, while the "line current" refers to the current in the mesh formed by two lines.