Unbalanced 3-phase load calculations

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anomaly

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I have a photocopy of a procedure to calculate line currents for unbalanced three-phase loads and it incorporates some steps and multiplication factors that are not explained. AND....there is no information on the photocopy to enable me to trace the procedure back to the source.

Does anyone have a suggestion where this procedure can be found in a textbook or other document??

Thanks for the help!!
 
I strongly suspect that the technique you are looking at is the one called "Symmetrical Components." The basic idea is that if you have a set of three voltages (and three currents) that are not balanced, you can model the system with a set of nine voltages (and nine currents). The nine voltages would be,
? A balanced set of Phases A, B, and C voltages, with a magnitude that you have to calculate, rotating in that order (i.e., A, then B, then C). These are called the "Positive Sequence Voltages."
? A balanced set of Phases A, B, and C voltages, with a magnitude that you have to calculate, rotating in the opposite order (i.e., A, then C, then B). These are called the "Negative Sequence Voltages."
? A balanced set of Phases A, B, and C voltages, with a magnitude that you have to calculate, and that do not rotate (i.e., the three are in phase with each other). These are called the "Zero Sequence Voltages."

The technique requires the use of "Matrix Algebra," in order to calculate the values mentioned above. It also makes use of "sequence networks," a collection of boxes that you fit together in order to match the characteristics of the original system.

This is no easy technique to learn. It takes up about a third of a semester-long, graduate-level course on electrical power systems engineering. The text book I used for my first course on the topic is "Elements of Power System Analysis," by William D. Stevenson.

I regret that this will not answser all your questions. But have I at least given you a start?
 
Do a search for "unbalanced loads" and look at the post by JLANN 6/7/06
 
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I've done them using the component data sheets, which list peak loads, and are therefore wrong by definition.

What are your loads?

I always try to balance things that will be on at the same time, like heaters and AC. Then I try to spread out the rest by duty cycle and expected usage times. Then a power logger tells you how good a job you did - which is usually a "Gentlemen's C" (ask a politician who graduated from the Ivy League). Then re-balance.

Heaters and AC draw heavy as they ramp, then idle at low draw. Motors tend to be the same, and some motors (conveyers) run all the time, while others are sporadic (Elevators).

Three-Phase components are nice - already balanced. It's the 2-phase loads, single phase loads, and sporadic uses that give you issues.

Matt
 
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