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Second this week... But was it easy I wouldn't be here...

A problem asked me to identify the diagrams from this picture:

Diagrams

The following are the correct answers, acording to the test developers.

a. The first diagram may represent the voltage over the load of a 4-wire Unbalanced Star.

b. The first diagram may represent the voltage over the load of a 3-wire Balanced Star.

c. The second diagram may represent the voltage over the load of a 3-wire Balanced Star.

d. The second diagram may represent the voltage over the load of an Unbalanced Delta

Again: all the above assertions are to be taken as true


I need help understanding these truths/assertions.


  • As far as I know, the line voltage is 30 degrees ahead of the phase voltage on a star connection (a neutral wire enforces this in case one of the impedances is not equal)

This knowledge matches the a. and b., but doesn't seem to my eyes to make sense for c.. How is the assertion c. possible ?

  • As far as I know, the line and phase voltages on deltas are equal, with No angle between them. But that wouldn't match assertion d. How is such assertion possible ?

Maybe the trick is in the distinction between load and source...

Who has the patience for this ? If I think any longer I lose my sanity

This is an adaptation of the actual question. The original is longer and not in english. Here, question number 29: https://arquivo.pciconcursos.com.br/provas/21572117/1fc8bd07ea0/tecnico_eletrotecnica.pdf

*I believe the D letter in the diagram is a typo

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    \$\begingroup\$ show source in references \$\endgroup\$ – Tony Stewart EE75 Dec 11 '19 at 19:44
  • \$\begingroup\$ If the source of the question is what you mean, I've added now the link at the bottom. \$\endgroup\$ – Lucas BS Dec 14 '19 at 22:10
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    \$\begingroup\$ According to that ...d. (X) Only statements 2 and 4 are correct. or your b),d) \$\endgroup\$ – Tony Stewart EE75 Dec 14 '19 at 22:16
  • \$\begingroup\$ Not the case. It's hard to see without a translator, but each of the statements (number 2 and 4) from the PDF have two assertions within (total of 4 assertions then). They are badly mixed, and separated only by a comma. In my translation, I've separated in a. b. c. d. (I also changed the order, or else it would be a mess if I simply "carried" the sentences here) \$\endgroup\$ – Lucas BS Dec 15 '19 at 2:19
  • \$\begingroup\$ It's quite a hard question. Maybe because of how it was written, maybe because of my adaptation, which was made with the best of my ability. It's a question taken from a difficult test, meant to be difficult. I called a bounty, but don't burn your brains like I did. \$\endgroup\$ – Lucas BS Dec 15 '19 at 2:26
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This is just a partial answer that may help.

The diagram seems to me to be very badly drawn or the system is unbalanced.

enter image description here

Figure 1. VAB, VBC and VCA redrawn in conventional fashion for a balanced system.

If the system is balanced then UA, UB and UC would be equal length and UAB, etc., would be √3 times the length of UA. As drawn they don't translate properly. Again on a balanced system UC would be at right angles to UAB.

Good luck.

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  • \$\begingroup\$ Indeed, there should be 90 degrees there... Scratching the head now... If we don't solve the problem of the second picture, I'll start believing this is in fact our caveat... Wonder how you saw it... Good luck for me then... Will need. \$\endgroup\$ – Lucas BS Dec 15 '19 at 2:01

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