Voltage in three phase circuit

[KamilAussie]

Hi, I have a 3 phase symetric (230/400 V) circuit, Y-Y configuration as shown in the picture.

Could anyone tell me how to calculate voltage Ur or Us?
I don´t know how to start without knowing the exact value of Z.


Thank you :angel:

 

[ggunn]

Hi, I have a 3 phase symetric (230/400 V) circuit, Y-Y configuration as shown in the picture.

Could anyone tell me how to calculate voltage Ur or Us?
I don´t know how to start without knowing the exact value of Z.

View attachment 17118
Thank you :angel:

If the center points of the Y's are connected, Us = Uos

 

[GoldDigger]

If you are trying to figure out the currents you need to know Z.
If all you are concerned about are the voltages you will find that the value of Z cancels out when solving the equations.
If the center points of the two wyes are not connected and the magnitudes of the three leg voltages are not equal the analysis gets a little more complex, but is still doable.
Apply Kirchoff's Laws.

The total current at the center point of the load wye must be zero.

mobile

 

[KamilAussie]

If the center points of the Y's are connected, Us = Uos

Unfortunately they aren´t connected. :happyno:

 

[KamilAussie]

If you are trying to figure out the currents you need to know Z.
If all you are concerned about are the voltages you will find that the value of Z cancels out when solving the equations.
If the center points of the two wyes are not connected and the magnitudes of the three leg voltages are not equal the analysis gets a little more complex, but is still doable.
Apply Kirchoff's Laws.

The total current at the center point of the load wye must be zero.

mobile

You´re the best, thanks a lot!
I solved it.
:thumbsup:

 

[GoldDigger]

You´re the best, thanks a lot!
I solved it.
:thumbsup:
View attachment 17120

Congratulations, KA!

Now that you have solved it, I will let you in on a small trick that comes with experience in problem solving:

To solve the problem with the switch closed, start with the assumption that the two wye points are in fact connected.
Then look at the resulting current in the wire that connects the two wye points.
If that current is zero, then it does not matter whether they are actually connected or not.
That in turn tells you that since the wye point of the load is at the same voltage as the wye point of the source, the load voltages are just identical to the corresponding source voltages. But that is not actually the problem that was posed.

The original problem does not ask you what the voltage across the load Z is when the switch is closed, it asks you what the voltage across the switch is when it is open. That changes the whole analysis, and makes your solution, the appropriate one.

With the switch open, U_T will equal zero since I_T equals zero.