NCEES Prob # 111

[ros]

very simple problem.. .but its some how bugging me.... Vab = IaA*Z + VAB simple ....

but in the solution provided in the book they have converted delta connected load Voltage VAB to Van ???? why is this necessary.... can someone please explain.

thank you

 

[Flyer_PE]

Not quite so simple. It would work that way if it were a single phase problem. Since it's 3-phase, the currents IaA and IbB are not equal and opposite, they are out of phase by 120o. The way the math works, you either have to reduce the current or the voltage by the square root of three to arrive at the right answer. The easy way to keep the math straight (at least for me) is to just reduce the problem to an equivalent single phase circuit by converting the voltage into a phase value.

There's a discussion of this problem here that may also help.

 

[ros]

Not quite so simple. It would work that way if it were a single phase problem. Since it's 3-phase, the currents IaA and IbB are not equal and opposite, they are out of phase by 120o. The way the math works, you either have to reduce the current or the voltage by the square root of three to arrive at the right answer. The easy way to keep the math straight (at least for me) is to just reduce the problem to an equivalent single phase circuit by converting the voltage into a phase value.
There's a discussion of this problem here that may also help.

Thanks a lot.... Makes sense.... But just to make this problem spicy!!

If in delta load impedence valie 'Z' was given along with line current then? How do we find Vab?

 

[Flyer_PE]

Not quite so simple. It would work that way if it were a single phase problem. Since it's 3-phase, the currents IaA and IbB are not equal and opposite, they are out of phase by 120o. The way the math works, you either have to reduce the current or the voltage by the square root of three to arrive at the right answer. The easy way to keep the math straight (at least for me) is to just reduce the problem to an equivalent single phase circuit by converting the voltage into a phase value.
There's a discussion of this problem here that may also help.

Thanks a lot.... Makes sense.... But just to make this problem spicy!!

If in delta load impedence valie 'Z' was given along with line current then? How do we find Vab?

If you are given the line current and line impedance, the voltage drop calculation is the same regardless of any other information given including the configuration of the load.