I gotta question for those who know AC circuits. I'm stuck on one of my question from a problem set our instructor gave us. Its friday so I won't be able to ask him until monday so I thought I'd ask you guys. Here's the question.

A 600 volt, three-phase system has phase sequence, A-B-C.

(a) What is the peak line-to-line voltage?
answer: 848.5 volts

(b) At a particular instant in time, Vab = 750 volts and is increasing. Determine the corresponding instananeous values of Vbc and Vca.
answer: Vab = 750.0 volts at 62.11 degrees
Vbc = -715.7 volts at -57.89 degrees
Vca = -31.3 volts at -177.89 degrees

I don't have a problem with part (a), its part (b) I can't figure out how to do. I'm not sure if he covered it yet in the lectures cause there's nothing in my notes.

Any help would appreciated
Thanks

Originally Posted by Kurupt

I gotta question for those who know AC circuits. I'm stuck on one of my question from a problem set our instructor gave us. Its friday so I won't be able to ask him until monday so I thought I'd ask you guys. Here's the question.

A 600 volt, three-phase system has phase sequence, A-B-C.

(a) What is the peak line-to-line voltage?
answer: 848.5 volts

(b) At a particular instant in time, Vab = 750 volts and is increasing. Determine the corresponding instananeous values of Vbc and Vca.
answer: Vab = 750.0 volts at 62.11 degrees
Vbc = -715.7 volts at -57.89 degrees
Vca = -31.3 volts at -177.89 degrees

I don't have a problem with part (a), its part (b) I can't figure out how to do. I'm not sure if he covered it yet in the lectures cause there's nothing in my notes.

Any help would appreciated
Thanks

Using result from part A, you understand Vab sine-wave with 848 magnitude. Draw this and the other 3 phase waves on a graph.
Now, part B says at a particular instant Vab = 750. Look at your graph and find when is Vab = 750 ? (it is still climbing towards 848) solve for the angle that it is at 750V. theta = arcsine (750/848) = 62.11 that part is just trigonometry. Now the other two phases are 120 & 240 degrees apart from this. Do a similar calculation but solve for their magnitudes at those angles (-57.89 and -177.89) thats it...

It will all become more clear to you if you draw the three waves on a graph with magnitudes and 120 degree relationships then just do trig to fill in the remaining unknowns..