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Principles of Symmetrical Components Part 1b.
In this part we'll talk about the balanced set of three phases voltages and currents, as well as phase rotation or phase sequence.
Now these two concepts are very, very simple and we won't spend a lot of time here but if you feel like this is way too fast, then there's another video tutorial that's presented by General Pac and we go through these concepts in great detail in that particular video tutorial.
The link can be found below.
Okay, now suppose that we have these three phasors, right? Let's call this phasor IA, this phasor IC and this phasor IB. Now by inspection I can tell you that this is a balanced set of three phase currents.
Now how do we know that this is a balanced set of three phase currents? We would have to verify three rules. Now rule number one has to do with the angle. Rule number two corresponds to the magnitude and rule number three is the sequence.
So with respect to the angle, we would have to have equal displacement between all three phasors and what that means is that the angle between IA and IB has to be 120 degrees.
Okay? The angle between IA and IC, that also has to be 120 degrees and the angle between IB and IC, well that also has to be 120 degrees. If the angle displacement between all three phasors is 120 degrees, we have completed rule number one.
Now rule number two has to do with equal magnitude between all three phasors. So if the magnitude of IA is equal to magnitude of IB, which is equal to the magnitude of IC, then we have completed rule number two.
Now we know that the magnitude of IA, IB and IC is equal to each other because the distance between this origin point here and the end of the tip here for IA is equal to that origin and the tip of IC, which is equal to the origin and the top of IB.
So the length of phasor A is equal to the length of phasor B, which is equal to the length of phasor C. So if that is true, then we've complete rule number two.
Now the sequence has to do with the phase sequence of these three phasors. So let's clean this up a little bit.
The sequence is a little bit more trickier, now in all of the engineering standards, in a lot of the papers out there we always assume that the phasors are rotating in the counter-clockwise direction.
Okay? Now if all three phasors are rotating in a counter-clockwise direction, then the question is what is the sequence of these three phasors?
Now by inspection I can tell you that it's A-C-B. Now how do I know that? Now suppose that you place a stationary mark here, if you place a stationary mark here,
if all three phasors were rotating in the counter-clockwise direction then we know that phase A is going to hit the stationary mark first.
Now as they rotate, we'll see that phase C is going to hit the stationary mark second and then phase B is going to hit it last. So by inspection we know that the phase rotation, or phase sequence is an A-C-B phase sequence.
Now before we conclude here I just wanted to say one more thing about this phase sequence. Now suppose we had an A-B-C phase sequence, now how would these phasors change to make this an A-B-C phase sequence, right?
So now we're not looking at A-C-B anymore we want A-B-C but we also have to note that the phasors would still have to rotate in the counter-clockwise direction.
So as the phasors rotate in the counter-clockwise direction we want phase A to hit this stationary mark first and then phase B to hit the stationary mark second and then lastly phase C,
which means IA's going to be here and as IA rotates in a counter-clockwise direction, in this direction here, then we know it's going to hit the stationary mark first.
Now we want IB to hit the stationary mark second, now the second phasor is also going to be rotating in the counter-clockwise direction,
now notice that the second phasor was IC, so all we would have to do, is we would have to move IB here and IC there, so all we're doing is we're swapping IB and IC.
Now as IB rotates in a counter-clockwise direction, it's going to hit the stationary mark second and then as IC rotates in a counter-clockwise direction.
It's going to hit the stationary mark third. So this balance set of three phase current has a rotation of A-B-C.
And that concludes this video tutorial. Now if you want more details just click on the video tutorial link that's found on the bottom of this particular video, that video goes into much great detail on these two concepts.
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