Phase Angle

[Mike01]

Sorry for the rudimentary question but I guess I never really understood this. Just curious how you determine the phase angle? Is it derived from the transformer? Does it re-set itself so to speak when you derive a new source? what would cause you phase angle to change in a system?

 

[mivey]

Sorry for the rudimentary question but I guess I never really understood this. Just curious how you determine the phase angle? Is it derived from the transformer? Does it re-set itself so to speak when you derive a new source? what would cause you phase angle to change in a system?

Not sure exactly which phase angle you mean.

If you mean the one between the voltage and current, the time of the voltage waveform peak is compared to the time of the current waveform peak and that shows how much of a shift in phase is between the two.

If you mean the 120? between the windings, it is initially set at the generator. The zero degree point is arbitrary as the voltages are relative in time. We just pick one to be the reference.

As the power is delivered out through the system, varying loads and impedances can shift the angles of the delivered voltage to something other than 120? but it is generally close enough to say that is the angle between the phases for a three phase system.

Transformers can also cause a phase shift due to the relative orientation of the windings, what voltages are used from the transformer bank, and what we use as a reference point.

 

[richxtlc]

Sorry for the rudimentary question but I guess I never really understood this. Just curious how you determine the phase angle? Is it derived from the transformer? Does it re-set itself so to speak when you derive a new source? what would cause you phase angle to change in a system?

Phase angle usually the relationship between the current and voltage in an electrical circuit (it can also be the relationship between voltages in different parts of the system. The phase angle can be derived mathmetically but taking the arccos of Watts/VA's, or for the reactive power angle, VARs/VAs. It can also be measured by using a phase-angle meter and measuring the relationship between the current and voltage of the same phase. (or the voltage and voltage on the same phase of a transformer, or the current and current of the same phase).
The system phase angle will change according to the type of load that is connected to it, capacitive or inductive or resistive.
In a transformer, the phase angle is derived from the physical relationship of the winding in the primary to those of the secondary and the type of connections each winding has.
Most Y-D transformers have a 30 degree shift between the primary and secondary. The secondary can lead or lag the primary depending on how the primary (delta) is connected. (A-B, B-C, C-A OR A-C, C-B, B-A). There are also other types of connections zig-zag to provide a ground reference on a delta system, phase-shifting transformers that can change the phase relationship by adjusting it taps.
The reason our three-phase system has each phase 120 degrees apart is that the windings on the generator are physically placed 120 degrees apart. the direction of rotations is such that A is followed by B is followed by C in a counterclockwise direction for a positive sequence and A followed by C followed by B in a counterclockwise direction for a negative sequence.