Transformer Taps and Phases
Date: Summer 2013
I am interested in the phase differences in A-B, B-C, and A-C. What might they be?
Since A-B and B-C are part of the same winding as A-C, it should be
very clear that there can be no phase differences.
But if you consider A-B versus C-B (note the reversed order of the
connection!), there will now be a 180 degree phase difference between
C-B and either A-B or A-C. This, by the way, is the most common way
to connect a transformer with a center tapped output; that is, the
center tap (B) is grounded, and thus terminal A is 180 degrees out of
phase with respect to terminal B, and this arrangement is very
commonly used as part of so-called full-wave AC-to-DC rectifier
The center tap ("B") is used as a ground or reference, and A-B and C-B will be 180 degrees out of phase with each other.
In other words, when voltage A measured w/ respect to B (A-B) is going positive, voltage C measured w/ respect to B (C-B) will be going negative. When A-B is passing through zero volts, C-B is also passing through zero.
To discuss phase difference in A-C has little meaning. Either A or C could be treated as a ground reference, and the other end will then swing both positive and negative in sinewave fashion.
Bob's example was splitting an 800V output into two 400V "legs", where phase is not particularly important. But in the case of a rectifier circuit, the phase A and C is important.
Thank you for your question. I am an electrical engineer who specializes in power.
If B is the center tap, then the voltages measured at A and C (relative to B) will be of equal magnitude and opposite phase with respect to each other. Opposite phase is the same as saying that they are 180 degrees out of phase with each other.
You may view DC as having 0 frequency. If you have 400 volts DC on either side of center tap, then as viewed from the center tap B you will have +400 on A and -400 on C (or vice versa). Opposite polarity DC voltages are analogous to opposite phase AC voltages. Of course a transformer cannot practically provide DC voltage but this example may help to explain.
As viewed from one end then the other end has 800 Volts and the center tap has 400 Volts. If A and B look positive as viewed from C, then C and B will look negative as viewed from A.
I hope this helps.
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Update: November 2011