It's still only one phase, with the center tap providing multiple voltages.
My posts from previous threads:
"Center-tapping a secondary doesn't change anything except available voltages. It's still only single-phase. Looking at either line from the neutral point exhibits a difference of
polarity, as we debated a while back."
"We had quite the debate about this a few years ago. We concluded that it is a polarity issue that, because it involves AC,
merely resembles a 180-degree phase shift. If it involved DC, there would have been no polarity-vs-shift debate."
"It's analogous (in one frozen moment in time) to two batteries in series supplying both 1.5v and 3v loads. The DC is additive because of the polarity of each cell (in phase); if you reversed one, you'd still have two 1.5v sources, but no 3v source (out of phase).
What's the real point of the discussion is polarity, plain and simple. A secondary comprised of two 120v windings must be connected "in phase" to provide 240v, and at the same time, mimic a center-tapped 240v secondary, and provide two 120v sources."
"Let's start by using one end of the secondary as the reference (ground) instead of the center tap, and call the tap L1 and the far end as L2. Any argument as to phasing or polarity now? We're all in phase now, right? Only the magnitude of voltage is different. If this were DC, as per my battery analogy, we would be discussing polarity. To me, that it's AC changes nothing.
With two batteries in series, and measuring with a DC voltmeter, one would place the black lead on the negative terminal, regardless of whether testing either one or both batteries, right? The center-tapped-neutral argument is like keeping the black test lead on the center-point of the two batteries, and then wondering why one battery seems to have a reversed polarity."
"How much can we complicate the simple?
Egad! My opinions and observations, in no particular order:
We don't
have to use the neutral as a reference; if we did, we'd never be able to make line-to-line voltage measurements.
If using L1 as the reference, nobody would argue there is a phase or timing issue when measuring to N or L2.
What changes when using N as the reference is instantaneous polarity, nothing else. The only thing shifted is probe location.
A dual-secondary transformer with the secondaries in series is electrically identical to a center-tapped secondary.
To function correctly, two secondaries must be wired "in phase" when wired in parallel, as well as when wired in series.
When using the neutral as the reference, the polarity is inverted, and resembles, but
is not the same as, a 180-degree time shift.
If a person is standing, and does an about-face, they have made a 180-degree change, but that has nothing to do with time.
The D.C. analogy
is suitable for this discussion, because the absolute polarity does not matter as long as both batteries are reversed.
If the two halves of a center-tapped secondary were genuinely out-of-phase, the line-to-line output would be zero volts."
""
if you have a scope, observe polarity and use 2 probes, L1-n and L2-n, you WILL get 2 waveforms 180 deg out of phase; that is incontrovertible fact, no matter what spin is applied"
If, however, you probe L1-N and then N-L2, then you would consider the two to be in phase, correct? Keeping one probe on the center tap is a measurement choice, just as grounding that point is a design choice.
Technically speaking, because of the
choice to ground the center tap, and the
choice to use the center tap as your reference (grounded probe), you're inverting the probes as you apply them to the terminals.
If you were scoping a pair of batteries, would you keep the grounded probe on the center point between them, or move both probes, keeping the polarity consistent, with the grounded probe toward negative?
I will agree that, if I were scoping a dual-polarity DC power supply, I would keep the grounded probe on the grounded center-tap, but I would acknowledge that I'm seeing an expected polarity difference."
The long thread:
https://forums.mikeholt.com/threads/may-i-ask-a-question-about-the-single-vs-two-phase-stuff.144013/
