Isolation transformer

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gar

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210626-0908 EDT

If I have a transformer labeled X1 and X2 as the primary, and X3 and X4 as the secondary, and apply a dot to X1, and a dot to X3, then what is ---
(1) the voltage phase relationship of X3 - X4 to X1 - X2 ?
(2) the voltage phase relationship of X4 - X3 to X1 - X2 ?

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(1) zero
(2) 180

When you have dot on X1 and X3, the current entering X1 leaves X3 in phase. So if this transformer would have a turns ratio of 1:1, then it would be like the transformer isn’t even there. Measuring X3 -X4 and X1-X2 would be just like measuring L-N at two different points along an unbroken cable. So there would be no phase shift.
 
Another way of looking at it is that current entering at the dot terminal will cause the same direction (but not necessarily amount) of flux in the core regardless of which winding it flows on.
 
210626-1737 EDT

Assuming a 1 to 1 ratio. If I apply power to X2-X1 and connect X4 of the un-directly-powered X4-X3 winding to X2, then what is the voltage between X1 and X3? And why?

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gar said:
210626-1737 EDT

Assuming a 1 to 1 ratio. If I apply power to X2-X1 and connect X4 of the un-directly-powered X4-X3 winding to X2, then what is the voltage between X1 and X3? And why?

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Click to expand...

Zero.

No phase shift between X1-X2 when compared to X3-X4. (Answer to your original question)

And now X2 and X4 are tied together so both circuits share a common connection. For voltage measuring purposes, you could substitute X2 and X4. So X1 to X2 is the same as X1 to X4.

And since it is 1:1 turns ratio, there’s no increase or decrease in potential from primary to secondary, X1 to X2 is exactly the same as X3 to X4, therefor X1 to X3 is zero.

Summarize:
- no phase shift
- share common reference
- magnitude of X1 is same as magnitude of X3 due to 1:1
 
210627-1615 EDT

Next change the connections to:

(1) Assuming a 1 to 1 ratio. If I apply power to X1-X2 and connect X3 of the un-directly-powered X3-X4 winding to X2, then what is the voltage between X1 and X4? And why?

(2) Now use X2X3 as the reference point, then what is the phase relationship of X1-X2 to X4 - X3? How many phases are there?

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(1) Voltage is 2x what X1-X2 is. Pri and sec are equal in magnitude, share a common reference but the reference is low wrt the other winding while it is high wrt to the winding on the other circuit... similar to 180 phase shift, and magnitude is same since 1:1.

(2a) Phase relationship of X1-X2 to X4 - X3 is equal in magnitude but opposite polarity.
(2b) I'm not sure that "How many phases are there?" is a valid question. As best I can answer it, I would say there are (2) circuits with (1) phase in each.


I'm not quite sure why I'm still answering these questions as it has become obvious that they are not legitimate questions you are wondering about, but rather you trying to demonstrate something. Curiosity for your end-game, I suppose.
 
not legitimate questions you are wondering about, but rather you trying to demonstrate something

None of the above, Gar's post is an educational quiz<G>

How about an extra credit quiz: I have about 120 each 3 phase delta wye isolation transformers, each transformer is fed to a different delta tap. If I add diodes to the secondary, now many pole rectifier is the possible and what is the ripple frequency? How many degrees separation are the taps?
FWIW, typical setup for first stage rectification for DC supply to megawatt powered phased array radar.
 
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