seriestransformer
I need a 1:1 transformer, 25VA 600VAC. I'm not having much luck finding such a thing. It occurs to me that I could take five smaller 1:1 120VAC transformers (like this), series all the primary windings and all the secondary windings, and get the same effect.
It seems to me that this is little different than taking a single multi-winding transformer and connecting its windings in different arrangements. Is this valid? Recommended? Are there problems?
I've never seen a seven-winding transformer but I have dealt with three-winding transformers.
The nameplate information for one particular three-winding transformer is as per below. (Graphic redrawn from photo to avoid showing the full extent of the very large nameplate.)
The impedances given are all on 60 MVA base. In this case tap no. 5 is the principal tapping, so consider the impedances as HV-LV 19.32%, HV-TV 33.08%, LV-TV 9.93%.
The impedance quoted for each pair of windings is with a voltage applied to the first winding, the second winding shorted, and all other windings open circuit. Refer IEC 60076.1:2005 Power Transformers - Part 1: General:
Therefore, assuming --
-- your fault current depends only on the impedance from the HV winding to the particular winding under fault.
In this example, a three-phase fault on the 33 kV winding would produce a maximum three-phase fault current of 60 MVA ÷ 19.32% ÷ 33000 V ÷ √3 = 5.43 kA. A three phase fault on the 11kV winding would produce 60 MVA ÷ 33.08% ÷ 11000 V ÷ √3 = 9.51 kA.
The trifilar transformer you have drawn will couple all three of its windings together. In the ideal transformer case, that means the voltage on each set of windings will be identical.
It will therefore effectively parallel all the signals together. The fact that you have drawn load impedances of Z on the end of each winding means that the transformer will not actually short your two noise sources together, which without the Zs could have made them malfunction. The average noise source voltage, ie half the sum, will be added to your signal.
Nevertheless, effectively connecting your two noise sources in parallel like this is a rather 'dirty' way to do it.
You can design a three input combiner, that isolates each signal from the other. Alternatively cascade a pair of two-input combiners. The design equations for these things are readily available, or you can buy the finished part, see MiniCircuits for instance.
If the sources are low impedance, then it would be better to cascade a pair of simple transformers, adding one noise source in each. The transformers will put the noise voltage in series with the signal voltage, so simply adding them. As the two noise series are then in series, they won't load each other.
Best Answer
I think it will work fine if they're connected properly (if they share voltage unevenly the one that gets more voltage will begin to saturate and the voltage across it will be limited).
However the transformers may not be sufficiently rated to be safe on 600VAC, so isolation and creepage distances may not be good enough to meet requirements. At least they're split-bobbin type which is inherently pretty good for isolation.
Since 600VAC is a common industrial voltage in Canada, there are a lot of control transformers available that will step down from 600VAC to 120VAC or 240VAC. I'd suggest using two of those transformers (600:120-> 120:600 or 600:240 -> 240:600) to get what you want.
For example, a Hammond B356633 is a 50VA 600VAC:120VAC transformer.