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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?

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I think your idea is valid but you might need balancing resistors, like when you use multiple series caps to increase the voltage rating. I'm not 100% sure though... – Vladimir Cravero Aug 27 '14 at 14:09
I am far from sure. I can imagine that working for two transformers (say across 240V) because the 'centre-connection' would be at a nominal zero volts, but 6 transformers make me feel nervous. Part of the cause of my nervousness is what is each transformer doing to the phase of the power. Would it all be in phase, and what happens if one is slightly out of phase? What happens with a reactive load? Two transformers look like a centre tap, and so seem simpler. However, I am definitely not an AC mains kind of guy. – gbulmer Aug 27 '14 at 14:22
@gbulmer I think that the phase thing would not be a problem, that's just like a big long single (mainly)inductive load. – Vladimir Cravero Aug 27 '14 at 14:31
@VladimirCravero - I am totally happy with the theory. However, I am less happy that actual manufactured products, especially ones with a high degree of mechanical manufacture, obey theory until I have parts in my hands, and I can measure them. I have never had a batch of transformers and measured them. I even imagine these things are within a usable tolerance. I am still conservative. IMHO, there are several things that might go wrong. I would want to test and measure before believing it would work. – gbulmer Aug 27 '14 at 15:01
up vote 3 down vote accepted

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.


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If any one of those transformers develops a short across its primary winding, the fault would cascade until it ate all five transformers; lose one and suddenly the remaining four would each get 150V (and get much hotter than before), so a second would fail soon afterwards. The second would fail, and now the remaining four would each get 200V. A third would fail, and the remaining two would get 300V each. Might get pretty spectacular.

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It is perfectly feasible to implement the series connection of the transformers, but you should be very careful with the phase relationship.
In the diagram below, I show how the connection of the transformers must be performed, for two transformers.


simulate this circuit – Schematic created using CircuitLab

I recommend you previously Determines homopolar terminals (marked with a dot) by using an oscilloscope, entering a known signal phase and verifying it.
You must check all transformers before connecting.

Edit: After writing this response, I consulted with the staff of transformer test of where I work. The answer was that no matter whether the phase relationship if the transformer is single phase, since in that case there is no concatenation of magnetic flux. I can not assure this assertion. Personally I connected groups of up to three transformers as described above, and I can assure it works.

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You seem very confident of what you say, but can you provide some evidence (articles), explanations or at least a "I've been doing this for decades!"? There's some 600VAC involved. – Vladimir Cravero Aug 27 '14 at 14:33
A (very very) quick look around lead me to discover that inductance and resistance of the windings are not always specified... – Vladimir Cravero Aug 27 '14 at 14:37
If he gets the phase relationship wrong, because the windings are all in series (and the transformers can be assumed to be non-magnetically connected), what is the worst that could happen? – Andy aka Aug 27 '14 at 14:43
@Andyaka, you'd lose 2x a transformer's output... one for the reversed phase, and a second one to neutralize the reversal. Your secondary would only deliver 360V instead of 600V. – TDHofstetter Aug 27 '14 at 14:59
@VladimirCravero I currently work in a factory for power transformers and rectifiers. We have sometimes made ​​such connections. – Martin Petrei Aug 27 '14 at 17:07

With identical theoretical transformers there is no problem.

With supposedly identical real transformers there could be problems. I can envisage that the primary inductance (this drives the transformer action) could be different by up to 30% compared to another and if 4 were 15% low and 1 were 15% high, this transformer would see significantly more primary voltage when wired in series.

Think of it like 4 resistors of value 85 and one resistor of value 115 - together they add up to 455 and the fraction of voltage across the 115 resistor will be 25.3% or, as a fraction of 600 VAC, will be 157 volts. Not a vast deal more on the face of it but, if that transformer starts to saturate then it's going to get warm and may in fact burn out.

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What about balancing resistors? Can that actually work? – Vladimir Cravero Aug 27 '14 at 14:38
@VladimirCravero The primary inductance might be 3 henries (because they are small transformers) and at 60 Hz this is an impedance of about 1131 ohms - think about how warm the balancing resistors are going to get. I don't think the primary inductance of a small transformer may get much above 10 henries but it's all down to what the OP is looking at - he may have something that is 30 henries but still, balancing with "R" is gonna fry-up a lot of watts LOL. – Andy aka Aug 27 '14 at 14:41
So the way to go is "use enough transformers to be safe". – Vladimir Cravero Aug 27 '14 at 14:43
@VladimirCravero - I'd match them by connecting two across 120VAC and checking the centre point is 110VAC then step and repeat to get 5 reasonably matched primaries. – Andy aka Aug 27 '14 at 14:45
I don't know why you think the characteristics would vary so much. The inductance is primarily (pun intended!) a function of the number of turns in the winding and the permeability of the core, both of which are rather tightly controlled in the manufacturing process. Also, core saturation is a function of current, and since the windings are in series, the current must be the same throughout. – Dave Tweed Aug 27 '14 at 15:00

Your solution will work (is valid), except for the safety aspect of it. To take care of this, you must add, in series with the input windings, a 15 ma 800V AC fuse. This will prevent a possible "runaway" condition which could destroy the transformers and possibly cause a fire or an explosion!

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