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Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I just picked up a 3 phase 15A variable transformer. It is essentially 3 single phase 15A transformers. I was wondering if I rewired them in parallel to be a single phase 45A variable transformer, how close does each voltage have to be to the other. They need to track together, but if they go out a bit, how critical is it? If one was at 150V, and the others at say 151V and 148V, will I have a lot of losses? This is what the guy I bought them from lead me to believe.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You're supposed to use three paralleling chokes. This page tells all: (you want to wire it up ala "Connection 3, page 30" schematic on that page)
Note there's a mistake: he says ferrite core, but it should be an ordinary laminated iron transformer core.
If you can get three identical 6.3V or 5V 15A filament transformers, they would probably do just as well as the proper chokes. You'd put the three variac outputs through the low voltage windings, and connect the high voltage windings in a circle as shown in the Connection 3 schematic. (We use the low voltage windings because the transformers only need to supply the differences in voltage between the three variacs.)
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Many thanks Steve.
Edit. Wondering what else I could use? Perhaps some MOT cores? or is that overkill? Would be a waste of a good filament transformer or three, especially since they seem quite rare around here. He says to use a small variable transformer and take out the core, but that would also be a waste. The picture shows what seems to be a small choke, I estimate to be about 50mm diameter. Could that be right?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Shouldn't be too hard to find three small transformers that'll suit. For instance a 100VA toroidal transformer with 240V primary and two 6V secondaries. Or I guess you could get three transformer cores and make your own. The core should be about 100VA sized, I guess.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You're supposed to use three paralleling chokes. This page tells all: (you want to wire it up ala "Connection 3, page 30" schematic on that page)
That's a nice connection, I wondered whether a current balancing transformer could be extended to N>2 inputs.
The three secondaries in series force the same current to flow through each secondary, and therefore the same in each primary.
The three secondaries in series also keep the sum of the secondary voltages equal to zero, which means the primary sum will also be zero. This allows a voltage to be developed between each variac output and the combining point, with the condition that all three voltages add up (vectorially) to zero, so forcing the combining point to be at the average of the three variac output voltages.
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3PH variable transformer
