As my system produces 120VAC but I occasionally need 240VAC for a well pump, I explored the world of transformers.
I came across a 5000VA ST-5000 step-up/step-down transformer for cheap. After I opened the unit and looked inside, I was horrified. This unit has switches and breakers and even an on light but all those can be off and power can still be present on some of the sockets. THIS DESIGN IS EXTREMELY dangerous!
I filed with the Consumer Product Safety Commission but they were unconcerned.
I would never use one of these units as delivered. However, I did strip out the giant transformer to determine if it could be made safe to use and reasonably US NEC compliant.
I have no idea how the transformer is officially wired but by poking and prodding with a DVM and an oscilloscope I did come up with the following:
BELOW ARE BETTER AND SAFER ALTERNATIVES!
THE BAD 5000VA 3000VA step-up / step-down 110/120 <-> 220/240 transformer
This chart/wiring diagram is as you are looking from the end with no wires and down from the top. The wires are facing away from you.
L2 N L1
220 110 +H
X X X X X
1 2 3 4 5
____/-------\____
| |
| |
| top |
| |
| |
----\_______/----
(not to scale)
Transform 110 -> 220
110 in -> (5) L1 + H 110
110 N -> (4) N 110
110 N <- (3)
220 L1 <- (5) L1 + H 110
220 L2 <- (2) L2
(1) float (warning powered)
Transform 220 -> 110
220 L1 in -> (5) L1
220 L2 in -> (1) L2
110 H <- (5) L1
110 N <- (3) 110 N
(2) tied to (1) L2
(4) float (warning powered)
3000VA toroidal step up transformer
2000VA toroidal step up transformer
Both of these units came with woefully undersized wires and will overheat in the included enclosure. They may overheat outside of the provided enclosures. I do not know. I installed fans and thermal overload disconnects on mine.
However, if you provide adequate cooling and heavier duty wire they will perform reasonably well. Toroidal transformers are quite a bit more efficient and smaller than the equivalent E style transformer.
I am running the 2000VA toroidal at 1032watts continuous and 1920watts surge and it is working fine with a cooling fan and 10 gauge wiring.
I am running the 3000VA toroidal at 1344watts continuous and 2688watts surge and it is working fine with a cooling fan and 10 gauge wiring.
There is an EDISON 3-wire transformer somewhere that steps the really dangerous voltage to a lower mostly dangerous 220/240VAC.
Additionally, the transformer has a center tap which relative to each side has 110/120 Volts on it. This allows you to connect both 220/240 devices and 110/120 devices.
The 220/240 part I call L1 and L2. The center tap I call N.
Between L1 and L2 you will find 220/240VAC.
Between L1 and N (or L2 and N) you will find 110/120VAC.
The toroidal transformers I linked to above ARE NOT EDISON STYLE transformers!
You **will** be able to convert 110/120 -> 220/240, however, you **cannot** get 110/120 out of the transformer by referencing one side to ground or neutral! DO NOT DO THIS! YOU WILL DIE!
YOU CAN ONLY DO THE SIMULTANEOUS 110/120 220/240 TRICK ON THE OUTPUT OF EDISON 3-wire TRANSFORMERS!
THE TRANSFORMERS MENTIONED THUS FAR ARE **NOT** EDISON 3-wire DESIGNS!
A couple of companies sell what I believe are 110/120 -> 220/240 Edison 3-wire step-up transformers but I have not been able to confirm this. They are very expensive ($400+) for what should otherwise be a much less expensive transformer.
"autotransformers" as I call them are **not** isolation transformers. I believe "THE BAD" transformer discussed above is an "autotransformer" style.
However on a theoretical basis all simple (not Edison 3-wire) isolation transformers E or toroidal may work in either direction.
So yes there are more options, but you **must** know what you are doing: DO NOT EXPERIMENT!