Connecting Torodial transformer in Buck-Boost mode

[Jester]

I'm on the prowl for a 240 to 220V transformer with about a 1kVA rating.

I found this rather large 1kVA transformer in my transformer box, and I think I can use it in a buck-boost configuration, using one of the 21V secondary windings.

The winding sketch, is my interpretation based on the label.

Initially I tried with the secondary winding reversed (Blue/Black) and the voltage at Red-Yellow increased. So I assume the spots on the secondary side are as shown. so with it wired as shown I'm getting very close to 220V at Red-Yellow, with 240V in.

Based on the 1kVA rating, am I correct to assume that I should be able to run it at at least 1kVA and probably more? 

As I understand most of the load current is flowing in the Blue-Black winding and the secondary windings are #12, while primary are #16

[johansen]

yeah you can run all 4 windings in parallel and pull 50 amps out of that transformer at 220v.

the transformer itself is only handling 50 amps at 20 volts, or 1 kva

[Ian.M]

Not quite - it also has those 14V secondaries, so the full VA cant be drawn from only the 21V secondaries.  We don't know if they are all the same wire gauge and thus current rating, but if so the available VA is proportional to the secondary voltage (i.e. all rated for the same current), which would put the permissible full load current at 220V when wired as an autotransformer at 31.3A not 50A.

[Jester]

I did a load test to get a sense for temperature rise in the transformer, the only thing getting warm is the room from the 2200W of heat from the heaters, transformer has no perceivable temperature rise after 20 minutes. I only used one Blue-Black winding for this test.

Thanks for the comments.

[johansen]

Not quite - it also has those 14V secondaries, so the full VA cant be drawn from only the 21V secondaries.  We don't know if they are all the same wire gauge and thus current rating, but if so the available VA is proportional to the secondary voltage (i.e. all rated for the same current), which would put the permissible full load current at 220V when wired as an autotransformer at 31.3A not 50A.

lets examine the information provided by the OP:
"As I understand most of the load current is flowing in the Blue-Black winding and the secondary windings are #12"

#12 wire.

4 in parallel.

you really think they can't handle 50 amps?

[Jester]

Not quite - it also has those 14V secondaries, so the full VA cant be drawn from only the 21V secondaries.  We don't know if they are all the same wire gauge and thus current rating, but if so the available VA is proportional to the secondary voltage (i.e. all rated for the same current), which would put the permissible full load current at 220V when wired as an autotransformer at 31.3A not 50A.

lets examine the information provided by the OP:
"As I understand most of the load current is flowing in the Blue-Black winding and the secondary windings are #12"

#12 wire.

4 in parallel.

you really think they can't handle 50 amps?

IF I had a big enough breaker at the source, I have no doubt this would source 50A. BTW some of the other secondary windings (orange/Grey/brown) actually have smaller gauge wire. So the Blue-Black  windings appear to be responsible for most of the 1kVA rating.  These Buck mode configurations are interesting in how little heat actually gets dissipated by the transformer. The normal I-R losses associated with the primary winding become negligible in this mode and that certainly helps. The transformer I pictured above was a bit large to fit in the designated control cabinet so I kept digging in my transformer box, and found another smaller 600VA with a single 22V secondary, instead of the many taps on the previous transformer. I ran the "little guy" at 2200W for about an hour and for the first 20 minutes it was cold to the touch because it was stored in a cool location. After an hour under 10A load feels to be about 30 degC so I'm going with this one, save the big daddy for something else.

[Ian.M]

I'm not surprised as wired in an auto-transformer buck configuration, your 600VA should be good for fractionally under 30A load current continuous 24/7.