Quick help with transformer configuration

[fastline]

We have a very temporary need to create 240V from a 120V generator. We have a 2kva 120/240x16/32 transformer onsite. I have contacted Acme and of course they "don't advise this" but in the paste we have simply connected our 120V to only the primaries H1-H2, connect H2/H3, and 240V load on H1 and H4. My question is what is the KVA capacity with this configuration? I can sort of see where each winding is capable of 1kva so we 'might' be able to get full 2kva capacity? Or do we have the half the capacity?

 

[retirede]

My quick answer:
You are using one winding as input, the other as output. So half capacity.

 

[bwat]

I don't know that you can just turn a dual voltage transformer into an autotransformer and expect winding ratings to be kept the same, but it's very possible that you do get the full capacity. Under normal circumstances with that transformer it is 8.3A through each primary winding. When wired up for 120V, they're in parallel which gives you a total of 16.6A x 120V = 2kVA (but still only 8.3A through each winding), and when wired up for 240V they are in series and so it's 8.3A x 240V = kVA.

Drawing up a quick sketch for a transformer like yours connected as an autotransformer, you get the full 2kVA while only having 8.3A through each winding.




Again, not sure that you can do this, but it seems possible that you could get the full rating based on the sketch. But again, I don't know if you can just turn your transformer into an autotransformer and not have other things to consider.

 

[retirede]

I don't know that you can just turn a dual voltage transformer into an autotransformer and expect winding ratings to be kept the same, but it's very possible that you do get the full capacity. Under normal circumstances with that transformer it is 8.3A through each primary winding. When wired up for 120V, they're in parallel which gives you a total of 16.6A x 120V = 2kVA (but still only 8.3A through each winding), and when wired up for 240V they are in series and so it's 8.3A x 240V = kVA.

Drawing up a quick sketch for a transformer like yours connected as an autotransformer, you get the full 2kVA while only having 8.3A through each winding.

View attachment 2552154


Again, not sure that you can do this, but it seems possible that you could get the full rating based on the sketch. But again, I don't know if you can just turn your transformer into an autotransformer and not have other things to consider.

You are right - that’s what I get for formulating a “quick answer”.

The power supplied by the autotransformer is the output current multiplied by the boosted voltage or 1 kva in this case.

In a sense, my answer was correct - you only get 1/2 capacity from the transformer, but that is half of total circuit capacity!

 

[Jraef]

You are going to try using the two primary windings as an autotransformer then? Not the intended use, so it would be a 110.3 violation IMHO.

Oops, sat in the queue for a while and I didn't notice the subsequent postings. But I still think it may be a 110.3 violation. An inspector may wantot o see that this transformer is rated to be used this way.

 

[topgone]

We have a very temporary need to create 240V from a 120V generator. We have a 2kva 120/240x16/32 transformer onsite. I have contacted Acme and of course they "don't advise this" but in the paste we have simply connected our 120V to only the primaries H1-H2, connect H2/H3, and 240V load on H1 and H4. My question is what is the KVA capacity with this configuration? I can sort of see where each winding is capable of 1kva so we 'might' be able to get full 2kva capacity? Or do we have the half the capacity?

From what you have there, you can't get 240V from that transformer! You have a transformer with two primary windings at 120V rating each and two secondary windings at 16V each. Wiring the transformer to serve as an autotransformer with available 120V generator will, at least give a maximum out put of 120+ 32V = 152V! (parallel the 120V windings of the primary and connect the secondaries in series to serve as the autotransformer output leg).

 

[retirede]

From what you have there, you can't get 240V from that transformer! You have a transformer with two primary windings at 120V rating each and two secondary windings at 16V each. Wiring the transformer to serve as an autotransformer with available 120V generator will, at least give a maximum out put of 120+ 32V = 152V! (parallel the 120V windings of the primary and connect the secondaries in series to serve as the autotransformer output leg).

He wants to use the two 120V ‘primary’ windings as an autotransformer. The secondary will not be used.

 

[LarryFine]

From what you have there, you can't get 240V from that transformer! You have a transformer with two primary windings at 120V rating each and two secondary windings at 16V each.

If you energize any single winding on a multi-winding transformer at its design voltage, every other single winding will put out its design voltage (ignoring losses). So, if you energize one primary winding, the second primary winding will also have a voltage output.

So, if you wire it as shown in post #3, the primaries will function as a 1:2 auto-transformer. If you can wire the secondaries in series with the primaries to boost voltage, why couldn't you wire one primary in series with the other to also boost voltage? Answer: You can.