Tranformer 277V / 220


Thread Starter


Hi All,

Here in our plant all the lighting transformers are 480V primary (Delta) and 277V secondary (Star-Y). I want to convert this transformer to give 220/230/240 secondary voltage without rewinding it. Transformer primary input tapping are 500-492-480-469-456-445-435. If I connect 480VAC to 500volt tapping what would be my secondary voltage? If the secondary voltage is within limit, is it safe to use like this. What are the effect on total KVA and is there any loss expected?

You are misinformed as to the nature of your transformer. The secondary is 480Y277, so the Line to Line voltage is still 480V, only the Line to Neutral voltage is 277V. The only way to get 240V Line to Neutral is to feed the transformer with 415V, which would require another transformer anyway. If you want 240V single phase, just buy a 480-240V single phase transformer. If you want 240V 3 phase, buy a 3 phase transformer. But understand that the 240V secondary will be Delta, and there are ramifications of that with regard to feeding anything electronic.
Yes. Phase to phase is 480 V and phase to neutral is 277. We are taking neutral and one phase (277V) for lighting. Transformer KVA ranges from 45 KVA to 100KVA.
Putting 480 onto the 500 Volt primary tap will result in 266 on the secondary. If you have access to the secondary windings you may be able to remove a few turns from each.
>Putting 480 onto the 500 Volt primary tap will result in
>266 on the secondary. If you have access to the secondary
>windings you may be able to remove a few turns from each.

From what I can see you have two options, both require fiddling with the transformers. Your best bet is to speak to the transformer manufacturer to get their suggestion and then evaluate the cost implications

Option 1: What RoyMatson suggested. The theoretical maximum voltage with thos method comes up to 266V however the actual load flow voltage may be lower.

For example, if the main distribution transformer is loaded up to 50% and the isolation transformer (the one in question) is also loaded to 50%, the load flow voltage at the secondary transformer can be around 260V.

The next step will be to see the maximum operational voltage range of the load (lights in your case) to see if the load can take 260V
You can then look at removing some windings which may void any warranties and will also impose OH&S risks

Option 2: Change the secondary winding connection from Star to Delta and ground the high leg. Do some research on "high leg of delta transformer". This will give you 240V at L-N which is what you are after. Down side of that is some transformer manufacturers will have a maximum load limit if you use this method. Again, speak to the manufacturer to see if they can allow the load that you have on the 1:1 isolation transformer.

Hope that helps

Let us know how you end up resolving this.

Bob Peterson

if what you are trying to do is change the L-N voltage on the secondary to 230V, the bottom line is you can't with what you have told us about the transformer. depending on what taps are available (if any) on the secondary side and what range of voltage is safe to use on what you want to power you might be OK. This is a very odd question though. In the US there just are not many lighting loads that are 230V L-N. It just is not a real common thing.