Deriving 3Ph wye system from single phase

[electrofelon]

Hypothetical/academic question that I thought of from the recent thread on phase converters. What if I wanted to derive a three phase wye system from a single phase source. Could I feed a Delta wye transformer with the single phase, ground the secondary wye point, and connect the phase converter to the transformer secondary? Could the derived phase be connected to the "third phase" on the transformer or should it remain separate? Would this work?

 

[jim dungar]

I often think of transformers as motors that don't turn.
My last design like this was for 120/240V fed to a phase convertor, to make 240V. My transformer had a 240V delta input/primary and a 480Y/277V output/secondary feeding a 480V grinding machine.

 

[electrofelon]

I often think of transformers as motors that don't turn.
My last design like this was for 120/240V fed to a phase convertor, to make 240V. My transformer had a 240V delta input/primary and a 480Y/277V output/secondary feeding a 480V grinding machine.

Ok right that makes sense: just make the three phase FIRST and THEN use a transformer to derive a new grounded Conductor in the correct "spot". Would using the transformer first work?

 

[retirede]

I think the more conventional way would be to feed the primary from the phase converter.

 

[winnie]

A delta:wye transformer could be used to derive a neutral. So in theory you could feed single phase to the rotary converter and then derive the neutral as you describe.

But unless your single phase source was floating you couldn't _ground_ that derived neutral.

Generally you have 120/240 grounded single phase going to the rotary converter, and what you get out looks like a 'high leg' delta. If you derive the neutral of this delta, it would be at about 69V relative to the 120/240V ground point. Grounding it again would be bad.

Note: you can't use a stand alone wye as a grounding transformer. But you can use a wye:delta with the delta simply left floating. Neutral current in the wye causes circulating current in the delta, which is why it works.

Jon

 

[electrofelon]

A delta:wye transformer could be used to derive a neutral. So in theory you could feed single phase to the rotary converter and then derive the neutral as you describe.

But unless your single phase source was floating you couldn't _ground_ that derived neutral.

Generally you have 120/240 grounded single phase going to the rotary converter, and what you get out looks like a 'high leg' delta. If you derive the neutral of this delta, it would be at about 69V relative to the 120/240V ground point. Grounding it again would be bad.

Note: you can't use a stand alone wye as a grounding transformer. But you can use a wye:delta with the delta simply left floating. Neutral current in the wye causes circulating current in the delta, which is why it works.

Jon

Right, you need isolation somewhere so you can derive a neutral where you want it. You can't reground a grounded conductor in a different "vector spot" without isolation.