Three phase autotransformer 2

[pragmatist]

What problems can arise if I use a three phase autotransformer with a center tapped delta power source? Sinse each phase is 240 Volts, I am told the one tapped phase will cause my autotransformer or its' load to short out.

 

[jbartos]

Suggestion: Please, would you elaborate on transformer connections, e.g. Autotransformer connection, center tapped delta power source connection, (open delta versus closed delta)?

 

[pragmatist]

Hello jbartos,

My sources can be three phase 60 Hz.Y or delta at 208V, 240V, 480V, & 575V(Canada). I assume that I won't be able to use corner grounded 240V delta.

My load will be a three phase Y connected refrigeration compressor operating at 440 V 60 Hz. (European design)

The autotransformer has all of the above taps for the input voltages and for the 440 V output. It also has a non-grounded neutral.

I plan to tap one of the 440 V phases to develope, via another single phase autotransformer, 230 V for control boards and ventilating fans.

 

[busbar]

Suggest obtaining a commercial 480∆-240∆ dry-type transformer reverse fed with taps adjusted for lower ratio.

 

[pragmatist]

To: busbar
From: pragmatist

It looks like you are recommending an isolated transformer to get around the different kinds of three phase systems; grounded and ungrounded, that might be found in the US. Is this correct?

 

[jbartos]

Suggestion: The dedicated transformer secondary load, e.g. a compressor, does not have to have grounded transformer secondary neutral.
The primary voltages are just a matter of correct taps.
The range of transformer primary voltages is suitable for an autotransformer. It will be smaller than a regular two winding transformer. There is a chance to save some money on shipping.

 

[Shortstub]

While the compressor motor's power circuit doesn't have to be grounded, the control circuit does per the NEC. Therefore, if the power source is ungrounded a control-circuit transformer will be required.