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I'm in a course where we wired up a single phase transformer as delta/delta and delta/wye and tested the current on the secondary side, Is. I noticed that we needed more than twice as much input power in the delta/delta configuration as we did in the delta/wye configuration to generate the same secondary current.

Delta/delta

Delta delta

Delta/ wye

Delta wye

Does anyone know why this is?

Cheers.

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    \$\begingroup\$ You need to describe us the exact experimental setup and procedure (by amending your question, not replying to this comment), as your current question leaves a lot of ambiguity: Did you adjust the primary voltage to keep the secondary current at same value in both (delta and wye) instances, or did you do something else? Was the load a fixed resistance, a motor, a power supply, a constant current sink or something else, and how was it connected (delta or wye)? How exactly did you measure the input and output power, current and voltage? There could be measurement errors. \$\endgroup\$ – jms Nov 22 '16 at 4:34
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    \$\begingroup\$ Switching one side between wye and delta changes the transformer voltage ratio by 1.7ish (sqrt(3) IIRC). Now note jms's comments about how you're mixing up power and current, and not specifying what's held constant. \$\endgroup\$ – Neil_UK Nov 22 '16 at 6:19
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You are confusing power and voltage in your experimental setup.

Let's assume you use three single phase transformers with a 1:1 turns ratio to build your three phase solutions using 415 V 3 phase as the input.

Star-Star would give 415v on the secondary

Delta-Delta would give 415v on the secondary

Star -delta is a stepdown transformer with secondary output voltage of 415/sqrt(3) = 244 V

Delta-Star is a step up transformer with secondary output voltage of 415*sqrt(3) = 705 V

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