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I just witnessed a 3 phase control transformer in the mcc that steps down 400V to 230V connected only with 2 phases on the 3 400V terminals, and on the other side (secondary) are your live and neutral wires. Does only using 2 of 3 wires affect the transformer or secondary circuit in any way?

Any information would be appreciated

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Does only using 2 of 3 wires affect the transformer or secondary circuit in any way?

Due to the nature of magnetization currents that flow, the centre common part of a 3-ph transformer core may have very little magnetic flux and hence can be made quite small without worries about core saturation. If you remove one of the three phase voltages, the flux in the centre-limb is no longer zero and has a value that is significantly higher than the individual fluxes. This could easily saturate that centre-limb and cause over-heating.

enter image description here

Picture taken from here. Other types/constructions of 3-ph transformers may not be affected.

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