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I have a 3-phase transformer with primary and secondary windings both star(neutral connected). When connected to 3-phase source whose line volts is 415V, I got the following readings at secondary winding:

    phase A to neutral :  105V
    phase B to neutral :  110V
    phase C to neutral :  220V

Please I want to know the cause of lower phase voltages on lines A and B. Thank you. i will appreciate your answers.

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    \$\begingroup\$ Can you give us the complete specs of the transformer, e.g. attach an image of the nameplate? \$\endgroup\$ – realtime Apr 19 '14 at 12:09
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A 3-phase transformer, like any transformer is more naturally inclined to be an isolation transformer therefore, measuring the secondary voltages with respect to neutral may be totally meaningless because the secondaries will not normally be galvanically connected to neutral.

However, assuming they are galvanically connected AND there is no fault on the transformer, the voltages measured can only be because the secondary turns are different thus they produce 220V, 110V and 105V.

It would be wrong to have a three-phase transformer with different primary windings because the ampere-turns wouldn't be balanced and the core would likely saturate due to this.

It could also be that there is just one secondary winding and this has got tap-off points.

A picture might help.

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Start by following the IEC 60067 standards. Measure winding resistance, turns ratio.

Most likely there is a winding short or miswired during assembly. Winding resistance results and measurement conditions will add clarity to the question.

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  • \$\begingroup\$ To make this answer more useful it'd be worth including more information on why you think the IEC 60067 standard would help. Especially when it seems to be a non-free standard, maybe if you have a copy you could at least give a brief summary of the areas it will help with. \$\endgroup\$ – PeterJ Apr 19 '14 at 14:22

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