How can the locations and polarity of the windings be tested on the various legs of a core when it has 2 open(circuited) three pahse windings (III and iii)?what measurements are required , assuming that the transformer is so enclosed that only the 12 winding terminals are visible?Ohmmeter , voltmeters , a single phase voltage supply are available.
It is assumed that the primary and secondary voltages are within the range of standard voltmeters,example 440/220 Volts AC.Transformer terminals as you mentioned are 12 terminals.Label the terminals: 1; 2; 3; 4; 5; 6; 7;8;9;10;11 and 12.Let 3 concide with terminalII. With an ohmmeter,check for the end of 1, it should be 4;likewise for 2, the end should be 5; and 7 should end with 10, 8 should end with 11.Look for the end of 3 or III, and label it with 6; iii or 9 should end with 12. Assume for a primary wye connection. With this connection, connect terminal 4 and 7, 5 and 8, and the wye terminal 10,11 and12.With the primary supply disconnected, disconnect also, the load end secondary connection. Impress a single phase 220 volt supply between terminals 1 and 2 of the secondary. Test the voltage between 1 and 2 in the primary. A voltage of 440 volts should register, assuming the transformer ratio is 440/220volts. ATline 3 and 1, voltage should be 380 volts only and so with 2 and 3.Switch off test voltage supply and connect terminals iii,6 and 9 with a small magnet wire, No.20 AWG.Impress the test voltage and measure voltages across :1 -2; 2-3; and 3-1. An equal voltage of440 Volts should indicate if not, change the polarity of III and iii,6 and proceed with the same procedure.If the voltages at the primary are equal, replace the jumper across 6 and 9 with an original jumper wire.
It sounds like you have a three phase transformer. In the cover plate there normally is a drawing to guide you on what you want to do. Most trans formers can have input voltages of 440 volts in to 240 ouput. but some can provide 115 volts on the output if wired correctly. Any transfomer compnay website provide wiring drawings that pretty match everyones transformers. Contact the manufacturer of the transformer for instructions.
