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The Basics of 3-phase transformers — part 2

Aug 1, 1999 12:00 PM, By Paul Frank, Tel-A-Train

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Most large power (and many small distribution) transformer installations provide all three phases. Sometimes they do so with three single-phase units, sometimes with one 3-phase unit. In either case, there are six windings, three on the primary and three on the secondary side.

Each set of the three windings will connect in either wye or delta. Each winding of a wye-connected transformer connects to a neutral junction. Power lines connect to the other ends of these windings. Windings of a delta-connected transformer connect in series. Power lines connect to each pair of winding ends, where the ends join.

You can calculate output voltage in the wye-to-wye and delta-to-delta connections simply by applying the turns ratio rules for single-phase transformer connections. In Part 1, we explained the secondary voltage is the primary voltage divided by the turns ratio and the secondary current is the primary current times the turns ratio.

However, in the wye-to-delta and delta-to-wye arrangements, two other rules also apply:

• Current divides in delta (CD), and

• Voltage divides in wye (VW).

You can remember these relationships by thinking "I like to play CDs in my VW." "CD" means that in a delta connection, current in any winding is less than the current in the phase by a factor of 1.73. To find the current in a winding, divide the current in the phase by 1.73. Voltage across a delta winding is the same as the voltage going into the winding.

"VW" means in a wye connection, voltage across a winding is less than the voltage in the phase by a factor of 1.73. To find the voltage across the wye winding, divide the voltage in the phase by 1.73. Current in the windings is the same as the current in the phases.

Using these rules and the rules for turns ratios, you can calculate the input and output voltages for any 3-phase transformer connection.

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