Can You Answer This Three-Phase Transformer Exam Question?
What is the turns ratio of a three-phase 4-wire delta-wye transformer, with a 480 volt primary and a 208 volt/120 volt secondary?
Solution:
A turns ratio is a way of quantifying the relationship between the number of turn of the coil making up the primary verses the secondary phase coil of a transformer.
The side with the greater number of turns of coil will be the side with the higher voltage. For example a turns ratio of 2:1 indicates that the voltage on the primary side is twice as high as that on the secondary side of the transformer.
Since the question indicates that the primary is a delta-phase and line voltage is the same. So primary side phase and line voltage is 480 volts.
However since the secondary is a four wire wye, which means that the secondary has three line conductors and one neutral conductor, the phase voltage (which would be read between any one line and the neutral conductor) is 120 volts and the line voltage (which would be read between any two line conductors) is 208 volts In a wye phase voltage x 1.732 = line voltage.
Since when determining turns ratio only phase voltages are used, simply divide the primary phase voltage by the secondary phase voltage to determine the turns ratio of the transformer.
480 primary phase volts/120 secondary phase volts = 4:1 turns ratio