Determining turns ratios for three-transformers is directly related to understanding that the number of turns primary verses secondary phase determines the turns ratio of the transformer.
Can you answer the following question with ease?
Question:
The turns ratio of a 480v/208v/120v three-phase, 4-wire delta-wye transformer is which of the following?
A. 1:2
B. 2:1
C:1:4
D. 4:1
Solution:
D. 4:1
The turns ratio is based on comparing the primary to the secondary phase voltage. The phase with the greater number of turns will have the higher voltage which means that the number on the left represents the primary and that on the right the secondary. Since the primary side of the transformer is delta configured, and in a delta phase volts equal line volts, the phase voltage on the primary side is 480 volts. The secondary side of the transformer is wye configured and in a wye phase voltage times 1.732 equals line voltage. Since 120 volts times 1.732 equals 208 volts, the phase voltage is 120 volts.
480v/120v = 4:1 turns ratio
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