A step down transformer with a ratio of 25:1 and primary voltage of 6.6kV is described. The document calculates the secondary voltage at: (a) the principal tap of 264V, (b) the +5% tap of 251.4V, and (c) the -5% tap of 277.9V. An increase in turns ratio results in lower secondary voltage, while a decrease in turns ratio increases secondary voltage.

1. Example 1
A step down transformer has a ratio of 25:1 and a rated primary voltage of 6.6 kV.
Calculate the secondary voltage:
a)At the principal tap.
b)at the +5% tap, and
c)at the -5% tap.
(a)
E2 at principal tap (V2) V1 = N1
V2 N2
6600 = 25
V2 1
V2 = 6600 = 264V
25

2. Example 1
A step down transformer has a ratio of 25:1 and a rated primary voltage of 6.6 kV.
Calculate the secondary voltage:
a)At the principal tap.
b)at the +5% tap, and
c)at the -5% tap.
(b)
+5% the turns ratio = 105 x 25 = 26.25:1(Increases turns ratio)
100
E2 at +5% tap V2 = 6600 = 251.4 V
26.25

3. Example 1
A step down transformer has a ratio of 25:1 and a rated primary voltage of 6.6 kV.
Calculate the secondary voltage:
a)At the principal tap.
b)at the +5% tap, and
c)at the -5% tap.
(c)
-5% the turns ratio 95 x 25 = 23.75:1 (Decreases turns ratio)
100
E2 at -5% tap V2 = 6600 = 277.9 V
23.75
Note that an increase in the turns ratio results in a lower output voltage and a decrease in the
turns ration will result in an increase in the output voltage.