Brochure

ESG
8.638.63
8.638.63
8.63
Fig. 20: Transformer tab switching
Secondary winding
Primary winding
AC
power
SSR1
SSR2
SSR3
AC or DC
control inputs
1
2
3
COM
R
L
R
X
R
X
R
X
A
Sec
Primary winding
AC
power
120/240
SSR1
SSR2
R
L
R
X
R
X
AC
input
AC
input
120 VAC
CT
B
R
L
Secondary winding 120 VAC
SSR2
SSR1
AC
input
AC
input
R
X
R
X
CT
Primary winding
AC
power
120/240
C
Transformer Tap switching
(Fig. 20) (Fig. 20)
(Fig. 20) (Fig. 20)
(Fig. 20)
If a momentary interruption in power is acceptable, a time delay on operate is suggested to prevent overlap and the
resulting high current surge form a shorted winding. Two times Rx plus the winding resistance must be sufficcient to
limit the surge current to the one cycle surge rating of the SSRs.
As an additional precaution, the
SSR blocking (breakdown)
voltage should exceed the main
winding voltage plus the
highest tap voltage. For multi-
tap switching the SSRs are
generally logic driven dc
controlled ac output types
without special requirements.
For Figs. 36B and C, they are
250 Vac output with 120 Vac
input for SSR1 and 240 Vac
input for SSR2 in each case. An
important requirement of SSR2
is that it must be off below the
highest expected 120 Vac line
swing, say 150 Vac. When SSR2
is off, SSR1 will be on and vice
versa, thus activating the
apporopriate winding.