Brochure

ESG
8.478.47
8.478.47
8.47
output pair is capable of operating at much higher
frequencies. Wowever, because of circuit time cons-
traints in the drive circuitry, other SSR parameters
become the limiting factors (e.g. the zero switching
window may be extended and/or turn-delayed each half
cycle with eventual lock.on or lockout).
Zero switching
Zero voltage turn-on (or zero crossing), as illustrated
in Fig 4, is used in some ac SSRs to reduce electroma-
gnetic interference and high inrush currents. Without
zero crossing, the load voltage is applied randomly to
the load at any point in the line voltage cycle.
With the zero crossing feature, the line voltage is
switched to the load only when it is close to zero,
typically specified with a maximum value of ±15V volts
peak. Thus, a very small change in power results, and
proportionally lower EMI levels are generated. After
zero crossing, the "Zero" switching voltage, which
defines the switching window limits, may also be
expressed in terms of phase angle, or time, converted
as follows:
Voltage to phase angle (15 volts) or phase angle to
time (5°):
Zero voltage turn-off is an inherent characteristic of the
thyristors used in ac SSRs, whether zero votlae is emp-
lyoed or not. Once triggered, the thyristor stays on for
the balance of the half cycle, until switching load current
drops below ist "holding" level, where it turns off. For a
resistive load, this point is also close to zero voltage, as
shown in Fig. 5. With an inductive load, the amount of
stored energy
in the load is a function of the current flowing through it,
which in this case is so small that inductive kickback is
virtually eliminated. This is probably the most desirable
feature of the SSR, when compared to the destructive
effects of "arcing" contacts when switching inductive
loads with an EMR.
φ = sin
-1
15
120
x
1,41
φ = 5°
φ = sin
-1
Z sw. max.
Line V RMS ( 2 )
T =
½ cyc. ms
½ cyc. deg
T =
8,3
180
T = 0,23 ms
x
φ
x
5
AC SSR
Terminals
output
Off state
DC control
voltage
Turn-on signal
Actual
Turn-on
On state
Turn-off signal
Actual
Turn-off
Off
On
Fig. 4: Zero voltage turn-on