Publication 700-AT001A-EN-E June 2002
Considerations When Designing SSR Control Systems
2-11
The supply voltage plus the charge voltage of the capacitor is applied
to both ends of the SSR when it is OFF. Therefore, use an SSR model
with an input voltage rating twice the size of the supply voltage.
Limit the charge current of the capacitor to less than half the peak
inrush current value allowed for the SSR.
Inrush Currents From Transformer Loads
The inrush current from a transformer load will reach its peak when
the secondary side of the transformer is open, when no mutual
reactance will work. It will take half a cycle of the power supply
frequency for the inrush current to reach its peak, the measurement of
which without an oscilloscope will be difficult.
The inrush current can be, however, estimated by measuring the DC
resistance of the transformer.
Due to the self-reactance of the transformer in actual operation, the
actual inrush current will be less than the calculated value.
I peak = V peak/R = (
✕
V)/R
If the transformer has a DC resistance of 3
Ω
and the load power
supply voltage is 220 V, the following inrush current will flow.
I peak = (1.414
✕
220)/3 = 103.7 A
The inrush current resistance of Bulletin 700-S_ SSRs is specified on
condition that the the SSR is in non-repetitive operation. If your
application requires repetitive SSR switching, use an SSR with an
inrush current resistance twice as high as the rated value (Ipeak).
In the case above, use a 700-S_ SSR with an inrush current resistance
of 207.4 A or more.
The DC resistance of the transformer can be calculated back from the
inrush current resistance by using the following formula.
R = V peak/I peak = (
✕
V)/I peak
For applicable SSRs based on the DC resistance, refer to the tables on
page 2-12.
These tables list SSRs with corresponding inrush current conditions.
When using SSRs in actual applications, however, check that the
