I am building a Spot Welder with a big powerful transformer. I have to switch a big inductive load of 70mH (input of a transformer) The switching will be done with an SSR (Opto Triac Zero crossing SSR-90DA).
When the transformer is under load the input impedance is almost resistive. My problem is this transformer is also open circuit output sometime. This is equivalent to switching a 120VAC into a 70mH inductance. The zero crossing feature is great but with at purely inductive load it become a problem. Switching ON is OK. Switching OFF is not OK because the inductive load will shoot a big reverse pulse much higher than the MAX voltage of the SSR. I want to put an bipolar AC capacitor in parallel with the inductor, reducing the pulse to acceptable voltage. I am missing math involved necessary to calculate the Cap value. What is the energy stored in the coil at Switch OFF ? What voltage this energy will charge the capacitor at Switch OFF ?
How can I calculate the pulse voltage .vs. Capacitor value (at switch OFF time) ?
How can I calculate the necessary value of this capacitor ?