How to design individual commutation for SCRs in H Bridge configuration?
Here is a basic SCR chopper circuit that is described in detail in B. D. Bedford &amp; R. G. Hoft, Principles of Inverter Circuits, John Wiley &amp; Sons, 1964. It is in an Appendix by W. McMurray.



Forced commutated SCR circuits with switching frequencies up to about 400 Hz have been successfully used in products. Such products were successful in some applications for a short time. They were not very efficient, so they produced a lot of heat. It was very difficult to make them rugged and reliable. It was difficult and expensive to get SCRs with sufficiently short turn-off time specifications. The dv/dt of reapplied voltage was a problem.

Here is a diagram of a reversible DC motor drive using a saturable reactor in the commutation circuit. TRC is time ratio control. I think that is the same as PWM. The circuit is probably described in: W. McMurray, "Analysis of thyristor DC chopper power converters including nonlinear commutating reactors," in IEEE Transactions on Magnetics, vol. 6, no. 1, pp. 16-21, Mar 1970.
doi: 10.1109/TMAG.1970.1066710

The commutation circuit is described here: R. E. Morgan, "Basic Magnetic Functions in Converters and Inverters Including New Soft Commutation," in IEEE Transactions on Industry and General Applications, vol. IGA-2, no. 1, pp. 58-65, Jan. 1966.
doi: 10.1109/TIGA.1966.4180615