Nitin,

The previous posts have given you some good pointers with regard to implementing zero crossing detection and the need to adjust timer presets.

I suggest the soft start function you want to implement can be done by placing a rate limit on the input (command) to the PID. If your application can use a single control loop, and accept the limited bandwidth on the input, I think you'll find it an easy implementation.

However, if your application needs soft start and quick response to changes in the input once operating, you might need an alternate approach. Some alternates for you to research via the web include multiple PIDs adjusting a single input variable to control multiple output variable of the plant (ac motor, in your case). The mechanisms that enable this approach to work extremely well include multiple PIDs (each having three signal inputs - command, feedback and process) and a LSS Bus (Low Signal Select) or HSS Bus (High Signal Select). Arbitration between the PIDs is accomplished via the feedback to each PID from the LSS or HSS. Control of the plant (process) variable is accomplished via the process output variable that is fed back to the process input of the PID.

Good Luck,

Bruce