I want to make a soft starter for 3 phase induction motors. TRs are SCR pairs and ZCs are the zero-cross timing respect to line voltages (400V), not phase voltages (230V). I control the firing angle of the SCRs with their ZC timing:
- TR1 with ZC-12
- TR2 with ZC-23
- TR3 with ZC-31
When I turn fully ON all of the SCRs, there is no problem. However when I control the firing angle there are some range in the firing when the motor starts to vibrate and makes bad noise.
I don't want to share my program but I'm almost sure that it works as it should, because I tested it with a resistive load and I measured the current whit a scope.
My question is: is it right how I firing the SCRs respect to zero-cross (TR1 with ZC-12 and so on)?. How the big companies trigger the SCRs? I mean are they firing the SCRs separately like I did, or firing in pairs at once, for example: turning on TR1 and TR2 at the same time, respect to ZC-12. TR2 and TR3 respect to ZC-23...?