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:

  1. TR1 with ZC-12
  2. TR2 with ZC-23
  3. 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...?

3 phase controlled soft starter

  • \$\begingroup\$ Post the scope trace. The correct sequence is described in books and scientific articles. \$\endgroup\$ May 28 '21 at 10:11
  • 2
    \$\begingroup\$ I’m voting to close this question because it shows a lack of understanding that requires more than just the answer to a single question. We can not present, in one answer, material that requires a number of textbook pages to explain. The answer can be found on web sites that have tutorial presentations, in text books, in online lecture notes and perhaps Wikipedia. That material is too extensive to repeat here. \$\endgroup\$ May 28 '21 at 11:16

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