That particular soft starter is of the type I hate, because it uses a concept of “2-phase control” in which there are SCRs in only 2 of the phases (4 SCRs instead of 6), the center phase is just a piece of bus bar straight through. There is a big risk of losing the motor in that design, because if any single SCR is damaged and shorts, you would have uncontrolled current flowing to one phase of the motor*. So to avoid having this happen, they recommend (although it’s really required) that you then add an isolation contactor ahead of the soft starter so that if it detects that one SCR is shorted, which is what R2 is tied to, it will drop out that contactor to save the motor.
So the control circuit is set up in that case to where your stat/stop energized the contactor and the soft starter immediately begins ramping when it sees voltage from the contactor. R2 is held closed whenever the soft starter does NOT detect a shorted SCR and drops out when it does. The Run selector wired to the other terminal is there in case you want to implement Soft Stop, for example on a pump. But that has other negative consequences too.
This cheap type of soft starter is also very hard on the motor because it works by purposefully delivering severely unbalanced voltage to the motor since one phase is not being controlled. You cannot use this type on heavy loads without risk of damaging the motor. It’s good for centrifugal pumps and fans only with short ramp times of 10 seconds or less.
The irony of this is that the ONLY reason to use only 4 SCRs instead of all 6 is to save cost, then you add that cost back in, and more, by adding that line isolation contactor!
* In a standard 6 SCR design, one shorted SCR does not result in current flow through the motor because there is no return path; you need at least 2 shorted SCRs in different phases for that and that is very rare. In the 2-phase design, the return path, through that bus bar phase, is always there waiting...
