# Question regarding a resistor in optocoupler/TRIAC circuit

I'm building a ac motor starter with a optocoupler (FOD4218) and a TRIAC (BTA16), one for each phase in a three phase 400V system. I have a question regarding the circuits that can be found all over the web, like this one: circuit http://www.electronics-tutorials.ws/articles/opto6.gif?81223b

What is really the purpose of the 100R resistor? Is it for voltage dividing or what? In my case where the circuit is between two phases, is it necessary? Some circuits do not have that resistor at all. I have planned to use 600R for the upper resistor.

Thanks.

The 100$\Omega$ resistor may help prevent the triac from triggering with high dv/dt (for example, with a somewhat inductive load- which you have with a motor load). It's generally not necessary unless you are using a sensitive-gate triac. For motor loads you should consider using an alternistor (Edit: which the BTA16 is), which has higher dv/dt immunity.
You did not ask about the 600$\Omega$ resistor, but increasing that part value from the minimum will cause unnecessary EMI and possibly other problems.