Quote Originally Posted by BobEdge View Post

At the moment I am detecting the zero crossing point by feeding the 3 phase into the input through high resistance chains.
Bob...

So, it sounds like you need a good zero crossing detector, not a PLL - correct?

I have lots of applications where I need to detect zero crossings and I use an opto-coupler like a MOCD207M (Fairchild) in a circuit with a 15K 2W resistor and a bridge rectifier. The transistor in the opto reliably turns off within 5 degrees on either side of the zero crossing. The LED is getting about 1mA at that point. If you increase the resistor's rating to 5W, you can use the same circuit for 120 and 240V. Use a 10K resistor in the collector of the opto.