I want to turn heating elements that are connected in star formation across 3 phases on and off according to zero crossing. I'm not sure if this is possible. as when 1 phase is turned on at its zero crossing, the other 2 phases need to be on to close the circuit. But then, they would nor be turning on at their zero crossing. This is getting me quite confused.

 

Rowan... zero-crossing is not useful in varying a resistance-type load by varying the "on" time per cycle.

However, it is used for "burst-control" that is, several cycles (per-phase) "on"; several "off!"

regards, Phil Corso

 

Rowan... further to my earlier post, controlling the 3-elements of a wye-connected resistive-load requires the supply to be 3-phase, 4-wire!

Phil Corso

 

I think what you are saying is never are all 3 phases zero. Perhaps if you think of it from a current perspective. At the instant the current in phase A drops to zero turn that off then 120 later when the current B-C drops to zero, turn those off.

Many of the solid state controllers only have 2 elements, the third phase is on all the time. Turning back on you would do the opposite switch A-B at zero then C at the appropriate time.

Mind you, I am only surmising here, I haven't actually studied the circuit, just seems to make sense.

Roy