# zero crossing switching in three phase systems

#### m121212

Joined Jul 24, 2011
89
In a single phase AC system, it would make sense to disconnect the load at a zero current crossing (and to activate it at a zero voltage crossing).

What about in a three phase system? If one phase is at zero, the other two are not. Is one phase just selected to spare the switching element for that phase?)

Are all phases switched simultaneously?

If zero crossing switching is indeed useful for three phase fixed frequency systems, is it also useful for three phase variable frequency systems?

#### GopherT

Joined Nov 23, 2012
8,012
In a single phase AC system, it would make sense to disconnect the load at a zero current crossing (and to activate it at a zero voltage crossing).

What about in a three phase system? If one phase is at zero, the other two are not. Is one phase just selected to spare the switching element for that phase?)

Are all phases switched simultaneously?

If zero crossing switching is indeed useful for three phase fixed frequency systems, is it also useful for three phase variable frequency systems?

See page 8
https://www.fairchildsemi.com/application-notes/AN/AN-3004.pdf

#### Lestraveled

Joined May 19, 2014
1,946
Please note that zero crossing circuits only trigger the triac at zero volts. The triac itself turns off when the current drops below a threshold.

#### m121212

Joined Jul 24, 2011
89
Thanks for the comments.

I guess if it's a heater or a motor, it doesn't matter if the phases are disconnected at different times.

In figure 16 on that app note, why do the switches go on when the voltage is non-zero?

#### Johann

Joined Nov 27, 2006
190
Thanks for the comments.

I guess if it's a heater or a motor, it doesn't matter if the phases are disconnected at different times.

In figure 16 on that app note, why do the switches go on when the voltage is non-zero?
Well, the circuit is under power (functional), therefore whenever any phase is not zero, its appropriate switch will turn on as the triac will be triggered after the zero transition. Remember, the 3 phases follow up at 120 degree intervals in time.