To Zero Cross or Not To Zero Cross - John Dunn, Consultant, Ambertec, P.E., P.C.

Turning on AC power to some load by making switch closure at the zero-crossing of the input voltage sine wave sounds intuitively like the thing to do, but that may not be proper or wise.

We use recursive differential equations to examine the settling behavior as input power is applied to a sample circuit.  

 

If the load has an inductance to which the AC input voltage will be applied, an inductance such as that of a power transformer's primary winding, the current in that inductance will want to settle out to be in quadrature, at ninety degrees of phase shift, versus the phase of the applied input line voltage.

If you apply the input AC voltage by closing the switch at an exact instant when the input line voltage is zero, you will start out with the inductor's starting current being zero too and that is not the steady state condition that the voltage and current will eventually come to.

The time constant to the steady state condition will be very nearly Tau = L / Rs as in the above sketch and if the line resistance is really low, that time constant will be really long.

Ever hear a power transformer being turned on and going: HHMMMMMmmmmmmm.........?

This is why that happens.

Instead, it would be better to close the switch at the peak of the input AC line voltage. Since the inductor's current is initially zero as before, switching this way puts the applied voltage and the inductor's current immediately in quadrature with each other (or really close to it) and there is no transient event or current settling phenomenon.

The current starts out already settled.

Comments

[Edwin Pettis]

Some years back, I was the project engineer on a large component burn-in system. The system was supplied off of a 3-phase power transformer so that the loads could be as balanced as possible. I had designed TRIAC controllers to ramp up the line voltage to minimize the line surge with +90 degree phase shifting. Each power supply was a very large, 50lb, power transformer with multiple secondaries. To tell a long story in short, the 'chief' engineer insisted on replacing the three small power transformers I had specified with the one big monster and he also insisted on using zero cross over phase control to minimize the current suges despite my arguing for the other method. To him, zero voltage across the primary meant zero current....

The PC boards were finished per the Chief's changes and a few power supplies were built. When they were switched on, there was a snap and flash of light in each one of them, a PCB trace was vaporized due to the huge magnetizing current resulting from the zero cross over phase control. Since the boards had already been built, modifying them was out of the question, the only thing I could do was to place a large ballast resistor in the primary to limit the current surge. The Chief refused to accept the premise that it was his zero cross over and monster power transformer causing the problem.

[Andreas Fechter]

Thank you for helping me avoiding a classic, but big mistake!
I was looking for a good idea to switch over (on the fly) two power supplies: one coming from a photovoltaic driven line inverter (24V DC -> 230V AC),the other one is the common line supply, both of each 230 V AC. And naturally, as "open source" supply one can never exclude the inductive components - up to the
USV driven PC station....

tx again!