What does this mean?
“To ensure longer service life and compatibility with electronic ballasts, the device carefully times the primary relay switching contact opening and closing with the zero crossing point of the AC power curve. This minimizes contact wear caused by in-rush currents from electronic ballasts.”
From this site.
Are they saying that the contacts close exactly when the sine wave is at zero? Is this even possible?
That would be a real trick with 3 phase.
That’s exactly what it means. I don’t see it as being terribly difficult, technically. The line frequency holds pretty well at 60 Hz, to within a few percent, so it’s simply a matter of timing. One could, for example, employ a peak detector circuit, and a timer would close the circuit 1/4 cycle later.
I guess, 1/240th of a second should be easy to do consistently.
It’s the opening of the contacts that is the really important bit as far as relay contact wear goes.
Yes you can get a little bit of contact bounce on closure and hence a small amount of arc splash, but on opening any inductive circuit, the back emf can be many times the supply, depending upon the rate of contact separation and the instaneous magnetic fields stored in the load.
In extreme cases, contactors are actually slowed down when they open to allow for some electrical discharge to take place, otherwise massive back emfs can flow and cause huge problems, this is an effect called current chopping and is a matter that has to be considered in the design and installation of 500KV switchgear as found on grid distribution networks.
“This minimizes contact wear caused by in-rush currents from electronic ballasts”. “Electronic ballasts” have a rectifier followed by a capacitor, no inductance.
It’s much much much much more accurate than a few percent.
Well, after doing some research, let me back off some and say just “much more accurate.” Cycle-to-cycle can vary as much as 0.1% or so, although long-term the four “muches” would apply. Close-in phase noise is excellent.
This caused the General Electric Company mucho bucks when they first introduced vacuum power breakers (15kv, 2000amp). The breaker would interrupt at a non-zero current point, and generate overvoltages of tens of thousand of volts…. Power transformers were blowing up all over the place.