Re: Volt-amps BS

Jon Slaughter wrote:

Can you explain a little about how the AC relays work a bit? I'm confused because it would seem that only DC would work unless there was some sort of rectification inside. What are these shading rings and stuff? (I tried searching but really can only find info on "DC" relays)
If a relay was pulled in by a simple AC coil, there would be two moments every cycle (when the current passed through zero) that there would be zero pull in force applied to the armature, and the whole thing would buzz, badly.

This is fixed by slotting the end of the pole piece that attracts the armature, and surrounding one half or so of the pole piece with a thick copper shorting ring. You can think of this ring and the part of the pole piece passing through it as a phase delay mechanism. When the flux level is rising in the other part of the core, the ring circulates current in the direction that bucks that flux to slow its rate of rise. But when the flux level is falling in the rest of the core, the ring circulates current in the direction that delays the drop of flux in the surrounded part. The net effect is that the armature is attracted by two poles excited by 2 phases of current, with the peak of one roughly at the zero of the other. So the attraction force is smoothed out, much like the way a two piston engine smooths the torque, compared to a 1 piston engine.

The shorting ring does generate considerable heat (from the large circulating current), so AC relays are often less energy efficient than similarly constructed DC relays.
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