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I have a circuit which turns a 230V 50hz AC motor on and off by use of a relay.

I've tried to arrange it so that the relay would be turned off when the current through the motor is zero. Of course, it turns out this doesn't work, since the release time of my relay is 3-5 ms, and if I arrange to open the contacts at 0A current, the current will be at its maximum at the end of the release time (5ms). This is because at 50hz there are zero crossings every 10 ms, and the peaks are 5ms after the zero crossings.

I then experimented with turning the relay off at different phases. It turns out that the best results seem to be obtained when releasing the relay at maximum current. Then I consistently get clean waveforms with no voltage spikes.

If I try to release the relay at 3ms before the zero, it sometimes turns off cleanly, but sometimes there are some pretty big voltage spikes (>500V) just after the zero-crossing, and then the relay keeps conducting all the way through to the next zero crossing.

Turning the relay off at maximum current seems to give consistent good results.

What is the physical process that keeps the relay conducting until 0 current? If I switch a very light load the relay turns off "when asked to", and doesn't keep on conducting until the next zero-crossing.

Is turning the relay off at maximum current in the AC cycle advisable?

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    \$\begingroup\$ Is there a reason why you don't want to use a triac for this? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 5 '15 at 6:59
  • \$\begingroup\$ I wanted to have reinforced insulation between control circuitry (coils) and switched load. And I wanted to keep things simple. \$\endgroup\$ – avl_sweden Aug 5 '15 at 7:06
  • \$\begingroup\$ But I think the next version of my gadget will use triacs. Or possibly rectification + IGBT:s for frequency control :-). \$\endgroup\$ – avl_sweden Aug 5 '15 at 7:07
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    \$\begingroup\$ @avl_sweden: You could also use triacs for fast safe switching, and then additionally a relay after it has switched off for your warm and fuzzy feeling of safety. \$\endgroup\$ – PlasmaHH Aug 5 '15 at 8:04

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