An NVR (No Volt Relay or No Volt Release) is a safety device commonly found on most workshop equipment, that acts as an on-off switch, but will default to off if the power fails.
I thought it would be interesting to take one apart, and it was interesting to find that the off button also applies force to the contacts to make sure they part reliably even if they have welded together slightly due to the switching current.
The shading ring on the magnet core is used to cause a phase shift so the AC powered magnet doesn't lose force as the sinewave passes through the zero crossing point. That also reduces the risk of buzzing from the coil. It's similar to the approach used in shaded pole motors to provide rotational direction. Here's the Wikipedia page for the shading ring:-
https://en.wikipedia.org/wiki/Shading_coil
This unit came from a UK seller. https://www.ebay.co.uk/itm/115501562742
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