Zero Current Synchronization of Opening Switch Contacts

Thermostatic controls switching ac loads at 220 V, 50 Hz consist of a slowly moving thermally activated prime mover operating randomly on a small snap-action switch. An experimental study on the opening characteristic of the snap-action switch has shown that the point on the current wave at which contact separation occurs becomes nonrandom on its occurrence for current greater than 6 Arms. This is attributed to the electromagnetic force of repulsion at the contacts being greater than the holding force exerted on the contacts by the switch mechanism as it approaches the toggle point. A possible means of utilizing an electromagnetic force to synchronize the instant of contact separation near the current zero is discussed and then verified experimentally. This leads to a significant reduction in the total energy dissipated at the contacts by the break arc, over many operation.