I ordered 2 of these from Amazon. Claims to support 1.5HP 250V motor on the NC, but I'm using a 1 HP 120V (=2HP @ 240) motor so not quite a fit.
Both sides of the relay are powering about 1HP induction motors that are running mutually exclusive. The NC motor runs a 1HP air compressor, the NO motor runs a 1HP motor connected to a 40:1 gear reduction drive moving a high pressure bicycle style pump up to 300 bar! The pump runs a about 43 RPM and for less than 90 degrees of the pumps rotation is under load. Most of the time the NO motor has minimal load on it since the pump has high resistance for less than 1/4 (probably only 1/8) of its rotation, or 0.35 sec out of 1.4 secs. The reason I monitor the current is to get most output from the pump and motor, I vary the input pressure to the bicycle style pump until I reach over than motors rated current of 9.8A (got to 14.8A) without stalling the motor. In the beginning when the output pressure is low the input pressure can be high, as the output pressure rises the input pressure is reduced to keep the peak motor current below where the motor would stall.
Since the project is already using a micro controller to monitor the current going through the relay, I could add a zero detect on the AC line and try to time closing the contacts at a zero crossing when closing a contact. Likewise I can use the current level to time the opening of the contacts to when the current is approaching 0. Off course this assumes when I measure the time it takes to open and close the contacts, that time remain reasonably constant.
When switching from NC to the NO, the NC motor will always be off, but when switching from the NO to NC motor I can't insure that the NC motor won't be active. So it is impossible to do both closing on zero voltage detection and opening on zero current detection. Unless I use 2 relays. Which is more important for relay lifespan opening the contacts at near zero current, or closing the contacts at zero voltage?
Worst case this will cycle in 10 min. intervals up to 20 times twice per week. So contact lifespan of over 10,000 (5 years) would be enough for me to call it OK.
https://www.electrokit.com/uploads/productfile/41015/NT90.pdf