Re: I Quit

"Bob La Londe"  wrote in message news:v6filg$mrg8$1@dont-email.me...

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I quit screwing around with those 5HP motors.  Now I have not one, but two unused single phase 230V "5HP" spare motors.  ...

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Single phase. I've been looking for a reasonably priced 3 phase double throw contactor rated for over 40A. Maybe two solid state relays (SSR) operated by a small double throw relay would be better for your XOR air compressor configuration. A DPDT center off switch could select which compressor runs when the pressure is low, or shut both off. The water heater circuit won't automatically revert to the high pressure compressor if the low pressure one fails, otherwise it should provide backup and enable only one at a time as you asked.
The simplest version has the control relay coil in parallel with the LP compressor motor and its NC contacts operating a single throw contactor or SSR to the HP compressor. This retains the single throw pressure switches and uses the relay for the logical inversion. Swapping main and backup compressors requires rewiring them, unless you have a DPDT switch rated for the motor current. I don't know your exact needs, priorities or intentions so I've given multiple possibilities.
I used this to power the time delay relay and SSR on my 120V/240V spot welder timer.
https://www.amazon.com/NOYITO-Precision-100V-264V-Indicator-Step-Down/dp/B0B7J8DT2G/ref=sr_1_10?