From: muratlanne@gmail.com   
      
   "Bob La Londe"  wrote in message news:v6filg$mrg8$1@dont-email.me...   
   >   
   > I quit screwing around with those 5HP motors.  Now I have not one, but two   
   > unused single phase 230V "5HP" spare motors.  ...   
      
   -------------------------   
      
   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?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)