I have two large air handlers that share a common duct system. Supply Fan vfd's maintain a static pressure in the supply duct. Each machine has its own separate pressure transducer. Because it is impossible to calibrate these absolutely perfectly, the ahu that reads the lesser amount of pressure ramps up. This causes the other to ramp down because it begins sensing a higher than setpoint pressure. It continues like this until the one unit gets to minimum rpm and then the other ahu ramps up and does all the work. I need a 0-10 vdc repeater with energy isolating characteristics so I can run both ahu's on the same transducer without sharing any power between controllers. Anyone know a part number that fits that bill?
Need 2 AHU's to share a pressure transducer
nine o
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I have two large air handlers that share a common duct system. Supply Fan vfd's maintain a static pressure in the supply duct. Each machine has its own separate pressure transducer. Because it is impossible to calibrate these absolutely perfectly, the ahu that reads the lesser amount of pressure ramps up. This causes the other to ramp down because it begins sensing a higher than setpoint pressure. It continues like this until the one unit gets to minimum rpm and then the other ahu ramps up and does all the work. I need a 0-10 vdc repeater with energy isolating characteristics so I can run both ahu's on the same transducer without sharing any power between controllers. Anyone know a part number that fits that bill?
Can you just choose one of the AHU's to be the one that controls both fans? Leave the other signal and transducer connected but run both VFD signals through a DPDT so that when the main one is running the VFD uses that signal but when it is not running it switches over to the backup signal.
That would require, at times, for the slave machine to use the lead machines control power. That is unacceptable. The control powe cannot be shared under any circumstances, I need to maintain the isolation via some component.
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You may have to rethink your requirements.
Any of these transducers will need power to work. Either you need a 3rd source of power to supply the transducer power....or it will have to steal it from one or the other unit.
If you go with a 3rd source, what would be the difference between that and just paralleling the 0-10vdc to both controllers? If you shutdown unit A it will still have power on this input.
Any of these transducers will need power to work. Either you need a 3rd source of power to supply the transducer power....or it will have to steal it from one or the other unit.
If you go with a 3rd source, what would be the difference between that and just paralleling the 0-10vdc to both controllers? If you shutdown unit A it will still have power on this input.
My thought exactly. Without a "Network Manager/Supervisor" the only way I have ever had it work reliably is to sync the Fans/Drives. As long as your wire distance is fairly short (< 150') it should work just fine. A little relay logic and you should have it working in short order (easy for me to say, I'm not the one doing it LOL !).
Good Luck.
Option 1 - if you have access to the PID tuning parameters and integration can be set to zero (the latter makes some Carriers a no go) try this. On both AHUs: Set the VFD acceleration to five minutes (possibly more) and deceleration to one minute (or less). Zero the integration term. Narrow the proportional band (greatly). This works surprisingly well.
Option 2 - if the VFDs are pretty flexible (ABB et al) try this. Take VFD-1, AO-x and wire to VFD-2, AI-x. Use a signal isolator if desired. Take VFD-1, DO-x and wire to VFD-2, DI-x. Signal isolation will not be required if the DOs are dry contacts. Set up VFD-1, AO-x to mirror the speed of the drive. VFD-1, DO-x is set up to close when the drive is running. VFD-2, AI-x will be set up to match the VFD-1 AO point. VFD-2, DI-x will be set up to change the speed input signal when a closed contact is detected. (ABB calls this External Commands - EXT1/EXT2).
Option 2 - if the VFDs are pretty flexible (ABB et al) try this. Take VFD-1, AO-x and wire to VFD-2, AI-x. Use a signal isolator if desired. Take VFD-1, DO-x and wire to VFD-2, DI-x. Signal isolation will not be required if the DOs are dry contacts. Set up VFD-1, AO-x to mirror the speed of the drive. VFD-1, DO-x is set up to close when the drive is running. VFD-2, AI-x will be set up to match the VFD-1 AO point. VFD-2, DI-x will be set up to change the speed input signal when a closed contact is detected. (ABB calls this External Commands - EXT1/EXT2).
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