Got a couple AHU's that have the return VFD's lockout on over current anytime they lose power. So I got a chance today, to spend some time with them.
Problem 1- If supply fan stops, it de energizes return fan contactor. But the return VFD is still getting run command from BMS. When supply restarts, return contactor energizes and over current lock out.
Problem 2- Can't start return fans in bypass. They pull 50-75amps and trip starters. Voltage drops 100 volts. Only way to keep them running in bypass, is to start them on the inverters and manually increase Hz then throw them to bypass.
Problem 3- Return fans hardly get any signal from BMS. Supply fans have the return fans spinning at 500-600rpms, even when de energized. When energized, the VFD's sit at 33% SP, 10hz, and 25% of rla.
These are belts drive plenum fans. The rotation is right. Mostly factory wiring. I did take static readings down the AHU's, but they didn't put up a red flag.
It's almost like there is way to much fan for application, but the supply fans run between 80%-100% to maintain 2.0wc on the supply.