On a couple of vfd's with bypass we have been having major problems restarting a return fan from a running AHU [ AIR HANDLING UNIT ]. 2 of the VFD'S are less than 7 years old but are obsolite so the vendor can not help. At the Hospital where I work we can not shut down both AHU'S or ramp them down too far due to negative or positive pressures required in some patient rooms. When the return fan [100 HP ] goes down the 2 125 HP AHU'S cause the return fan to run at over 800 RPM in the wrong rotation. Even when we do a quick ramp down to 30% of full speed the return fan still runs pretty fast. Building Engineers tried adjusting dampers to slow return fan but that only helped a little. Years ago we use to wire in a full wave 30 amp cube rectifier from the 480 volt line side of a starter and thru a timer to give DC power to 2 motor leads for 0.5 to 1.2 seconds to stop explosion proof motors. Those motors were only 1 to 7.5 HP. Any body have any sugestions and what ampere for a rectifier if I go that route. Does any company sell a kit for this? Thanks tom grabowski