Why motor trip at 20% speed running with AC drive
Install a blower with a sail switch to interlock on and moving the required quantity of air. Use the AC drive and motor for what they are designed and rated for. Remember the motor and AC drive are current devices and this is their limit.
As mentioned you could also put in a reduction drive and if necessary run the motor above base speed, but I doubt that this is necessary at 20% of base line speed.
As to the spikes generated by the AC drive the inverter duty motors have extra insulation at the input to the winding to prevent chronic discharge and the damaging effects and punch through of the insulation. After a few turns the inductive reactance reduces these spikes and they are no longer a problem.
Also when using a constant speed you should use a motor with an insulated ODE bearing or a shaft grounding brush to eliminate arcing from induced currents in the motor shaft. Most large motors and all synchronous motors will have this insulated opposite AC drive end bearing for this reason, even before variable frequency drive were invented.
The constant speed applications tended to have the discharge at the same point on the bearing and accelerated the pitting of the bearing when used in clean room fan applications.
