The speed depends on the size of the disks. If you have big disks (3 to 4 m diameter), then 1 to 2 rpm seems to be normal. For a smaller version, it can go until 5 rpm.
But don't worry, this doesn't play a big role. The biggest issue with a RBC, it is when you stop it for somes hours, and run it again. You will have a big unbalance, and it can break the motor or the shaft. So take care of that.
Second issue : the bearing. do you forget to maintain it well. Without grease, the bearing won't resist a year. And it costs a lot to remplace.
For the inverter Duty, it is a plus but not needed if the conception of your RBC is ok. Do you have a plastic coupling? What is the brand of your RBC? Is it a hollow shaft or a full shaft?