I've been thinking about building a roller mill and have often wondered the same thing.
Essentially you're now talking about a direct drive?
I have no experience with the speed controllers; my grinders work with the stepped pulley arrangement.
I'm pretty ignorant concerning anything electronics; matter of fact, I'm just now learning how to wire a motor!
Never had a need to before because everything was always plug and play. Just goes to show that ignorance is not always bliss.:sad:
I've also wondered about using a gearbox as a reducer.
I think it comes down to accomplishing two objectives-enough torque and proper speed. Yet another issue is economics- which method is most economical yet still performs the job?
Speed (slowing it down enough) isn't the problem. This could be accomplished by your idea of speed-controlled motor, gearbox, sprockets, or even pulleys (although pulleys could slow it down enough, I scrapped that idea due to possible belt slipping issues- too bad, because I have a HUGE selection of big pulleys).
I have no idea what amount of torque is required for the rolling mill. I have heard that one of the advantages of the speed controllers is that torque is stabilized even at the lower speeds- I don't know enough about it.
If one could figure out how much torque is needed for this application, the motor (for a direct drive setup) could then be sized accordingly. Just guessing out of ignorance, but using a direct drive motor for this may need a bigger motor than our typical belt grinders. From what I gather, the controllers become exponentially expensive as the motor size increases. Again, I don't know enough about it to make the call.
But, something to take into consideration is that sprocket and gear arrangements have long been used to "amplify/increase" torque. I don't think it's a matter of increasing torque, but rather using the same amount of torque more efficiently. In other words, I think we are increasing leverage.
When you're riding a ten-speed bicycle and encounter a hill, you change gears (sprocket size) to make it easier to move the same weight (your body and the bike).
Your "motor" size (the strength of your body) does not change. The load (weight of your body and the bicycle) does not change either. But the work to be done is now "easier" because of a different sprocket size.
I'm certainly not an engineer of any type, so I do not understand the principles involved, but I have to wonder if the sprocket and chain setup is what allows us to use a "standard" motor on the roller mill.
I'll watch this thread with interest.
Robert