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Trying to make a buffer or a belt grinder? You can also use belts and wheels, calculate ratio for size to speed and if a single slower speed is what you want- you'll have more torque at lower rpm.

correct, while there are workarounds such as a variac- would work for speed- with all of these you are suffering loss of power with loss of RPM. What you would do best with is a tri phase, DC perm...