The problem is that the pulleys give you a speed reduction and torque amplification.
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To run your motor that slow and to expect the torque you need you will most likely overheat it. For light stuff it will probably work.
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To get a motor to do what you want you will need to up the size and add some sort of external cooling mechanism to the motor.? Three phase motors designed to run low rpms usually have a fan run by a separate motor to keep it cool. Now you are looking at a larger physical size, so you will still need some sort of drive setup to run the spindle.
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Keep the motor where it is and run a single pulley speed keeping some sort of reduction to up the torque.
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What is the difference if you have a short belt or a long one you will need some sort of drive setup to the spindle.?
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A cog belt setup may be the optimal solution.
