Jeff covered all the options well. I agree with his recommendation to use a 3 phase motor and VFD. The one thing I wanted to add is that the 3 phase motors you'll buy have a fan to cool them, and are designed to run at rated speed. When you run them at slow speed, the fan is not effective for cooling the motor so if you plan to do a lot of slow speed operation, get a small fan and point it at the motor to keep it cool. Otherwise, the motor can overheat and fail.
Even though you're running it at a slow speed, the motor is generating about the same heat as when it's running at rated speed.
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