For best results you need to go VFD and 3-Ph or DC Gearmotor
by Bruce Bergman
Getting a consistent low speed in the 200-RPM range will be difficult on a direct drive motor, they usually don't have a lot of power left when they get down that slow, and if it stalls you have a problem. If it stalls while unattended (nobody there to kill the power) that can turn into a Huge Problem.
First thing, if the speed doesn't have to be exact or precisely controlled, plan on going with some sort of a gear or belt drive to get the speed reduction you want. You might be able to use one of those induction motors at the 1800 RPM native with the reduction doing the work of dropping the speed.
If space and packaging are an issue, you can get Inline motor and gearbox combos where the shaft comes out parallel to the motor shaft, and they slide right into the same spaces.
If you need true variable speeds, the easiest way is a Variable Frequency Drive - an electronic power supply that puts out 3-Phase 240V to run a small 3-phase "Inverter Duty" rated motor - that part is important because basic motors can't handle the widely varying speeds and voltages the VFD puts out at the far ends of the speed spectrum.
VFD's are available in all sizes you might need (1/4 HP to 100 HP) where they'll take Single Phase 120VAC or 240VAC as input, very useful for running a regular 3-Phase motor where you can't get it from the local utility - but you have to read the spec sheets before ordering.
You can even do tricks like control it from a computer, or set automatic ramp-up and ramp-down speeds, and a maximum torque alarm and shutdown if something jams, instead of just Off and On.
The "Inverter Duty" motors will gladly run at the extreme high and low speeds a VFD can put out, where a cheap 3-phase motor will just let out the "Magic Smoke" and die. But read the instructions, they do have limits.
The other way is standard industrial gear, the 90V DC motor and a commercial speed control power supply - Grainger (Dayton) makes a ton of those too.