C Motor Low RPM Problem

With those specifications I can only envision that this is a Universal motor. In a Universal motor, torque increases as speed decreases, but at the same time current increases, so at some point the motor design will allow the magnetic flux to saturate at low speeds and torque capability drops off significantly once that happens. If you have ever tried to use a 3000RPM drill motor at really low speed like 150RPM, you can see and feel this issue; the motor cogs and jumps a little rather than run smoothly and quietly. But if you use a 300RPM drill motor and run it at 150RPM, no problem at all. So it is best to use a motor which is designed to run at the same top speed as your application, because using a much faster motor sacrifices torque stability and capability at low speeds.

So bottom line raf23, the problem here may be that you are attempting to get greater than a 6:1 speed range from a motor that is best used at no more than 4:1. If you never use it at 6000RPM, consider matching your top application speed to the motor design so that you get better performance at the lower end. If you need a range that is fully 6000RPM down to below 1000RPM with full performance throughout, you may need to use a different strategy on your motor selection.