Frequency Reduction Through VFDs

Keep in mind that your gear or pulley system reduces speed but INCREASES torque.

If you reduce speed using a VFD you will only get the rated motor torque.

Also, when reducing speed it is best to fit an externally powered fan as an integral fan will not cool the motor adequately.

I think it is the best option as motors at lower frequency to those are wound for continuous duty cycle is not recommended and reduction by gears is the best option.

As for as VFDs are concerned these soft-starters [means from ~zero Hz] and to take the motors to their operating speed in preset time at a linearly rising slope [Hz/Sec].

How can this be off topic when he asked:

"Cant we use only a VFD reduce RPM to the lowest and hence remove the gear or the pulley mechanism."

The safest slowest speed of any Nema design B motor is 40% rated speed. Be low that, you need to look more closley as to the MFG's lowest speed rating. Like other posters, there is a lot to consider when using VFD's.

In the constant torque mode you can normally go in the ratio of 10:1.If the operation is likely to be for a longer duration you cannot have a standard fan cooled motor operating at 5Hz as the shaft mounted fan will not be able to give sufficient air.So additional cooling by blowers is required .VFD specially designed motors with built in blowers can be used.It is important to know the duration at each speed while selecting a motor to be used with VFD.

If you're looking for slow, controllable rotation, why not look at stepper motors?

another option is use motorised gear units (Geared motors) with VFD if u are looking at very low speed for longer duration.

Actually I have an invention that allows to develop a new type of motor at one millionths of rpm. It is free of ferromagnetic materials and its free of all faults of induction motor. Would you like to invest into my invention? Perhaps buy a license to produce it?

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ordinary motor does not run well when frequency lowers below 20 Hz. It become noisy and heats up. What exact rpm do you need? What is the needed torque for that rpm? Do you want it to be light?

I think the very low rpm is easy to achieve with stepper motors that are used in motion controls. You can get one with resolution 1000 steps per full revolution. so the rpm could be 1/100000. if you have proper stepping controls. In addition the precision would be much better than that of reduction gears.