If i run a 50 hz three phase motor in very low speed eg: 5-10 hz control by vfd, will it damage the motor?

Answer 1

You would only damage the motor if an excessive amount of current were allowed to flow through it, thus overheating it and burning out the coils or melting the insulation on the wiring. If your vfd limits the current, then things should be fine.

I once used a 150-watt incandescent light bulb in series with a chopped Electrolux vacuum cleaner, in order to slow the motor and cut the noise. (This was to power an old pedal-pump reed organ.) Because the bulb limited both the voltage and the current, the motor worked fine for many years.

Answer 2

running a motor at higher frequency is generally not damaging. running at lower will cause problems real fast. eddy currents in the iron can cause overheating even without the motor connected to a load. same is true of transformer devices.

Answer 3

the motor winding will be overheated.
there is hardly any counter emf if you run the motor at almost dc current.