In order to prevent saturation in the motor's iron core, VFDs will reduce the voltage proprtionally to the reduced frequency. This means that the availabel power will be reduced by aproximately the square of the reduction in speed. (P=V^2/R)
I am hopeing that some of you will call me on over simplifying the motor model, and well I did. It is true that the apparent resistance of a motor is affected by the loading, a higher loaded motor will have a lower apparant resistance becuase of the increased power needed to feed the load. This variability has limits, that is why a 1 HP motor wil not turn a 15 HP load.
VFDs typicaly work quite well on blowers since blower loads typicaly have very linear torque speed curves, asuming that a frozen bearing does not throw a wrench in the works.
