A report by U.S Department of energy ( Read first two pages)

http://www.p2pays.org/ref/40/39569.pdf

describes the effect on the efficiency and Power Factor according to the decreasing load.

Efficiency decreases dramatically below 50 % of rated Load

Power Factor decreases dramatically below 65 % of rated amperage

Are these effects on efficiency and P.F all valid in case of a VFD?

Thanks in advance

 

Salar... Ignoring losses of the VFD itself, motor losses will increase, although not significantly.

The reason is that the VFD introduces electrical phenomena not present when the motor is powered from a sinusoidal power source. The curves in the DOE report you listed are the result of appication of a sinusoidal voltage supply.

Two of the deleterious effects of the VFD output being a chopped-DC waveform are:

a) higher magnetic-core loss due to higher harmonic frequencies contained in the distorted waveforms.

b) higher copper-loss due to the skin-effect present in stator and rotor conductors.

In conclusion, resultant lowering of motor efficiency is insignificant in light of the overall savings resulting from speed adjustment to match process need!

Regards, Phil Corso