VVVF drive systems

vvvf means Variable voltage variable frequency...... this system varies frequency but in such a way that Volatge/frequency ratio to be stay constant

A little further explanation . . .

Motors are inductive and therefore resist the flow of alternating current. The higher the frequency, the less current will flow. The lower the frequency, the more current will flow. At some point, the amount of current and the size of the wire (and hence its resistance) in the windings will cause the motor to heat up to a point that it will burn the insulation off the wire (that's bad).

Wire has a DC resistance and is not a perfect conductor, and if enough current flows through a wire of a given size, it will self heat. Copper is a pretty good conductor, so making heaters out of copper doesn't make much sense. Nickel-Chromium wire has a high DC resistance, and that's what you find in hot plates, foot warmers, etc.

In order to keep the current to a safe level, the applied voltage is reduced when the frequency is reduced. To keep it simple, the amount of current that will flow in a circuit is a function of the applied voltage and the circuit's resistance (reactance is the term used for AC circuits, but it is, nonetheless, practically the same -- reactance implies frequency-dependant resistance).