vfd means varieble frequency device, use to change the 
speed of motor as per requirement. power is directly 
proportional to frequency and to reduce the speed you are 
reducing frequency interms power is saving is being done.

In VFD the speed of motor is controlled by changing syn. 
speed of motor. This is done by giving variable frequency 
supply to motor. In order to keep flux constant (i.e. V/F) 
voltage is also reduced alongwith frequency for lower speed 
values thereby reducing input current & ultimately power 
saving

good amount of power can be saved when controling the speed 
of PUMP AND FAN,when using a VFD.
the pump and fan works on AFFINITY LAW ..ie..the speed is 
directly proportional to the cube of power.
if 20% of speed is reduced there will be a saving of 50%

VFD IS MORE POWERFUL THAN ANY TOOLS BECAUSE IT USES SOLID 
STATE DEVICES LIKE MOSFETS AND IGBTS WICH HAVE A LOWER 
POWER CONSUMPTION, BUT IF WE USE RESISTANCE ALOT OF POWER 
WILL BE DISSIPATED IN RESISTANCE

Variable frequency drive,if one motor working full speed ex-
chilled water pump but there is no full load if u fix a VFD 
in motor frquency propotional to speed when reduce 
frequently automaticaly reduce also speed.that time motor 
will run depending upon load,this type we can save power.