VFD is a device which can control the speed torque and direction of an AC motor.
Other common terms used for a VFD are AC drives,Inverters,Adjustable frequency drive or adjustable speed drive.
An AC motor speed is related to the frequency so that the higher the frequency the higher the RPM or speed of the motor.
Have a look at the schematic of VFD. Internal Architecture of VFD.
You can see 3 arrow looking devices on the left.These arrow looking devices in the electrical world called diodes.In the mechnical world we might think
of check valves.They basically let current flow only in one direction that is the direction of the arrows.So we have an AC to DC converter.These convert
3 phase AC to DC.DC that comes out of these is rather chopppy and not very smooth.So we put in a capcaitor in order to clean that DC.
The capacitor is like a reservoir in a plumbing circuit and that reservoir smooth out everything and gives us a clean DC.
Then going to the right wee see six switches.These six switches composed the DC to AC inverter.So by switching these switches ON and OFF we can create
any frequency we like and that frequency will regulate the speed that the motor goes.So this entire circuit is called a Variable Frequency Drive.