Variable Frequency Drive or VFD is the system that controls the speed of a rotating device such as motor by applying varying frequency AC voltage. It is a kind of adjustable speed drive widely used in domestic and industrial applications such as pumps, ventilation systems, elevators, machine tool drives etc. The Variable Frequency Drive is also called as Inverter drive or variable speed drive. By controlling the output AC frequency, it is possible to drive the device at different speed based on the requirements. It is essentially an energy saving system.
An Inverter system is meant for generating AC voltage from DC using an oscillator circuit and transformer. It generates high voltage Alternating Current from low voltage Direct Current. Oscillator circuit gives pulses of a specific frequency while the transformer produces the alternating current. The transformer induces an electric current in the coil and the changing magnetic field causes AC to flow in the other coil. The coil through which the current passes act like a magnet and produces a magnetic field. When the direction of current changes, polarity of the magnetic field also changes. If the number of turns in the secondary coil of transformer is twice of the primary coil, then the secondary voltage will be twice that of the primary voltage. So by increasing the number of turns in the secondary coil, it is possible to generate high voltage from a low voltage.
If DC is applied to the primary coil of the transformer, it will not induce current in the secondary since there is no change in magnetic field. If the direction of the Direct current in the primary is changing alternately, it is possible to get a square wave output at the secondary. This is the basic principle of the working of Inverter.
In short, the Inverter system converts low voltage DC to high voltage AC with a frequency close to the mains frequency (50Hz / 60Hz).