Variable frequency drive Working principle and circuit diagram

Variable frequency drives are most widely used for starting and running of Induction motors. The basic function of VFD’s is to control speed of motor according to load requirement which will also lead to power saving along with speed control. Although VFD’S cost initial cost is very high in comparison to Star- Delta and Soft- Starters but due to power saving application of VFD will leads to cover it’s extra cost very easily.

Now let’s discuss how VFD’S work;

VFD’S consists of following parts:-

1.Full wave rectifier

2.Filter circuit

3.Control circuit 

4. Inverter circuit. 

Circuit diagram consists of all parts is shown below:-

VFD Circuit Diagram

VFD circuit working is as discussed below:-

First of all AC 3-Phase supply given at VFD I/P that I/P will get converted into DC through full wave rectifier if VFD is given 430 Phase to phase voltage then DC voltage that we get after full wave rectifier will be around about 690V.

That voltage will appear across filter circuit which consists of capacitor and inductor called as choke. It is very heavy part in VFD usually mounted backside of VFD.

Then that DC voltage is converted into AC through control circuit and inverter circuit. Inverter circuit consists of IGBT’S which through controlled firing will give AC voltage at O/P as per requirement. At O/P circuit load is connected.