VFD stands for Variable Frequency Drive. VFD is used to speed control of AC induction motors and protect the motors. A VFD can also control the speed of motor during start and stop cycle, as well as throughout the run cycle. VFD is also known as VVVFD. The meaning of VVVFD is Variable Voltage Variable Frequency Drive. It means with change in frequency voltage can also be changed and vice versa. 

Working of VFD

The working of VFD is very simple. An AC supply given to the VFD which is first converted into DC supply then it is converted into AC supply again at desired frequency and voltage. Changed magnitude of frequency and voltage used to control of speed of AC induction motors. For better understanding, we write a relation between speed and frequency which are given below

N= (120×f)/P

NS = Synchronous speed in r.p.m

f = frequency of power supplied or input power or supply frequency

P = No. of Poles

In above relation we can see, with change of frequency of input power the speed of the induction motor is changed. Simply we can say that the speed of the induction motor is directly proportional to the supply frequency.  
variable frequency drive VFD

Rectifier Converts ac into dc. SCRs are used in rectifier. In three phase rectifier six SCRs are used. Control Unit control the supply frequency and voltage. inverter converts dc into ac.

Advantages of VFD

VFD have many advantages. some of advantages are given below:-

  • VF drive save energy.
  • It improves the power factor of the machines.
  • It gives smooth starting to the induction motors.
  • It reduces the power when not required.
  • We can change the direction of induction motor very easily.
  • It provides controlled starting and stopping. 
  • It provides protection to the induction motors against short circuit, overload, earth fault etc.

Disadvantages of VFD

  • The initial cost of VFD is very high
  • Skilled workers are required to operate it.

Applications of VFD

  • It is used in industrial application such for the control of speed of motors.