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Basics of VFD


What is Variable Frequency Drive: 

  • Variable frequency Drive (VFD) is also called Voltage variable frequency drive (VVFD) & Variable speed drive (VSD). 
  • Generally motor speed is controlled in two ways - either by controlling the voltage or frequency. Frequency controlling gives better control due to constant flux density than voltage control. This is where the working of VFDs comes to play. 

Fig. Delta MS300 VFD

  • It is a power conversion device that converts the fixed voltage, fixed frequency of the input power to the variable voltage, the variable frequency output to control AC induction motors.
  • VFD is a controller that drives an electric motor by varying the frequency and voltage supplied to electric motor. It is also use for both AC- induction motor as well as DC motors.
  • According to the variable frequency drive working principle, it's the electronic controller specifically designed to change the frequency of voltage supplied to the motor. The motor have its speed that can changed by changing the frequency of the input voltage used to power it. That means if the voltage applied to an AC motor that has rated frequency is 50 Hz, then the motor works at its rated speed, and if the frequency of the supply voltage is increased above 50 Hz then the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than its rated frequency, the motor will run slower than its rated speed.

                                                                
        the number of pole is constant the speed Ns can be varied by continuously changing frequency.

  • It consists of power electronic devices (like IGBT, MOSFET), high speed central controlling unit (such as a microprocessor, DSP) and optional sensing devices depending on the application used.
  • Compared to conventional motor drives, VFD has greater functionality and operation capabilities.
  • Software and interfacing options of the VFD’s allow the user to control the motors at desired levels.
  • In addition to adjustable speed control, variable frequency drives have other protections: 
  1. Phase loss protection
  2. Under voltage protection
  3. Over voltage protection
  4. Short circuit protection
  5. Open circuit protection
  6. Over current protection
  7. Low current protection
  8. Over heat protections.

Written by : Anuj Yadav
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