Why VFD is needed in a system and how can it control a motor

What is a Variable frequency drive

A variable frequency drive can be used to control the frequency and voltage which is supplied to the motor and thus it can control the speed of the motor and the system it’s driving. The rotational speed of the electric motor can be controlled by the VFD by controlling the frequency of the electric power supplied to the motor. The electric motor can be either an induction motor or synchronous motor VFD is also called as variable speed drives and variable frequency drives.

Why VFD is needed in a system

  • It can do better process control and regulation
  • It can speed up or slow down a machine or a process
  • Inherent power factor correction
  • Energy-saving can be done
  • Soft start and smooth acceleration can be achieved by using this
  • It can reduce power supply problems in a facility
  • It can reduce motor heating and stress
  • Protection from overload currents
  • Safe acceleration
  • It can produce the required torque to handle the load
  • It monitors the application or process effectively

Why VFD is used for HVAC applications

  • Improved energy saving
  • Simplified motor controlled panel design
  • Improved temperature regulation
  • Reduced mechanical component stress
  • Reduction in noise
  • Less energy consumed

It adjusts the speed of HVAC based on the demand to save energy and to increase the component life. VFD conserves energy when an HVDC control system senses that a motor can meet heating or cooling needs by running at less than 100 percent power


The main components of VFD are rectifier circuit unit, DC circuit, and inverter circuit unit, controller

The rectifier converts the AC input to DC and then charges the capacitor in parts of the circuit, the DC power from the rectifier is fed to the DC circuit and it has capacitors and inductors to smooth the ripples and then it stores the DC power. Then the inverter circuit section will receive this DC power and then it converts it into AC the components in this section consists of IGBT, thyristors …

The controller section varies the frequency of the received AC power and then the output of the converter is used to drive the electric motor

Types of VFD

On the basis of control, it is classified into

  • Constant voltage converter: the intermediate DC link voltage remains approximately constant during each output cycle
  • Constant current drives: large inductor is placed between the input rectifier and the output bridge so the delivered current is nearly constant
  • Cyclo converters: by the help of rectifiers and inverters each output terminal is connected to appropriate input phase

VFD operation

When an induction motor is connected to a full voltage supply it draws more current than it is rated, when the load accelerates the available torque decreases a little and rises to the peak and the current remains very high until the motor reaches its full speed

When a VFD starts a motor it applies low power and frequency to it and due to the low starting frequency the inrush current can be avoided after that the frequency and power is increased in a controlled manner without drawing excessive current

Why do we need to use line reactors in VFD

Due to the installation of a large number of VFD’s or other equipment will distort the current and the voltage waveforms of the power supply, in that case, we can use line reactors which provides additional smoothing to that provided by the DC choke and it will reduce the VFD disturbances


  • It is used to control motors, fans, pumps
  • Oil & gas, petrochemistry, oil transfer pumps
  • Water supply pump
  • HVAC fans and pumps
  • VFD can be used for any machining process which can be improved by varying speed or flow