Variable Frequency Drive (VFD)

Last updated on September 3rd, 2023 at 02:00 pm

Variable Frequency Drive controls the working of a motor, and it optimizes its working by taking advantage of different functions a VFD can offer. VFDs are only used with the AC motors, not with the DC type motors.

Electric motors have become an essential part of the industry; in almost all types and the ways, the motor drives different functions are uncountable. From high-speed mixers to a conveyor belt in a packaging line, the system remains incomplete without the involvement of the motor and its control is a very critical step. 

Previously, the most common method to control the motor was through Gearbox-based mechanisms. The gearbox only allowed limited capability of controlling, with difficulty in operation and maintenance. Moreover, the replacement procedure was also time-consuming and difficult to perform. 


Variable Frequency Drive controls the motor’s speed by varying the frequency supplied to it. The motor’s speed is directly related to the frequency, so we can effectively control the motor’s speed by changing the frequency according to our needs.

The Variable Frequency Drive working consists of main three main steps

1. AC to DC Conversion

In this stage, the incoming AC voltage converts to DC type. A Bridge Rectifier consisting of six diodes constitutes the circuit that performs this function. The DC output at this stage contains ripples that are of no use, and it attenuates the signal. The AC ripples are eliminated and sent to the next stage.

2. Filter Section

At this stage, any ripples in the DC voltage, from the previous must be removed. The output from the converter becomes the input to the capacitor circuit that serves as a filter. This filter removes the filter resulting in a smooth DC waveform.

3. DC to AC Conversion 

This is the final stage that results in an output, ready for motor use. In this stage, the DC converts back to the AC form and is called the Inverter stage, and thus the reason for another name of VFD i.e. Inverter. Here, the conversion takes place through the use of Power Electronic devices, called IGBT, and the technique is called Pulse Width Modulation.


The Variable Frequency Drive can be advantageous in many ways depending upon the particular application in which it finds its application. However, there are some advantages common in every industry resulting in its dominance and reliability.

Variable Frequency Drive can perform many functions for the motor. That means you can use a single VFD for more than one application, without the need for any additional hardware.

Flexibility in Speed Control 

VFD can be used to operate the motor at different levels smoothly. Depending upon the application, we can use a manual system through a potentiometer, through PLC or controller. We can also fix the speed at the same level.

Ramp-up and Ramp-Down

In some applications, mainly in the mechanical movement system, the motor’s speed should gradually increase to the maximum value and similarly gradually decrease to the zero levels. It is because it can cause wear and tear in the mechanical system. The VFD operates the motor in this procedure, often called Ramp Up and Ramp Down, respectively.

Easy Replacement

We can replace the VFD without any substantial and time-consuming procedure. Only the wiring needs to be re-connected after the new VFD installs back. The replacement procedure is also trouble-free as the VFD is in an accessible location, in the electrical panel.

The programming is also straightforward. You can parameterize the VFD with the buttons at the local control panel, without the need for additional software.

Built-in Safety System

The Variable Frequency Drive comes with many safety features, for both Motor and VFD itself. In case of any problem with the motor, it forces the operation to stop, until the fault is gone. This ability enables safe working without spending on additional safety components.

How to select a VFD for your application

When we talk about motor usages, there are many types of motors, each designed for a particular power requirement. When selecting a VFD for different types of motors, the following are the main points to consider

Input Voltage 

Input voltage is the voltage that is used to power-up the VFD. Some VFDs require three-phase 380 – 400V input while others require single phases 220 – 230 V.

Output Voltage

The output voltage is the voltage that directly goes out to the motor and is used to operate the motor. The output voltage can be single phase 220 – 230V or three-phase 380 – 400V. It is because some motor is single phase motor and others are three-phase motors

Load Capacity

Load capacity is the parameter that describes the load a VFD can bear. It depends upon the motor’s size. A small motor requires a small load capacity while a large motor requires a large load capacity. It is mentioned in kW (kilowatt) or HP (horsepower) or A (ampere).

Motor Control Method

This feature describes the method through which the VFD controls the motor. It includes a motor start/stop and commands for controlling motor speed.

Speed Control

For speed control, the following methods can be used

  • External Potentiometer: External potentiometer is used to control the VFD’s output, which directly controls the motor’s speed.
  • Reference Signals: In this method, a PLC or a controller is used to generate analog signals to the designated VFD’s input. These analog signals are then used to control the VFD’s output, resulting in different speed levels. The reference signals can be 0 to 10 volts or 4 – 20mA.

Motor Start / Stop 

The following methods are the most common for motor start and stop controls:

  • Start/Stop button: The motor starts/stops with an external button. 
  • PLC/Controller: the PLC or a controller is used to generate the digital output which lands in the VFD’s digital input.

Networking Methods

These allow the VFD to communicate on the network. They can be serial type or Ethernet type.