Variable Frequency Drive- Basics, Working and Selection of VFD

What is VFD?

VFD is variable Frequency Drive, it is a device that controls the voltage and frequency that is being supplied to a motor and therefore controls the speed of the motor and the system it is driving.

Why VFDs are required?

  • Better process control and regulation.

  • Speeding up or slowing down a machine or process.

  • Emergency bypass capability

  • Protection from overload currents

  • Safe Acceleration

  • Run a machine or process at a desired speed.

  • Produce adequate torque to handle the load.

  • Use power efficiently to produce the necessary torque at a given speed.

  • Effectively monitor the application or process.

Principle of VFD:

A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor. The drive also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio of voltage to frequency (V/Hz).

In first step, the VFD converts AC to DC using a rectifier. The DC power supply contains voltage waves that are smoothed with filter capacitors. This section of the VFD is often referred to as the DC link. This DC voltage is converted back to AC. This conversion is typically achieved through the use of power electronic devices such as IGBT power transistors using a technique called Pulse Width Modulation (PWM).

The output voltage is turned on and off at a high frequency, with the duration of ON- time, or the pulse width, controlled to approximate a sinusoidal waveform.

Selection of VFDs for AC motors:

The application of VFD can effect these things, which should be under consideration:

  • The ability of a motor to cool itself effectively is reduced as the motor is slowed down.

  • Different speed can cause mechanical resonance and These speeds should be identified and programmed out of the motor’s operating range.

  • The installation of filtering devices should be considered at the time of purchase of VFD to minimize the problems of power quality in the electrical system.

Electrical consideration:

  • The nominal supply voltage of the distribution system is normally higher than the drive nameplate voltage to allow for voltage drops from the distribution transformer to the point of utilization.

  • Single phase drives have standardized voltages of 120 and 240 volts. Three phase motors have standardized voltages of 200, 230, 460 and 575 volts.

Motor consideration:

  • Application of a PWM VFD can cause voltage transients well above the rated voltage of the motor which can lead to failure of the insulation system in a very short period of time.

  • Many VFDs provide for user adjustment of the switching frequency. This frequency can be adjusted over a range as broad as 500 Hz to 20 kHz. Switching frequency defines the number of voltage overshoots occurring at the motor in a certain amount of time.

  • Use lower range frequency.

Environmental issues:

  • VFDs must be selected to ensure that they have adequate protection from their environmental conditions.

  • VFDs are usually mounted into an electrical enclosure with other electrical devices, or as a standalone unit in its own enclosure.


  • It can limit torque

  • Starting and stopping with controlled acceleration/deceleration

  • Reduce maintenance cost

  • Control of process temperature and pressure.

  • A VFD can be adjusted to produce a steady 150% starting torque from the standstill right up to full speed

Constant torque applications include:

  • Conveyors

  • Extruders

  • Mixers

  • Positive displacement pumps and compressors

  • Exhauste fan application