Motor Applications involving VFD (Variable Frequency Drive)

VFD advent

The common method used to control the speed of motors is to allow them to operate at full speed however there are other applications where speed variation is required to get the desired results.

For this purpose, a variety of techniques were in place before the advent of variable frequency drive (VFD) to control speed. Some of the examples are dampers, control valves, fossil fuel engines, variable pitch sheaves, eddy current clutches, and DC solid-state control.

Dc motors were quite commonly used for achieving the speed required for the application and they were considered to be the best shape of VFDs suitable for this purpose in the past.

We need to understand the different facets to select the suitable VFD for motors.

VFD features and functions

Every VFD block comprises the following items as follows:


DC Bus or DC Link is used to connect the rectifier output to the input of the invertor. It has the important function to filter out the smooth from the uneven output which is close to pure DC.


It acts like a complete full-wave rectifier that is converting the applied AC to DC.


Invertor working comes after DC bus as it takes up the filtered DC from the DC bus changing it into a DC waveform. This pulsating waveform thus controls the inverter output and manages to simulate an AC waveform at various frequencies as desired.

Variable Frequency Drive (VFD) basic Principle

VFD has a basic principle of working that is adjusting the electrical supply to an AC application having a corresponding frequency and change of voltage in the motor speed.

VFD is used widely because it saves energy consumption as it matches the capacity with the actual load. It is also beneficial and supportive in reducing the wear and tear of the motors

Variable Frequency Drive (VFD) drawback

Well, VFDs has a drawback that it cannot perform well as required in no flow or near zero flow conditions and hence not at all suitable for specific appliances. However, appliance repairs if required with respect to VFDs are available to make your work easy going in this fast world.

VFD Types

VFD has the following three main types:

  • VSI,
  • CSI,
  • and PWM


VSI is defined as a voltage source inverter. It is the most commonly used VFD and a simple procedure is used for the conversion of AC signal into DC including a capacitor for storage of energy.

It has a good speed range with multiple control facilities with Red, Blue, and Yellow color circuit wires for connecting multiple motors. It has a simple design and is cost-effective too.

Its disadvantage is that its load motor faces jerks at the time of start and stop due to cogging effect with noises.


CSI is the abbreviation for the current source inverter. It provides smooth voltage output as per its variable frequency range as in CSI type VFDs the construction is depending on current and not voltage. It uses an SCR bridge converter instead of a diode bridge. The CSI acts just like the constant current generator. It is providing a square wave of current instead of a square wave of voltage.

CSI type is more durable and reliable as compare to VSI having a simple design yet good generation capabilities in it. It can manage to support the high horsepower induction motors whereas VSI is not at all supportive in this regard.

It also contains some disadvantages like VSI that due to cogging factor, we can face vibrations of the motor shaft running. At low RPM, the overall power factor is poor and is not fit for multi-motor operations.


PWM is considered as the improved version of VSI type VFDs. It is known as pulse width modulation. With this type, VFDs can provide a stable voltage output along with frequency being well maintained.

It is free of all kinds of clogging and jerks now having ideal speed under controlled range. It also contains various kinds of protection circuits with constant power with energy-efficient capability.

Its drawbacks include its complex design in terms of implementation. It also requires additional hardware along with expensive solutions in case of any fault.