Types of VFD- Variable frequency drive
and applications.

  1. Variable frequency drive.
  2. Types of variable frequency drive (VFD).
  3. Applications of variable frequency drive.

1 - Variable frequency drive :

A VFD- Variable frequency drive is electronic device which is used for controlling the speed of AC induction motor. It has two main functions that one is speed controller and second is energy savings. Motor speed is controlled according frequency control which is controlled by VFD. It is mostly used in water pump motor, air blower motor, fans etc.

2 - Types of variable frequency drive :

Different types VFD are used like as single phase VFD, three phase VFD etc other details of type below mentioned.

  • VSI- Voltage source inverter
  • CSI- Current source inverter
  • PWM- Pulse width modulation
1- Voltage source inverter:

VSI means voltage source inverter where the output voltage is depend on output frequency. In rectifier section the diode is connected as bridge type and a capacitor is also used for getting smooth DC then it is converted to AC with support of switching electronic components. Multiple motor can be controlled.

  • It is constructed in simple type design.
  • It can be purchased in low cost or cheaper rate.
  • It is good feedback of speed variation.
  • According requirement of group motors these motors can be used frequently.

During low speed the power factor is measured in low condition.

When motor starts and stops then it takes jerks due ot cogging effect.

During motor speed ramp up or ramp down, the power factor got hampered as low power factor.

2- Current source inverter:

CSI full form is current source inverter which is used for speed variation through current. Here the SCR based bridge circut is used in rectifier section in place of bridge rectifier.


It is better than VSI.

It is used in high current ratings motors

The regenerating capabilities are available.

Simple design and user interface.


The power factor in low specially when the RPM is low.

Multi motors can not be running in comparison of VSI drive.

It is expensive in comparison of VSI drive.

3- Pulse width modulation:

PWN- pulse width modulating is a advance updated model of VSI drive where a regulator is used for regulating the signal of output.

  • Power factor is improved.
  • Free from cogging during start and stop time.
  • The efficiency is good in comparison of VSI and CSI drive.
  • The design is very complex.
  • A additional hardware is required for output signal controlling.
  • It is expensive in comparison of VSI and CSI.

3 - Applications of VFD :

  • AC chiller system motors and blowers are controlled for temp and smooth running.
  • Water pump system motors are controlled for water pressure and smooth running.
  • Lift and escalators are controlled for start and stop smooth operation.
  • Now a days the VFD is using as soft starter for smooth start and stop motors, blowers and fans etc.

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