Frequency Inverter

The Frequency Inverter or Frequency Converter is sometimes also called VSD (Variable Speed Drive), VFD (Variable Frequency Drive) or simply Drives, which is probably the most common name.

This device is very common use in industrial sector because it user friendly and easy to install.It usually can control many equipment or machinery such as :

  • Air-Condition
  • Blower & fan
  • Water pump
  • Food processor machine
  • Conveyance machinery
  • Packing machinery
  • Textile machine
  • Machine tools
  • Furnace
  • or ETC

How the Frequency Inverter Operation ?

Basically this device control the speed of motor  ( RPM ) with manipulate the frequency ( Hz ).

If we refer the basic formula for speed of motor ( RPM ) :-

ns = 120 x Hz / p

ns = synchronous speed ( RPM ) ; Hz = frequency ; p = number of pole

The speed ( RPM ) of motor change when Hz ( Frequency ) is change too.No need to adjust the ampere or voltage to  change speed of motor can operate with full efficiency.

By controlling the frequency, the rated motor torque is available at a low speed and the starting current is low, between 0.5 and 1.0 times the rated motor current, maximum 1.5 x In.

The drive consists primarily of two parts, one which converts AC (50 or 60 Hz) to DC and the second part which converts the DC back to AC, but now with a variable frequency of 0-250 Hz.

As the speed of the motor depends on the frequency this makes it possible to control the speed of the motor by changing the output frequency from the drive and this is a big advantage if there is a need for speed regulation during a continuous run.

In many applications a drive is still only used for starting and stopping the motor, despite the act that there is no need for speed regulation during a normal run. Of course this will create a need for much more expensive starting equipment than necessary.

Soft STOP function

It is very common to install a filter together with the drive in order to reduce the levels of emission and harmonics generated.

For example when stopping pumps where the problem is water hammering in the pipe systems at direct stop.

The softstop function is also useful when stopping conveyor belts from transporting fragile material that can be damaged when the belts stop too quickly.


Advantages of inverter controller?

  • Energy Savings
  • Improved Process Control
  • Reduced Voltage Starting
  • Lower System Maintenance
  • Bypass Capability
  • Multi-motor Control


  • Initial Cost
  • Motor Heating at low Speeds
  • Maintenance
  • Output Harmonics
  • Induced Power Line Harmonics

For more detail information,please refer to VARIABLE DRIVE HANDBOOKS :


  1. Sometimes, frequency inverter may be a must, for example, precision control is needed.

    Of course, servo motor and drive are more suitable.

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