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VFD | What Is A Variable Frequency Drive?

Updated: Jan 14

An estimated 65 percent of industrial energy use is accounted for by electric motors therefore any solution that helps increase the motor performance efficiency has the potential to result in huge savings. This is where Variable Frequency Drives (VFDs) come into the picture.


VFDs are the most effective method of regulating motor speeds. By adding a VFD to the system we can take advantage of the motor's characteristics at multiple speeds instead of being limited to only 50/60Hz. VFDs allow us to match the speed of the motor-driven equipment to the load requirement and operates the motor at full-torque capability over a wide range of frequencies. Having full-torque available also allows us to use a standard or general-purpose motor in many applications.


1. Mains Input

Mains Voltage (230VAC / 380VAC 50/60Hz)


2. Rectification

This stage involves transforming the input AC voltage into the desired output for the motor. This is achieved by using a full-wave diode bridge rectifier which converts the AC voltage of the electrical supply into a DC voltage.


3. DC Bus

The DC Bus is a bank of capacitors that stores the rectified DC voltage. A single capacitor can hold a large charge, but arranging them in a DC Bus configuration increases the capacity. The stored voltage will be used in the next stage where the IGBT's produce the required power for the motor.


4. IGBT (Insulated-Gate Bipolar Transistor)

The IGBT is well-known for its high efficiency and fast switching and in this final stage, they are switched on and off in a pre-defined sequence to create pulses of varying widths from the DC Bus voltage to create the VFD's output waveform. By using Pulse Width Modulation, or PWM, the IGBT's can be switched on and off in a sequence that resembles a sine-wave superimposed on a carrier wave. This signal is then used to control both the speed and torque of the electric motor.


Controlling motor speeds has the additional benefit that sudden increases in production can be accommodated quickly without the need for extra equipment, as VFDs can handle RPM increases of 5–20 percent. By matching the performance of the motor to the needs of the process, VFDs can help cut down on costs greatly when compared to the wasteful practice of running the motor at full-speed.


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