How Variable Frequency Drive (VFD) Work

- May 31, 2018-

    It is important to understand how the frequency converters (VFDs) work because they are widely used in AC motor-driven applications such as variable frequency drives for motor control because they have a wide variety of characteristics. Compared with traditional motor drives, VFDs have greater functionality and operating capability. In addition to adjustable speed control, the frequency converter also provides phase protection, undervoltage protection and overvoltage protection. The VFD's software and interface options allow the user to control the motor at the required level.

    The two main features of the frequency converter are adjustable speed and soft start/stop functions. These two features make the VFD a powerful controller for controlling AC motors. The VFD is mainly composed of four parts; those are rectifiers, intermediate DC links, inverters and control circuits.


This is the first phase of the variable-frequency drive. It converts AC power from mains to DC. This part can be used on a one-way or two-way basis for applications such as four-quadrant motors. It uses diodes, thyristors, transistors and other electronic switching devices.

    If a diode is used, the converted DC power supply is an uncontrolled output when using a thyristor, and the DC output power is gated. Three-phase conversion requires at least six diodes, so the rectifier unit is considered to be six pulse converters.

DC bus:

The DC power from the rectifier section is fed to the DC link. This part consists of capacitors and inductors, which can smooth ripples and store DC power. The main function of the DC link is to receive, store and transmit DC power.


This part consists of transistors, thyristors, IGBTs and other electronic switches. It receives direct current from the DC link, converts it into alternating current, and sends it to the motor. It uses pulse width modulation such as modulation technology to change the output frequency to control the speed of the induction motor.

Control circuit:

It consists of a microprocessor unit that performs various functions such as control, configuration of driver settings, fault conditions, and interface communication protocols. It receives the feedback signal from the motor as the current speed reference, and adjusts the voltage to frequency ratio accordingly to control the motor speed.


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