AC Drive (VVVF) Block Diagram and Working Principle

VVVF means Variable Voltage Variable Frequency. The VVVF Drive is one type of AC drive used in so many applications and industries. This drive system is mainly used to control or operate the electrical motors (generally three-phase induction motors) according to our requirements such as controlled speed or controlled torque. Here in this article, you can see the block diagram of the AC Drive System or VVVF Drive System.

Here, you can see the VVVF Drive system has main three blocks or parts,

1. Voltage Controlled Rectifier or AC-DC Converter
2. DC Link or Filter
3. Frequency Controlled Inverter

Rectifier/Converter

The main function of a rectifier circuit is to convert the alternating current(AC) into direct current(DC). In this drive system, the rectifier circuit takes the three-phase AC supply as input and gives DC supply as output. Here, we used the term voltage-controlled rectifier because the rectifier not only converts the AC into DC in fact, it controls or changes the voltage level with the help of a logic control circuit. This converter or rectifier circuit is generally built with diodes, SCR, etc. Varying the gate pulse to the SCR can vary the voltage and current conduction.

Filter Circuit or DC Link

It is built with either a parallel capacitor or a series inductor or a combination of both inductor and capacitor. The function of this circuit is to filtrate the impure DC supply from the rectifier circuit and make a link between the rectifier circuit and the inverter circuit.

Inverter

Here, the inverter circuit converts the DC supply into a variable three-phase AC supply. It basically works with the pulse width modulation technique. 

The variable frequency can vary the speed of the induction motor. This inverter circuit is generally built with MOSFETs or IGBTs or any high-speed switching semiconductor device. 

Here turning on or off the individual components can vary the frequency of the supply. This inverter circuit is also controlled by the logic control circuit which is mostly built with a microprocessor.

In the above block diagram of the VVVF Drive or AC drive system, each block is explained with waveforms.

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