VFD is very useful for motors. It serves many features by saving electrical energy consumed by the AC electric motor, extending the equipment life, eliminating excessive inrush current, making flexible controlling for the induction motor, avoiding over temperature and short-circuits, etc.
What is VFD Drive?
A variable frequency drive (VFD) is a motor controller used to control the speed of an AC motor by varying the power supply frequency to the induction motor. It transmits modified AC voltage having harmonics to the AC motors by doing the internal conversion in an AC-DC-AC pattern. VFD is also known as AC drive, AFD (adjustable frequency drive), ASD (adjustable speed drive), VFI (variable frequency inverter), Microdrive, etc.
How Does a Variable Frequency Drive Work?
As discussed earlier, the VFDs control the speed of an electric motor by controlling the frequency of supplied voltage to the AC motor. Firstly, a variable frequency drive converts the input 3-phase or single-phase 50/60Hz Ac power to DC power by using diodes. Then, by various switching mechanisms, it inverts the DC power into PWM with an adjusted frequency and 3-phase alternative current for the connected motors.
VFD Block Diagram
Let us look at a detailed block diagram of how a variable frequency drive works. The VFD includes three main sections - the rectifier, DC-link/DC bus, and variable frequency control.
- AC to DC Conversion (Rectifier) - The first section of VFD (variable voltage control) not only converts the AC supply voltage into DC, but also controls the required pulsating DC output voltage to maintain V/F constant ratio by using SCR/IGBT/Diodes.
- DC Filter and Buffer - The second section of VFD (DC link/DC bus) consists of inductance and capacitors to convert the pulsating DC into pure direct current. Here, inductance smooths the current and capacitor smooths the voltage.
- DC to AC Conversion - The third section of VFD (variable frequency control) not only converts DC voltage back into modified AC voltage, but also controls the variable frequency to maintain the V/F ratio by using electronic devices, such as SCR/IGBT.
What Are the Types of Variable Frequency Drive Control Methods?
The control method determines how VFDs control motors. Generally, there are four types of motor control methods for induction motors powered by using VFD.
- V/F (Volts per Hertz) or V/HZ control.
- V/F with encoder or closed loop V/F (PG Card).
- Open loop vector control.
- Closed-loop vector control (PG card)