What is VFD (Veritable Frequency Drive)?

Thise Blog Is about is the basic information about VFD 

What is Variable Frequency Drive (VFD)?

AC motor speed is controlled in two ways

Ø Either by controlling the voltage.

Ø Frequency.

Frequency controlling gives better control due to constant flux density than voltage control. This is where the working of VFD’s comes to play. It is a power conversion device which converts the fixed voltage, fixed frequency of the input power to the variable voltage, variable frequency output to control AC induction motors. It is an assembly of Where us power electronic devices (like IGBT, MOSFET), high speed central controlling unit (such as a microprocessor, DSP) and optional sensing devices depending on the application used.

Most of the industrial applications require variable speeds at peak load conditions and constant speeds at normal operating conditions. Closed loop working of VFDs maintain the speed of the motor at a constant level, even in case of input and load disturbances.

Ø Working of VFD

The two main features of variable frequency drive are adjustable speeds and soft start/stop capabilities. These two features make VFD’s a powerful controller to control the AC motors. VFD consists of mainly four sections; those aa re rectifier, intermediate DC link, inverter and controlling circuit.

The basic circuit of VFD consist of Following circuits

Ø Rectifier

Ø DC bus

Ø Inverter

Ø Control circuit

·       Rectifier

It is the first stage of variable frequency drive. It converts AC power fed from mains to DC power. This section can be unidirectional or bidirectional based on the application used like four quadrant operation of the motor. It utilizes diodes, SCR’s, transistors and other electronic switching devices.

If it uses diodes, converted DC power is uncontrolled output while using SCR, DC output power is varied by gate control. A minimum of six diodes are required for the three phase conversion, so the rectifier unit is considered as six pulse converter.

·       DC bus

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

·       Inverter

This section comprises of electronic switches like transistors, thyristors, IGBT, etc. It receives DC power from DC link and converts into AC which is delivered to the motor. It uses modulation techniques like pulse width modulation by to vary output frequency for controlling the speed of induction motor.

·       Control Circuit

It consists of a microprocessor unit and performs various functions like controlling, configuring drive settings, fault conditions and interfacing communication protocols. It receives feedback signal from motor as current speed reference and accordingly regulates the ratio of voltage to frequency to control motor speed.

Ø Advantages

Variable frequency drives not only offer adjustable speeds for accurate and precise control applications, but also have more benefits in terms of process control and conservation of energy. Some of these are given below.

·       Energy saving

More than 65% of the power is consumed by electric motors in industries. Both magnitude and frequency control technique to vary the speed consumes less power when variable speed is required by the motor. So a great amount of energy is conserved by these VFD’s.

·       Closed loop controlling

VFD allows accurate positioning of the motor speed by continuously comparing with reference speed even at changes in the loading conditions and input disturbances like voltage fluctuations.

·       Limits starting current

Induction motor draws current which is of 6 to 8 times the nominal current at starting. Compared to conventional starters, VFD’s gives better results because it delivers low frequency at the time of starting. Due to low frequency, motor draws less current and this current never exceeds its nominal rating at starting as well as operating.

·       Smooth operation

It offers smooth operations at starting and stopping and also reduces thermal and mechanical stress on motors and belt drives.

·       High power factor

Inbuilt power factor correction circuit in DC link of VFD reduces the need for additional power factor correction devices.

Power factor for induction motor is very low for particularly no load application, while at full load, it is 0.88 to 0.9. Low power factor results in poor utilization of power due to high reactive losses.

·       Easy installation

Pre programmed and factory wired VFD’s offers easy way for connection and maintenance.

Ø Manufacturers of VFD in Market

1.    ABB

2.  Seimence

3.  Scheinder

4. Fuji

5.  Delta

For all readrs of the blog hopfully thise information is helpful and you all get the basic information about VFD.