Variable Frequency drive,Working Principle,Why we used VFD,types of VFD,Components of VFD System,VFD as a Soft Start,VFD MotorCommutation,Design of VFD for Motor

Introduction
                Variable frequency drive (VFD) or adjustable frequency drive(AFD),Variable -voltage/Variable frequency(VVF) drive,Variable speed drive(VSD),AC drive,Micro drive or inverter derive is a type of adjustable speed drive used in electro-mechanical drive system to control AC motor speed and torque by varying motor input frequency and voltage.
Working Of VFD
                 The first stage of a variable frequency AC drive or VFD is the converter.The converter is comprised of six diodes used in plumbing system.They allow current to flow in only one direction the direction shown by the arrow in the diode symbol..For example whenever A Phase voltage (voltage is similar to pressure in plumbing system) is more positive than B or C phase voltages, 


then the B-phase diode will open and allow current to flow .When B -phase become more positive than A -phase then the B-phase diode will open and the A-phase diode will close.The same is true for 3 diodes on the negative side of the bus .Thus , we get six current pulses as each diode opens and closes .This is called a "Sis pulse VFD" Which is the standard configuration for current variable frequency drive.
Why We used VFD
               There are many reasons why we may want to adjust the motor speed for example.
  • Save energy and improve system efficiency.
  • Convert power in hybridization application.
  • Match the speed of the drive to the process requirements.
  • Match the torque or power of a drive to the process requirements improve.the working environment , lower noise level for example from fans and pumps.
  • Reduce mechanical stress on machine to extend their life time. Shave peak consumption to avoid peak demand price and reduce the motor size required.
  • In addition , to days drives integrate networking and diagnostic capabilities to better control performance and increase productivity. So, energy saving ,intelligent motor control and reduction of peak current drawn are three great reasons to choose a VFD as the controller in every motor driven system.
Types Of VFD
                     There are two major types of variable frequency drive.
  1. Mechanical VFDs.
  2. Electrical VFD
Mechanical VFDs include the following sub-types.
  • Variable Pitch Drive.
  • Traction Drive.
  • Hydraulic Hydrostatic Drive.
  • Hydraulic Hydrodynamic Driver or fluid coupling.
  • Hydraulic Hydro viscous Drive.
           The most common mechanical VFD used with in industry is the fluid coupling.
           
           Electric VFDs include the following sub-types.
  • Eddy Current Coupling.
  • DC Drives and corresponding dc motor.
  • AC VFDs and associated AC motors.
  • Current Source.
  • Voltage Source.
Parts Of VFD system
                            The variable frequency device has three parts like an AC motor, a controller and Operating interface.
                             The AC motor used in VFD is generally a three phase induction motor even through single phase motor is used in some systems.Motors that are designed for fixed speed operation is generally used,but some motor designs offer better performance in VFD than the standard design.
                            The controller part is the solid electronic power converter circuit to convert Ac to DC and then to quasi sine wave AC.The first part is the AC and DC converter section having a full wave rectifier bridges usually a three phase/single phase full wave bridge.This Dc intermediate is then converted into quasi sine wave AC using the inverter switching circuit.Here MOSFET/IGBT transistors are used for inverting DC  to AC.
                           The inverter section converts Dc to three channels of AC to drive the three phase motor.The controller section can also be designed to give improve power factor , less harmonic distortion and low sensitivity to input transients.
VFD as Soft Start
                     In an ordinary induction motor , switched on using a AC switch,The current drawn is much higher than the rated value and can increase with increased acceleration of the load to attain full speed of the motor.
                     On the other hand in a VFD controlled motor,initially low voltage at low frequency is applied .This frequency and voltage increases at a controlled rate to accelerate the load.This develops almost torque than the rated value of the motor.
VFD Motor Commutation
                       The frequency and applied voltage are first reduced to a control level and then kept on decreasing until it becomes zero and the motor shut down.
Design Of VFD for motor

                    The rule of thumb for sizing the single phase input on a three phase drive is to use a VFD rated for 2 times the FLA of the motor .For example if yours motor is a 10hp motor with a FLA of 28 amps,then you would need to select a VFD with an amp rating of 56 amps which ends up being around 20hp.

Comments

Post a Comment

Popular posts from this blog

What is full-wave Bridge Rectifier?,Comparision b/w FW-Bridge Rectifier with FW-Center tapped transformer,Working principle of FW-bridge Rectifier ,Comparison of all types of rectifiers,FW-bridge rectifier applications.

Image
Full-Wave Bridge Rectifier                      A Full-Wave Bridge Rectifier is a circuit arrangement which makes use of both half cycles of input alternating current (AC) and converts them to direct current (DC) .In full-wave bridge rectifier we used four diode in the form of a bridge as shown in the diagram below.  Comparison of FW-Bridge rectifier With Center Tapped Transformer                      The FW-bridge rectifier uses four diodes instead of two but the it eliminated the need of a center-tapped transformer.The PIV-rating of the diode used in FW- Bridge rectifier should be better than Vmax rather than 2Vmax if we compared it with FW-rectifier with a center-tapped transformer.This means that the diodes used in FW-bridge rectifiers are cheap as diodes of high PIV-rating are always more expensive used in FW-Rectifier with center tapped transformer ,this is b...
Read more

What is Current Transformer(CT)? ,Function Of CT,Construction of CT,Ratio of CT,Working Principle of CT,Uses /Advantages and Disadvantages of CT

Introduction                A Current Transformer(CT) is a type of transformer that is used to reduce or multiply an alternating current (AC) (used along with measuring or protective device).It produces a current in its secondary which is proportional to the current in its primary. Function of Current Transformer                The current transformers are basically a step up transformers.In case of high magnitude supplies we can not connected the measuring devices directly.we used current transformer to the supply of those devices with currents of magnitude proportional to these of power.Current transformer also isolates the measuring instruments from the high voltage circuits.current transformers supply the protective relays with current of magnitude proportional to those of power circuit but sufficiently reduce in magnitude. Construction of Current Transformer          ...
Read more

What is Star Connections,Voltage ,Current and Power in Star connections,Summary of Star connections,Uses of Star connections,

Image
Star Connection           Star Connections is a method of connecting pohyphase circuit is such a way that one end of each phase line is connected to a common neutral point that may be connected to the earth as protection against lighting or to a wire to which all the other neutral points of the system are connected.            It is also known as three phase four wire system.It is denoted by Y.It is the most preferred system for AC power distribution.            If we connect the balance load across three phase voltage system in parallel,then the three currents will be flow in neutral and their quantities will be same .The angle between them is 120º.So we can say That the sum of all currents is zero.                                                       ...
Read more