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How Does a VFD Working

What is VFD

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive,adjustable speed drive, adjustable frequency drive and inverter.

How Does a VFD Working:

The first stage of a Variable Frequency Drive or VFD is the Converter. The converter is comprised of six diodes. 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 is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The same is true for the 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 “six-pulse VFD”, which is the standard configuration for current Variable Frequency Drives.


Block Diagram of VFD

The output from the VFD is a “rectangular” wave form. VFD do not produce a sinusoidal output. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor. If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output transistors more slowly. But, if we reduce the frequency to 30 Hz, then we must also reduce the voltage to 240 V in order to maintain the V/Hz ratio.This is called Pulse Width Modulation or PWM.

During the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480 V or 240 V. By pulsing the output, we can achieve any average voltage on the output of the VFD.

Block diagram of VFD

Capacitor Bank: 

We can get rid of the AC ripple on the DC bus by adding a capacitor. This capacitor absorbs the ac ripple and delivers a smooth dc voltage. The actual voltage will depend on the voltage level of the AC line feeding the drive, the level of voltage unbalance on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the drive.

DC to AC converter: 

Generally  in VFD case we use IGBT or MOSFET type transistor as a DC to AC converter. It has better output performance and also operable for higher ampere range applications. 

Output Choke: 

Output choke is optional block in variable frequency drive and must use when  higher range of output is connected or required. If distance between motor & VFD is more than 50 m than must be use output choke it regulate desired speed of motor, if we do not use output choke then desired output is not performed. 

Encoder:

It use as a feedback signal that provide actual RPM speed to PID regulator section to take feedback and calculate the difference from output and desired speed and must provide required speed.

Speed Reference control: 

This block control the speed of drive (VFD) from external reference signal. It may be different type of reference signal that depends on which type of VFD that you have used. Some reference signals are listed below.

Analog Input: Analog input is applied from external reference source that may be 0-20mA, 4-20mA or 0-10Vdc. This analog signal is further converted into digital signal that required by controller.

Digital Input: Digital input signal is 24V, 110V or 220V. It depends on which type of card that you have used.

LOCAL: IT is used from external card or HMI or other type of interface through we control speed, voltage and other parameter from external devices.

PLC: We also use PLC to communicate VFD and control speed of motor through PLC in counts format.

Trim/RAMP control: 

In this section we trim motor speed using necessary trim reference signal. For example. if we want to run this motor as same speed as different motor run so that time we have to apply a difference amount of signal through trim reference.

Speed PID Regulator: 

VFD applications require the AC motor to run at a specific speed as set by the keypad, speed potentiometer, or analog input. Some VFDs provide an alternative option that allows precise process control through a PID mode of operation. Many VFDs come equipped with a built-in proportional-integral-derivative (PID) controller. The PID loop is used to maintain a process variable, such as speed. The connection of PID desired set value and PID real value of the application with the functions of the variable frequency drive enables process control without further components. In this way, VFD applications such as pressure, volume flow or speed control can be implemented easily.

Torque Reference control: 

Direct torque control is one method used in variable frequency drives to control the torque (and thus finally the speed) of motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor.
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