Understanding “VFD

Variable Frequency Drives

Xubair khan19092(BE)-Electronics

Contents

• Introduction

• Block Diagram

• Building blocks

• Modes of operation

• VFD Parameters

• Braking Technique

• Start/Stop Command methods

Introduction

– 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, or an AC drive.

– Frequency (or hertz) is directly related to the motor’s speed (RPMs)

Energy Usage

Electric Motor Facts

• Electric motors account for more than two-thirds of all energy used in industry

• 63% of the energy consumed is linked to air & liquid flow

• A typical 50HP industrial motor, with an efficiency of 90%, costs over $25,000 to operate continuously over a year.

– Standard motors are constant speed and when they are energized they run at a 100% speed no matter the load.

– What if the speed of the driven machine (Fan, Pump) is to be changed?

What is a VFD?

– Variable Frequency Drive (VFD)

– Equation of motor speed

Speed= 120 x f /P

• P=No. of poles

• F=Line Frequency

– How to change line frequency?

Constant =50Hz

Block Diagram

VFD Fundamentals

50 Hz Power

Electrical Energy

ABB

Variable Frequency

To

Motor

VFD

RECTIFIER

(AC - DC)

INVERTER

(DC - AC)

AC DC AC

VFD

Variable Frequency50 Hz

VFD Explored

First, the Converter (usually a diode rectifier) converts three-phase AC power to DC power.

Next, the DC Bus stores and filters the DC power in a large bank of capacitors. Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC power in a

Pulse Width Modulated (PWM) AC waveform to the motor.

Rectifier

• Basic Building blocks of rectification

– Diodes (Uncontrolled)

Thyristors

• Controlled

• Output voltage can be controlled

Gate Pulse

Conduction after Gate

Pulse

Three Phase Rectifiers

Output Voltage

• Output Voltage (dc) = 3× 3×𝑉𝑚

∏x cos𝐴

–Vm=Peak Value of voltage

–A= Firing Angle

• Firing at zero gives maximum output dc voltage

Is it a perfect Direct Current?

• Conversion of AC into DC a perfect process?

– Ripples

• How to eliminate the ripples?

– Filters

DC bus in VFD

Inverter Action

• Switching DC voltage ON and OFF will make it AC

• Filtered output from DC bus is sent to inverter in VFD

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

How switching can convert DC into AC?

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

Pulse width modulation

• Such a waveform is not acceptable

– Nowhere near Sine wave

• Contains harmonics

– Multiples of fundamentals

• Solution

– Pulse width modulation

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

How Often You Switch From Positive

Pulses To Negative Pulses Determines

The Frequency Of The Waveform

Frequency

Vo

ltag

e

Area Under the Curve

VFD Control Mode

• Scalar mode• Vector mode

V/F Control Mode

• Scalar mode

• Drive is unaware of what is happening in the motor

Example:

• A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern, Voltage applied by the drive will also be half.

• Perfect when at no load.

• After loading, motor will run at less than 50% speed

• Drive is unaware of it

SolutionVector Control

MotorDrive

Speed setting=50% Speed at no load=50%

Load increase

MotorDrive

Speed setting=50% Speed=40%

No idea what is happening

Vector Control Mode

• Vector control with sensor

– Feedback through encoder

– Better speed regulations up to 0.01%

– Faster response to load variations

MotorDrive

Feedback

Enco

der

sen

sor

Drive INC/DEC

Speed AccdFb ..

Vector Control Mode

VFD input Parameters

• Max./Base frequency setting

• Motor rated output

• Motor rated voltage

• Motor rated current

VFD input Parameters

• Frequency Reference setting methods

• Stop Command method

• Start frequency

• Stop frequency (DC Braking starts)

• Torque Boost

Frequency Reference Setting Methods

• Potentiometer

• 0-10V input voltage

• 4-20(mA)

Frequency Reference Setting through Potentiometer

Min

Max

Min Max

Freq

ue

ncy

Main Control Panel

Potentiometer

VFD drives ……….

Stop Command methods

• Coast to stop

• Ramp to stop

Coast to stop

Coast to Stop

Frequency

Run Command

Motor speed

Command is removed

Ramp to stop

Frequency

Run Command Command is removed

DC Brake

Ramp to Stop

Auto Tuning

• Drive familiarizing itself with motor

VFD here I am M

Auto Tuning

MotorDrive

Start of Auto tuning

Primary resistanceLeakage reactanceDC Brake VoltageTorque Boost

VoltageSlip compensation

DC Injection Braking

• No mechanical Contact

• DC is applied at the stator winding

• DC causes stator to be become a magnet with constant field

• A voltage is induced inside the rotor causing current to flow

• According to Lens’s law, this current will cause rotor to stop

Resistive Braking

Dynamic Braking

Electrical Energy

DiscardUtilize

ResistiveElevators

RegenerativeElectric grid

Braking resistor

VFD

Braking Unit

Thank you…….

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