variable frequency drives basic vfd theory · pdf filewhat is a vfd ( variable frequency...

SALES & SERVICE FORABB AC VARIABLE SPEED DRIVES SOFT STARTERRSDC VARIABLE SPEED DRIVES POWER QUALITY PRODUCTS

Variable Frequency Drives

Basic VFD Theory


What is a VFD ( Variable Frequency Drive)

! Electrical / electronic piece of equipment that varies the speed of a 3 phase or AC motor.

! Also called ….! VSD ( Variable speed drive)! ASD ( Adjustable speed drive)! AFD ( Adjustable frequency drive)! Inverter


Basic VFD Operation

! Convert AC power to DC power.

! Filter DC power.

! Invert DC power to variable voltage and variable frequency AC power.


Voltage Source VFD

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Input Line Capacitor

InverterRectifier

AC Motor

PWM Power Diagram


Convert AC Input Power to DC

AC input power is rectified to DC through a six pulse diode bridge

DC voltage is equal to AC input voltage * 1.414 (root 2)

!230 * 1.414 = 325!460 * 1.414 = 650!600 * 1.414 = 850

DC power is filtered and stored through a capacitor bank called the DC bus


Diode Rectifier


Rectifier and Filter


Power Factor


Invert DC Power to Variable Voltage and Frequency AC Power

! 2 IGBT’s per output phase switch the positive and negative sides of the DC Bus utilising pulse width modulation

! IGBT’s will have a characteristic rise time depending on its generation and current rating

! Rise time is the time it takes the IGBT pulse when switched on to reach DC bus voltage level


Insulated Gate Bipolar Transistors


Inverter


Full Voltage, 60 Hz PWM Waveform

Half Voltage, 30 Hz PWM Waveform

1.0

-1.0

1.0

-1.0

PWM Waveform


Pulse Width Modulation


Modulation of Waveform


PWM Carrier Frequency


Volts / Hz Ratio


Variable Voltage Variable Frequency

100%

Amount

of Boost

Field Weakening

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60HzOutput Frequency


VFD Summary

! VFD Varies the speed of an AC Motor! Maximum output voltage = input

voltage! V/Hz ratio is controlled throughout the

range up to 60 Hz. ! Torque decreases after 60Hz.! Output wave form is Pulse Width

Modulated (PWM) .


Application Types

! Variable Torque

! Constant Torque

! Constant Horsepower


Variable Torque Curve

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Variable TorqueAffinity Laws:

! Flow is proportional to speed.

(Flow1 / Flow2) = (RPM1)/(RPM2)

! Pressure is proportional to the square of the speed.(Press1 / Press2) = (RPM1)2 / (RPM2)2

! HP is proportional to the cube of the speed.(HP1 / HP2) = (RPM1 )3/ (RPM2)3


Load Torque Proportional To The Square of Speed

60 100

200%

100%

63%

Torq

ue(T

ypic

al)

Continuous torquecapability

Speed in Hertz

Region of continuousoperation

Region of intermittentoperation

Peak torquecapability

Load torqueRequirement


Variable Torque - Flow

Flow is proportional to speed.(Flow1 / Flow2) = (RPM1) / (RPM2)

Flow RPM Second SecondGallons/min RPM Flow

10000 1800 900 5000.00


Variable Torque - Pressure

Pressure is proportional to the square of the speed.

(Press1 / Press2) = (RPM1)2 / (RPM2)2

Pressure RPM Second SecondLb/sq. in RPM Pressure (lb/sq. in)

1000 1800 3600 4000


Variable Torque – Horsepower

HP is proportional to the cube of the speed.

(HP1 / HP2) = (RPM1)3 / (RPM2)3

Horsepower RPM Second Second RPM Horsepower

10 3450 3000 6.58


Variable Torque Applications

Irrigation Water PumpsDown-hole Water PumpsSubmersible Water PumpsDomestic Water PumpsCentrifugal Sewage

Treatment Pumps

Centrifugal FansBlowersAir Handling UnitsChillers


Constant Torque

! Torque remains constant up to the motor base speed.

! HP is proportional to the speed.


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Constant Torque Applications

ConveyorsCompressorsPositive Displacement PumpsCrushersMixersElevatorsEscalatorsEtc.


Constant Torque Load

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100%

63%

Torq

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Speed in Hertz

Region of continuouso eration

Region of intermittenoperation

Load torquerequirement



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Speed in Hertz

egion of continuousperation


Load torquerequirement




Torque De-rating Due toMotor Cooling

6 0 1 0 0

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1 0 0 %

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F ie ld w e a k e n in g re g io n

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ical

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C o n tin u o u s to rq u ec a p a b ility lim ite db y m o to r c o o lin g

S p e e d in H e rtz


Torque Curve Below & Above 60 Hz, Constant Torque V/F Ratio

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variable frequency drives basic vfd theory · pdf filewhat is a vfd ( variable frequency...

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