‘VFD - Variable frequency drive (၂)'

AC line power ၿဖင္႔ fixed speed motor load applications ေတြကို တိုက္ရိုက္သံုးစြဲမယ္႔အစား၊ VFD မွတဆင္႔ variable - speed အၿဖစ္၊ အသံုးၿပၿခင္းၿဖင္႔ energy saving အေနနဲ႔ စြမ္းအင္ ေခၽြတာနိဳင္တာေတြ႔ရပါတယ္။ VFD အသံုးၿပဳထာတဲ႔ variable torque centrifugal blower fan ေတြနဲ႔ pump applications ေတြမွာ၊ load torque နဲ႔ speed ဟာ square ဆိုတဲ႔ နွစ္ထပ္ကိန္းၿဖင္႔ တိုက္ရိုက္အခၽိဳးကၽၿပီး၊ power နဲ႔ speed ကေတာ႔ သံုးထပ္ကိန္းနဲ႔ တိုက္ရိုက္အခၽိဳးကၽပါတယ္။ တနည္းအားၿဖင္႔ load torque တဆမွာ speed နွစ္ဆၿဖင္႔ လည္ပတ္နိဳင္ၿပီး၊ power consumption တဆမွာ speed သံုးဆရရိွနိဳင္တယ္လို႔၊ ဆိုနိဳင္ပါတယ္။ motor ဟာ (၆၃ %) speed ၿဖင႔္ load ထမ္းကာလည္ပတ္စဥ္၊ full speed power ရဲ႕ (၂၅ %) သာ၊ သံုးစြဲမွာၿဖစ္ပါတယ္။

ဒီေန႔ေခါါတ္ကာလမွာ VFD အသံုးၿပဳထားတဲ႔ AC drives ေတြကို၊ industrial နဲ႔ commercial applications က႑မွ၊ acceleration, flow, monitoring, pressure, speed, temperature, tension နဲ႔ torque တို႔အတြက္၊ အစားထိုးသံုးစြဲလာႀကပါတယ္။ fixed speed operated load motor ေတြဟာ၊ ၿမင္႔မားတဲ႔ high starting torque ၿဖင္႔စတင္ လည္ပတ္ၿပီး၊ full-load current ရဲ႕ (၈) ဆခန္႔ ရိွတဲ႔ current surges ကိုၿဖစ္ေပါါေစပါတယ္။ AC drives ေတြမွာေတာ႔ operating speed ကို၊ 'ramp up' အေနနဲ႔ တၿဖည္းၿဖည္း ၿမွင္႔တင္ယူသြားတဲ႔အတြက္၊ mechanical နဲ႔ electrical stress ေတြေလၽွာ႔နည္းသြားသလို၊ maintenance နဲ႔ repair costs ကုန္ကၽမွဳေတြကိုပါ၊ သက္သာကၽဆင္းေစနိဳင္ပါတယ္။ 

Fig. compares AC and DC drives according to certain key parameters

DC နဲ႔ AC drives ေတြကို၊ low speed ၿဖင္႔၊ four quadrant operation with regeneration, frequent acceleration and deceleration routines အစရိွတဲ႔ continuous operation အေနနဲ႔၊ hazardous area ေတြမွာ လိုအပ္သလိုေမာင္းနွင္တဲ႔အခါ၊ ရရိွလာမယ္႔ performance factors ေတြအား၊ အေပါါပံုမွာ ေဖာ္ၿပထားပါတယ္။ AC drives ေတြကို၊ VSI ဆိုတဲ႔ voltage source inverter drive, CSI ဆိုတဲ႔ current source inverter drive, six-step inverter drive, LCI ဆိုတဲ႔ load commutated inverter drive, MC ဆိုတဲ႔ cycloconverter or matrix converter နဲ႔ doubly fed slip recovery system ဆိုၿပီး၊ ခြဲၿခားထားပါတယ္။

 

Fig. VSI and CSI

VSI and CSI - VSI drive မွာ၊ diode bridge converter မွ DC output ကို၊ capacitor bus မွာ stores ၿပဳလုပ္ၿပီး၊ stiff voltage input အၿဖစ္ inverter သို႔ေပးသြင္းပါတယ္။ VSI type ရဲ႕ output ဟာ PWM voltage output ၿဖစ္ပါတယ္။ CSI drive မွာေတာ႔ SCR bridge converter မွ DC output ကို၊ series reactor bus မွာ stores ၿပဳလုပ္ၿပီး၊ stiff voltage input အၿဖစ္ inverter သို႔ေပးသြင္းပါတယ္။ CSI type ရဲ႕ output ဟာ PWM voltage output ၿဖစ္နိဳင္သလို၊ six step waveform output လည္းၿဖစ္နိဳင္ပါတယ္။

 

Fig. Six-step drive waveforms

Six step inverter drive - ယခုအခါ six step inverter drive ေတြရယ္လို႔ obsolete အေနနဲ႔ သံုးနံွဳးမွဳမရိွေတာ႔ပဲ၊ variable voltage inverter drives, PAM ဆိုတဲ႔ pulse amplitude modulation drives, square wave drives နဲ႔ D.C chopper inverter drives လို႔သာ၊ ေခါါေဝါါႀကတာေတြ႔ရပါတယ္။ SCR bridge converter မွ DC output ကို၊ capacitor bus နဲ႔ series reactor bus တို႔မွာ smoothed ၿဖစ္ေစၿပီး၊ 'darlington pair' သို႔မဟုတ္ IGBT ဆိုတဲ႔ inverter quasi - sinusoidal တို႔မွတဆင္႔၊ six step voltage/ current input အၿဖစ္ induction motor သို႔ ေပးသြင္းပါတယ္။

Fig. (a) PAM - pulse - amplitude modulation and (b) Square waveform with fundamental sine wave component

'PAM' ဟာ signal modulation ေတြၿဖစ္ၿပီး၊ single polarity PAM နဲ႔ double polarity PAM ဆိုၿပီး၊ ခြဲၿခားနိဳင္ပါတယ္။ single polarity PAM ရဲ႕ pulses ေတြဟာ၊ positive signal ေတြ ၿဖစ္ၿပီး၊ double polarity PAM ရဲ႕ pulse ေတြကေတာ႔ positive နဲ႔ negative signal ေတြၿဖစ္ပါတယ္။ 'square wave drives' ဟာ electrical power converter ၿဖစ္ၿပီး၊ direct current (DC) ကို၊ alternating current (AC) အၿဖစ္ေၿပာင္းလဲေပးပါတယ္။ 

 

Fig. Schematic of an inverter using a vibrator as a chopper

'chopper' ဆိုတာကေတာ႔ electronic switching device တခုၿဖစ္ၿပီး အၿခား device တခုအား၊ control လုပ္ရန္အတြက္၊ interrupt signal ထုတ္ေပးပါတယ္။ chopper ေတြကို On basis of I/P - O/p voltage levels chopper, On basis of direction of output voltage and current chopper, On basis of circuit operation chopper နဲ႔ On basis of commutation method chopper ဆိုၿပီး၊ ခြဲၿခားနိဳင္ပါတယ္။

step down chopper နဲ႔ step-up chopper တို႔ဟာ၊ 'On basis of I/P - O/p voltage levels chopper' ေတြၿဖစ္ႀကၿပီး၊ 'On basis of direction of output voltage and current chopper' ေတြကိုေတာ႔ Class A, Class B, Class C, Class D နဲ႔ Class E chopper ဆိုၿပီး၊ ထပ္မံခြဲၿခားနိဳင္ပါတယ္။ အလားတူ' On basis of circuit operation chopper' ေတြ ကိုလည္း၊ first quardant, two quardant နဲ႔ four quardant chopper ဆိုၿပီး၊ ထပ္မံခြဲၿခားထားသလို၊ 'On basis of commutation method chopper' ေတြကိုေတာ႔ voltage commutated, current commutated, load commutated နဲ႔ impulse commutated chopper ဆိုၿပီး၊ ခြဲၿခားထားတာေတြ႔ရပါတယ္။ chopper ေတြကို modern uses အေနနဲ႔ DC to DC converters ေတြအပါအဝင္ switched mode power supplies, DC motors speed controllers, Class D electronic amplifiers, switched capacitor filters နဲ႔ variable frequency drive အၿဖစ္အသံုးၿပဳႀကပါတယ္။

Fig. Darlington pair and IGBT - insulated - gate bipolar transistor

'darlington pair' ကို 'darlington transistor' လို႔လည္းေခါါႀကၿပီး bipolar transistors (၂) လံုးကို separated devices သို႔မဟုတ္ integrated အေနနဲ႔ ေပါင္းစပ္ထားတဲ႔ compound structure ၿဖစ္ပါတယ္။ ပထမ first transistor ဟာ၊ current ကို amplified အေနနဲ႔ ခၽဲ႕ထြင္ေပးၿပီး၊ ဒုတိယ second transistor မွ ဆက္တိုက္ amplified လုပ္ၿခင္းၿဖင္႔၊ ၿမင္႔မားတဲ႔ higher common/ emitter current gain ရရိွလာပါတယ္။ darlington pair ဟာ အမၽိဳးအစားတူညီတဲ႔ transistor (၂) လံုးၿဖင္႔ shared collector တခုတည္းကို၊ သံုးထားၿခင္းၿဖစ္ၿပီး၊ NPN နဲ႔ PNP transistors (၂) လံုးသံုးထားတဲ႔ complementary Darlington တနည္းအားၿဖင္႔ 'sziklai pair' နဲ႔ မတူညီတာ၊ ေတြ႔ရမွာၿဖစ္ပါတယ္။

darlington pair ဟာ sensitive ၿဖစ္လြန္းတဲ႔ semiconductor device ၿဖစ္ၿပီး၊ safe voltages အေၿခအေနမွာ၊ လူ႔အေရၿပားနဲ႔ထိရံု skin contact ၿဖင္႔ current passed အေနနဲ႔ ၿဖတ္စီးနိဳင္တာေတြ႔ရပါတယ္။ darlington pair ရဲ႕ အားနည္းခၽက္ disadvantages ကေတာ႔ 'drawbacks' ေတြေပါါေပါက္ၿခင္းၿဖစ္ပါတယ္။ 'drawback' ဟာ base/ emitter voltage ရဲ႕ (၂) ဆခန္႔ရိွတဲ႔ ပမာဏၿဖင္႔ေပါါေပါက္နိုင္သလို increased saturation voltage ၿမင္႔မားမွဳအေနနဲ႔လည္း၊ ေပါါေပါက္နိဳင္ပါတယ္။ ဒါ႔အၿပင္ switching speed လို႔ေခါါတဲ႔ response ဟာ၊ reduction အေနနဲ႔ ေလၽွာ႔နည္းကၽဆင္းသြားတဲ႔ drawback လည္း၊ ၿဖစ္ေပါါနိဳင္ပါတယ္။

 

 

Fig. (a) - IGBT-Module (IGBTs and freewheeling diodes) with a rated current of 1,200 A and a maximum voltage of 3,300 V, (b) - Opened IGBT module with four IGBTs (half of H-bridge) rated for 400 A 600 V, (c) - Small IGBT module, rated up to 30 A, up to 900 V.

'IGBT' ဆိုတဲ႔ 'insulated gate bipolar transistor' ဆိုတာကေတာ႔ three-terminal power semiconductor device ၿဖစ္ၿပီး၊ high efficiency နဲ႔ fast switching response အေနနဲ႔ ေဆာင္ရြက္နိဳင္တဲ႔အတြက္၊ VFD ေတြမွာ ပိုမိုအသံုးၿပဳလာႀကပါတယ္။ ဒါ႔အၿပင္ switching amplifiers အၿဖစ္၊ electric cars, trains, variable speed refrigerators နဲ႔ air conditioners ေတြအၿပင္၊ medium နဲ႔ high-power applications ေတြၿဖစ္တဲ႔၊ switched-mode power supplies traction motor control နဲ႔ induction heating ေတြမွာလည္း၊ အသံုးၿပဳႀကပါတယ္။ IGBT ေတြဟာ simple gate-drive characteristics ဂုဏ္သတၱိရိွတဲ႔ 'MOSFET' နဲ႔ low - saturation voltage capability အေနနဲ႔ ေဆာင္ရြက္ေပးနိဳင္တဲ႔ 'bipolar transistor' တို႔ကို၊ ေပါင္းစပ္ကာ၊ 'isolated gate FET single device' အၿဖစ္အသံုးၿပဳၿခင္းရယ္လို႔ ဆိုနိဳင္ပါတယ္။

 

Fig. Cross section of a typical IGBT showing internal connection of MOSFET and Bipolar Device

IGBT cell ရဲ႕တည္ေဆာက္ပံုဟာ၊ n-channel vertical construction power MOSFET ရဲ႕ တည္ေဆာက္ပံုနဲ႔ ဆင္တူၿပီး၊ MOSFET ရဲ႕ 'n+ drain' ေနရာမွာ၊ 'p+ collector layer' ကို အစားထိုးထည္႔သြင္းကာ၊ vertical PNP bipolar junction transistor ကဲ႔သို႔၊ ဖြဲ႔စည္းထားၿခင္းၿဖစ္ပါတယ္။ p+ collector layer ရဲ႕ 'p+ region' ဟာ PNP bipolar junction transistor နဲ႔ n-channel MOSFET ရဲ႕ surface တို႔ကို၊ cascade connection အေနနဲ႔ ဆက္သြယ္ေပးထားပါတယ္။

Load commutated inverter drive (LCI) - ကေတာ႔ CSI ကို၊ မြန္းမံထုတ္လုပ္ထားတဲ႔ drive ၿဖစ္ပါတယ္။ CSI ကဲ႔သို႔ SCR-bridge converter မွ DC output ကို၊ DC link inductor circuit မွာ stores ၿပဳလုပ္ၿပီး၊ ဒုတိယ SCR-bridge's inverter မွာထပ္မံၿဖတ္သန္းၿပီးမွ၊ stiff quasi-sinusoidal six step current output အေနနဲ႔ synchronous machine သို႔ေပးသြင္းပါတယ္။

Cycloconverter or matrix converter (MC) - ႀကားခံ intermediate အၿဖစ္ energy storage ၿပဳလုပ္ေပးမယ္႔ DC link ေတြမပါဝင္ပဲ၊ AC - AC converters ေတြအၿဖစ္၊ တည္ေဆာက္ ထားပါတယ္။ AC - AC converters ေတြကို indirect AC - AC လို႔ေခါါတဲ႔ AC - DC - AC converters, CCV ဆိုတဲ႔ cycloconverters, hybrid matrix converters နဲ႔ MC ဆိုတဲ႔ matrix converters ေတြအၿဖစ္၊ ခြဲၿခားနိဳင္ပါတယ္။ 

Fig. blocking mode type cycloconverter

'CCV' လို႔ေခါါတဲ႔ 'cycloconverter' ေတြကို၊ 'cycloinverter' လို႔လည္းေခါါႀကပါတယ္။ 'CCV' ေတြဟာ AC supply မွရရိွလာတဲ႔ constant voltage, constant frequency AC waveform ကို၊ intermediate DC link အသံုးမၿပဳပဲ ေနာက္ထပ္ lower frequency AC waveform အၿဖစ္၊ ေၿပာင္းလဲကာ synthesizing output အေနနဲ႔ ေၿပာင္းလဲေပးပါတယ္။ CCV ေတြကို circulating current type နဲ႔ blocking mode type အၿဖစ္ခြဲၿခားနိဳင္ၿပီး၊ high power applications ေတြမွာ blocking mode type CCV ေတြကို၊ တတ္ဆင္အသံုးၿပဳပါတယ္။ ေယဘုယၽအားၿဖင္႔ CCV ေတြမွာ၊ phase-controlled SCR switching devices ေတြကို အသံုးၿပဳထားၿပီး၊ resistive load applications ေတြမွာေတာ႔ ေစၽးနံွဳးသက္သာတဲ႔ low-power TRIAC - based CCV ေတြကို၊ အသံုးၿပဳပါတယ္။

 

Fig. Typical output voltage waveform for one phase of six-pulse cycloconverter supplying an inductive (motor) load.

CCV ေတြကို 1 - phase/ 1 - phase, 3 - phase/ 1 - phase နဲ႔ 3 - phase/ 3-phase input/ output configuration ေတြၿဖင္႔တည္ေဆာက္ထားၿပီး၊ 3 - phase/ 3 - phase input/ output configuration ကို၊ အသံုးမၽားပါတယ္။ three-phase CCV ေတြရဲ႕ output to input frequency ratio ဟာ circulating current mode CCV ထက္ သံုးပံုတပံုေလာက္သာ ရိွၿပီး၊ blocking mode CCV ထက္ တဝက္ခန္႔သာရိွတာ၊ ေတြ႔ရပါတယ္။ CCV ရဲ႕ input မွ switching-device bridges ေတြကို၊ phase-shifted configuration ပံုစံၿဖင္႔ တတ္ဆင္ထားၿပီး၊ switching-device bridges အရည္အတြက္မၽားစြာတတ္ဆင္ၿခင္းၿဖင္႔ output waveform quality လည္းၿမင္းမားလာနိဳင္ပါတယ္။

phase-controlled converters ေတြၿဖစ္တဲ႔ CCV ေတြေနရာမွာ၊ IGBT, GTO, IGCT နဲ႔ အၿခားေသာ switching devices ေတြကို၊ အေၿခခံတည္ေဆာက္ထားတဲ႔ faster PWM self-controlled converters ေတြအစားထိုး ေနရာယူလာတာေတြ႔ရပါတယ္။ ဒါေပမယ္႔ higher end of the power rating range application ေတြၿဖစ္တဲ႔ driving rolling mine hoists, mill main motors, ore processing, cement kilns ေတြနဲ႔ သေဘ္ာေတြရဲ႕ propulsion systems ေတြမွာေတာ႔၊ CCV ေတြကို တတ္ဆင္အသံုးၿပဳႀကပါတယ္။

 

Fig. Schematic Diagram of a Cycloconverter - based Propulsion System

သေဘ္ာေတြမွာအသံုးၿပဳတဲ႔ cycloconverter ေတြကေတာ႔၊ three-phase current source ကိုအသံုးၿပဳၿပီး၊ three anti-parallel connected SCR - bridges ေတြမွတဆင္႔၊ six-pulse configuration output ထုတ္ေပးပါတယ္။ တနည္းအားၿဖင္႔ cycloconverter phase တခုစီကို fixed line frequency AC voltage အၿဖစ္ေရြးခၽယ္ၿပီး၊ variable load frequency AC voltage သို႔၊ ေၿပာင္းလဲေပးၿခင္းၿဖစ္ပါတယ္။
 

  

Fig. (a) conventional direct matrix converter and (b) indirect matrix converter

 

Fig. direct matrix converter type VFD

'matrix converter' ေတြကို၊ conventional direct matrix converter နဲ႔႔ indirect matrix converter ဆိုၿပီး၊ ေတြ႔ရပါတယ္။ indirect matrix converter ေတြကို၊ indirect energy conversion အေနနဲ႔ အသံုးၿပဳၿပီး၊ sparse matrix converter လို႔လည္းေခါါပါတယ္။ DC - link based VSI နဲ႔ CSI drives ေတြ တနည္းအားၿဖင္႔ IGBT - based ကို၊ အေၿခခံၿပီးတည္ေဆာက္ထားတဲ႔ matrix converter ေတြဟာ၊ separate stages ေတြမွတဆင္႔ voltage နဲ႔ current conversion ေတြကို၊ လိုအပ္သလို ေဆာင္ရြက္ေပးပါတယ္။

Reference and image credit to : Bartos, Frank J. (Sep. 1, 2004). "AC Drives Stay Vital for the 21st Century". Control Engineering (Reed Business Information)., Cleaveland, Peter (Nov. 1, 2007). "AC Adjustable Speed Drives". Control Engineering (Reed Business Information)., "Energy Regeneration". Retrieved Apr. 20, 2012., Basics of AC Drives, pp. Hardware - Part 1: slides 9-10 of 11., Bose, Bimal K. (2006). Power Electronics and Motor Drives : Advances and Trends. Amsterdam: Academic. p. 22. ISBN 978-0-12-088405-6., Bartos, Frank J. (Sep. 1, 2004). "AC Drives Stay Vital for the 21st Century". Control Engineering (Reed Business Information)., Eisenbrown, Robert E. (May 18, 2008). "AC Drives, Historical and Future Perspective of Innovation and Growth". Keynote Presentation for the 25th Anniversary of The Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC). University of Wisconsin, Madison, WI, USA: WEMPEC. pp. 6–10., Horowitz, Paul; Winfield Hill (1989). The Art of Electronics. Cambridge University Press. ISBN 0-521-37095-7., Bose, Bimal K. (2006). Power Electronics and Motor Drives : Advances and Trends. Amsterdam: Academic. p. 126. ISBN 978-0-12-088405-6.

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