a voltage controlled adjustable speed pmbldcm drive using a single

8/13/2019 A Voltage Controlled Adjustable Speed Pmbldcm Drive Using a Single

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INTRODUCTION

PERMANENT magnet brushless DC motors (PMBLDCMs) are preferred

motors for a compressor of an ar!condtonng (Ar!Con) s"stem due to ts features

l#e hgh effcenc"$ %de speed range and lo% mantenance re&urements' Theoperaton of the compressor %th the speed control results n an mproed effcenc"

of the s"stem %hle mantanng the temperature n the ar!condtoned one at the set

reference consstentl"' *hereas$ the e+stng ar condtoners mostl" hae a sngle!

phase nducton motor to dre the compressor n ,on-off. control mode ' Ths results

n ncreased losses due to fre&uent ,on-off. operaton %th ncreased mechancal and

electrcal stresses on the motor$ thereb" poor effcenc" and reduced lfe of the motor'

Moreoer$ the temperature of the ar condtoned one s regulated n a h"steress

band'

Therefore$ mproed effcenc" of the Ar!Con s"stem %ll certanl" reduce

the cost of lng and energ" demand to cope!up %th eer!ncreasng po%er crss'

A PMBLDCM %hch s a #nd of three!phase s"nchronous motor %th

permanent magnets (PMs) on the rotor and trapeodal bac# EM/ %aeform operates

on electronc commutaton accomplshed b" sold state s%tches' 0t s po%ered

through a three!phase oltage source nerter (120) %hch s fed from sngle!phase

AC suppl" usng a dode brdge rectfer (DBR) follo%ed b" smoothenng DC ln#

capactor' The compressor e+erts constant tor&ue ('e' rated tor&ue) on the

PMBLDCM and s operated n speed control mode to mproe the effcenc" of the

Ar!Con s"stem' 2nce$ the bac#!emf of the PMBLDCM s proportonal to the motor

speed and the deeloped tor&ue s proportonal to ts phase current$ therefore$ a

constant tor&ue s mantaned b" a constant current n the stator %ndng of the

PMBLDCM %hereas the speed can be controlled b" ar"ng the termnal oltage of

the motor' Based on ths logc$ a speed control scheme s proposed n ths paper %hch

uses a reference oltage at DC ln# proportonal to the desred speed of the PMBLDC

motor' 3o%eer$ the control of 120 s onl" for electronc commutaton %hch s based

on the rotor poston sgnals of the PMBLDC motor'

The PMBLDCM dre$ fed from a sngle!phase AC mans through a dode

brdge rectfer (DBR) follo%ed b" a DC ln# capactor$ suffers from po%er &ualt"


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(P4) dsturbances such as poor po%er factor (P/)$ ncreased total harmonc dstorton

(T3D) of current at nput AC mans and ts hgh crest factor (C/)' 0t s manl" due to

uncontrolled chargng of the DC ln# capactor %hch results n a pulsed current

%aeform hang a pea# alue hgher than the ampltude of the fundamental nput

current at AC mans' Moreoer$ the P4 standards for lo% po%er e&upments

emphase on lo% harmonc contents and near unt" po%er factor current to be dra%n

from AC mans b" these motors'

Therefore$ use of a po%er factor correcton (P/C) topolog" amongst arous

aalable topologes s almost netable for a PMBLDCM dre' Most of the e+stng

s"stems use a boost conerter for P/C as the front!end conerter and an solated DC!

DC conerter to produce desred output oltage consttutng a t%o!stage P/C dre'

The DC!DC conerter used n the second stage s usuall" a fl" bac# or for%ard

conerter for lo% po%er applcatons and a full!brdge conerter for hgher po%er

applcatons' 3o%eer$ these t%o stage P/C conerters hae hgh cost and comple+t"

n mplementng t%o separate s%tch!mode conerters$ therefore a sngle stage

conerter combnng the P/C and oltage regulaton at DC ln# s more n demand'

The sngle!stage P/C conerters operate %th onl" one controller to regulate the DC

ln# oltage along %th the po%er factor correcton' The absence of a second

controller has a greater mpact on the performance of sngle!stage P/C conerters and

re&ures a desgn to operate oer a much %der range of operatng condtons'

/or the proposed oltage controlled dre$ a half!brdge buc# DC!DC

conerter s selected because of ts hgh po%er handlng capact" as compared to the

sngle s%tch conerters' Moreoer$ t has s%tchng losses comparable to the sngle

s%tch conerters as onl" one s%tch s n operaton at an" nstant of tme' 0t can be

operated as a sngle!stage po%er factor corrected (P/C) conerter %hen connected

bet%een the 120 and the DBR fed from sngle!phase AC mans$ besdes controllng

the oltage at DC ln# for the desred speed of the Ar!Con compressor' A detaled

modelng$ desgn and performance ealuaton of the proposed dre are presented for

an ar condtoner compressor dren b" a PMBLDC motor of 5'6 #*$ 5677 rpm

ratng'


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POWER QUALITY

The contemporar" contaner crane ndustr"$ l#e man" other ndustr"

segments$ s often enamored b" the bells and %hstles$ colorful dagnostc dspla"s$

hgh speed performance$ and leels of automaton that can be acheed' Although

these features and ther ndrectl" related computer based enhancements are #e" ssues

to an effcent termnal operaton$ %e must not forget the foundaton upon %hch %e

are buldng' Po%er &ualt" s the mortar %hch bonds the foundaton bloc#s' Po%er

&ualt" also affects termnal operatng economcs$ crane relablt"$ our enronment$

and ntal nestment n po%er dstrbuton s"stems to support ne% crane nstallatons'

To &uote the utlt" compan" ne%sletter %hch accompaned the last monthl" ssue of

m" home utlt" bllng8 ,9sng electrct" %sel" s a good enronmental and

busness practce %hch saes "ou mone"$ reduces emssons from generatng plants$

and conseres our natural resources'. As %e are all a%are$ contaner crane

performance re&urements contnue to ncrease at an astoundng rate'

Ne+t generaton contaner cranes$ alread" n the bddng process$ %ll re&ure

aerage po%er demands of 5677 to :777 #* ; almost double the total aerage

demand three "ears ago' The rapd ncrease n po%er demand leels$ an ncrease n

contaner crane populaton$ 2CR conerter crane dre retrofts and the large AC andDC dres needed to po%er and control these cranes %ll ncrease a%areness of the

po%er &ualt" ssue n the er" near future'

POWER QUALITY PROBLEMS

/or the purpose of ths artcle$ %e shall defne po%er &ualt" problems as8

,An" po%er problem that results n falure or msoperaton of customer e&upment$

manfests tself as an economc burden to the user$ or produces negate mpacts on

the enronment'.

*hen appled to the contaner crane ndustr"$ the po%er ssues %hch degrade po%er

&ualt" nclude8

< Po%er /actor


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< 3armonc Dstorton

< 1oltage Transents

< 1oltage 2ags or Dps

< 1oltage 2%ells

The AC and DC arable speed dres utled on board contaner cranes are

sgnfcant contrbutors to total harmonc current and oltage dstorton' *hereas 2CR

phase control creates the desrable aerage po%er factor$ DC 2CR dres operate at

less than ths' 0n addton$ lne notchng occurs %hen 2CR.s commutate$ creatng

transent pea# recoer" oltages that can be = to > tmes the nomnal lne oltage

dependng upon the s"stem mpedance and the se of the dres' The fre&uenc" and

seert" of these po%er s"stem dsturbances ares %th the speed of the dre'

3armonc current n?ecton b" AC and DC dres %ll be hghest %hen the dres are

operatng at slo% speeds' Po%er factor %ll be lo%est %hen DC dres are operatng at

slo% speeds or durng ntal acceleraton and deceleraton perods$ ncreasng to ts

ma+mum alue %hen the 2CR.s are phased on to produce rated or base speed' Aboe

base speed$ the po%er factor essentall" remans constant'

9nfortunatel"$ contaner cranes can spend consderable tme at lo% speeds as

the operator attempts to spot and land contaners' Poor po%er factor places a greater

#1A demand burden on the utlt" or engne!alternator po%er source' Lo% po%er

factor loads can also affect the oltage stablt" %hch can ultmatel" result n

detrmental effects on the lfe of senste electronc e&upment or een ntermttent

malfuncton' 1oltage transents created b" DC dre 2CR lne notchng$ AC dre

oltage choppng$ and hgh fre&uenc" harmonc oltages and currents are all

sgnfcant sources of nose and dsturbance to senste electronc e&upment'

0t has been our e+perence that end users often do not assocate po%er &ualt"

problems %thContaner cranes$ ether because the" are totall" una%are of such ssues

or there %as no economc Conse&uence f po%er &ualt" %as not addressed' Before

the adent of sold!state po%er supples$ Po%er factor %as reasonable$ and harmonc

current n?ecton %as mnmal' Not untl the crane Populaton multpled$ po%er

demands per crane ncreased$ and statc po%er conerson became the %a" of lfe$ dd


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po%er &ualt" ssues begn to emerge' Een as harmonc dstorton and po%er /actor

ssues surfaced$ no one %as reall" prepared'

Een toda"$ crane bulders and electrcal dre 2"stem endors aod the ssue

durng compette bddng for ne% cranes' Rather than focus on A%areness and

understandng of the potental ssues$ the po%er &ualt" ssue s ntentonall" or

unntentonall" gnored' Po%er &ualt" problem solutons are aalable' Although the

solutons are not free$ n most cases$ the" do represent a good return on nestment'

3o%eer$ f po%er &ualt" s not specfed$ t most l#el" %ll not be delered'

Po%er &ualt" can be mproed through8

< Po%er factor correcton$

< 3armonc flterng$

< 2pecal lne notch flterng$

< Transent oltage surge suppresson$

< Proper earthng s"stems'

0n most cases$ the person specf"ng and-or bu"ng a contaner crane ma" not be full"

a%are of the potental po%er &ualt" ssues' 0f ths artcle accomplshes nothng else$

%e %ould hope to

Prode that a%areness'

0n man" cases$ those noled %th specfcaton and procurement of contaner

cranes ma" not be cognant of such ssues$ do not pa" the utlt" bllngs$ or consder

t someone else.s concern' As a result$ contaner crane specfcatons ma" not nclude

defnte po%er &ualt" crtera such as po%er factor correcton and-or harmonc

flterng' Also$ man" of those specfcatons %hch do Re&ure po%er &ualt"

e&upment do not properl" defne the crtera' Earl" n the process of preparng the

crane specfcaton8

< Consult %th the utlt" compan" to determne regulator" or contract re&urements

that must be

2atsfed$ f an"'


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< Consult %th the electrcal dre supplers and determne the po%er &ualt" profles

that can be

E+pected based on the dre ses and technologes proposed for the specfc pro?ect'

< Ealuate the economcs of po%er &ualt" correcton not onl" on the present stuaton$

but consder the mpact of future utlt" deregulaton and the future deelopment plans

for the termnal

THE BENEFITS OF POWER QUALITY

Po%er &ualt" n the contaner termnal enronment mpacts the economcs of

the termnal operaton$ affects relablt" of the termnal e&upment$ and affects other

consumers sered b" the same utlt" serce' Each of these concerns s e+plored n

the follo%ng paragraphs'

1. Economic Impact

The economc mpact of po%er &ualt" s the foremost ncente to contaner termnal

operators' Economc mpact can be sgnfcant and manfest tself n seeral %a"s8

a. Po!" Facto" P!na#ti!$

Man" utlt" companes no#e penaltes for lo% po%er factor on monthl"

bllngs' There s no ndustr" standard follo%ed b" utlt" companes' Methods of

meterng and calculatng po%er factor penaltes ar" from one utlt" compan" to the

ne+t' 2ome utlt" companes actuall" meter #1AR usage and establsh a f+ed rate

tmes the number of #1AR!hours consumed' @ther utlt" companes montor #1AR

demands and calculate po%er factor' 0f the po%er factor falls belo% a f+ed lmt alue

oer a demand perod$ a penalt" s blled n the form of an ad?ustment to the pea#

demand charges'

A number of utlt" companes sercng contaner termnal e&upment do not

"et no#e po%er factor penaltes' 3o%eer$ ther serce contract %th the Port ma"

stll re&ure that a mnmum po%er factor oer a defned demand perod be met' The

utlt" compan" ma" not contnuousl" montor po%er factor or #1AR usage and

reflect them n the monthl" utlt" bllngs ho%eer$ the" do resere the rght to

montor the Port serce at an" tme' 0f the po%er factor crtera set forth n the serce


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contract are not met$ the user ma" be penaled$ or re&ured to ta#e correcte actons

at the user.s e+pense' @ne utlt" compan"$ %hch supples po%er serce to seeral

east coast contaner termnals n the 92A$ does not reflect po%er factor penaltes n

ther monthl" bllngs$ ho%eer$ ther serce contract %th the termnal reads as

follo%s8

,The aerage po%er factor under operatng condtons of customer.s load at the pont

%here serce s metered shall be not less than 6' 0f belo% 6$ the customer ma"

be re&ured to furnsh$ nstall and mantan at ts e+pense correcte apparatus %hch

%ll ncrease the Po%er factor of the entre nstallaton to not less than 6' The

customer shall ensure that no e+cesse harmoncs or transents are ntroduced on to

the utlt" s"stem' Ths ma" re&ure specal po%er condtonng e&upment or flters'

The Port or termnal operatons personnel$ %ho are responsble for mantanng

contaner cranes$ or specf"ng ne% contaner crane e&upment$ should be a%are of

these re&urements' 9tlt" deregulaton %ll most l#el" force utltes to enforce

re&urements such as the e+ample aboe'

Termnal operators %ho do not deal %th penalt" ssues toda" ma" be faced %th some

rather seere penaltes n the future' A sound$ future termnal gro%th plan should

nclude contngences for addressng the possble economc mpact of utlt"

deregulaton'

%. S&$t!m Lo$$!$

3armonc currents and lo% po%er factor created b" nonlnear loads$ not onl"

result n possble po%er factor penaltes$ but also ncrease the po%er losses n the

dstrbuton s"stem' These losses are not sble as a separate tem on "our monthl"

utlt" bllng$ but "ou pa" for them each month' Contaner cranes are sgnfcant

contrbutors to harmonc currents and lo% po%er factor'

Based on the t"pcal demands of toda".s hgh speed contaner cranes$

correcton of po%er factor alone on a t"pcal state of the art &ua" crane can result n a

reducton of s"stem losses that conerts to a F to 57 reducton n the monthl" utlt"

bllng' /or most of the larger termnals$ ths s a sgnfcant annual sang n the cost

of operaton'


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C. Po!" S!"'ic! Initia# Capita# In'!$tm!nt$

The po%er dstrbuton s"stem desgn and nstallaton for ne% termnals$ as

%ell as modfcaton of s"stems for termnal capact" upgrades$ noles hgh cost$

specaled$ hgh and medum oltage e&upment' Transformers$ s%tchgear$ feeder

cables$ cable reel tralng cables$ collector bars$ etc' must be sed based on the #1A

demand' Thus cost of the e&upment s drectl" related to the total #1A demand' As

the relatonshp aboe ndcates$ #1A demand s nersel" proportonal to the oerall

po%er factor$ 'e' a lo%er po%er factor demands hgher #1A for the same #* load'

Contaner cranes are one of the most sgnfcant users of po%er n the termnal'

2nce contaner cranes %th DC$ F pulse$ 2CR dres operate at relatel" lo% po%er

factor$ the total #1A demand s sgnfcantl" larger than %ould be the case f po%er

factor correcton e&upment %ere suppled on board each crane or at some common

bus locaton n the termnal' 0n the absence of po%er &ualt" correcte e&upment$

transformers are larger$ s%tchgear current ratngs must be hgher$ feeder cable copper

ses are larger$ collector s"stem and cable reel cables must be larger$ etc'

Conse&uentl"$ the cost of the ntal po%er dstrbuton s"stem e&upment for a

s"stem %hch does not address po%er &ualt" %ll most l#el" be hgher than the same

s"stem %hch ncludes po%er &ualt" e&upment'

E()ipm!nt R!#ia%i#it&

Poor po%er &ualt" can affect machne or e&upment relablt" and reduce the

lfe of components' 3armoncs$ oltage transents$ and oltage s"stem sags and s%ells

are all po%er &ualt" problems and are all nterdependent'

3armoncs affect po%er factor$ oltage transents can nduce harmoncs$ the

same phenomena %hch create harmonc current n?ecton n DC 2CR arable speed

dres are responsble for poor po%er factor$ and d"namcall" ar"ng po%er factor of

the same dres can create oltage sags and s%ells' The effects of harmonc dstorton$


9/90


10/90

< Better speed ersus tor&ue characterstcs

< 3gh d"namc response

< 3gh effcenc"

< Long operatng lfe

< Noseless operaton

< 3gher speed ranges

0n addton$ the rato of tor&ue delered to the se of the motor s hgher$

ma#ng t useful n applcatons %here space and %eght are crtcal factors' 0n ths

applcaton note$ %e %ll dscuss n detal the constructon$ %or#ng prncple$

characterstcs and t"pcal applcatons of BLDC motors' Refer to App!n+i0 B

23#o$$a"&4 for a glossar" of terms commonl" used %hen descrbng BLDC motors'

CONSTRUCTION AND OPERATIN3 PRINCIPLE

BLDC motors are a t"pe of s"nchronous motor' Ths means the magnetc

feld generated b" the stator and the magnetc feld generated b" the rotor rotate at the

same fre&uenc"' BLDC motors do not e+perence the GslpH that s normall" seen n

nducton motors'

BLDC motors come n sngle!phase$ :!phase and =!phase confguratons'

Correspondng to ts t"pe$ the stator has the same number of %ndngs' @ut of these$

=!phase motors are the most popular and %del" used' Ths applcaton note focuses

on =!phase motors'

Stato"

The stator of a BLDC motor conssts of stac#ed steel lamnatons %th

%ndngs placed n the slots that are a+all" cut along the nner perpher" (as sho%n n

/gure =)' Tradtonall"$ the stator resembles that of an nducton motor ho%eer$ the

%ndngs are dstrbuted n a dfferent manner' Most BLDC motors hae three stator

%ndngs connected n star fashon' Each of these %ndngs are constructed %th


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numerous cols nterconnected to form a %ndng' @ne or more cols are placed n the

slots and the" are nterconnected to ma#e a %ndng' Each of these %ndngs are

dstrbuted oer the stator perpher" to form an een numbers of poles' There are t%o

t"pes of stator %ndngs arants8 trapeodal and snusodal motors'

Ths dfferentaton s made on the bass of the nterconnecton of cols n the

stator %ndngs to ge the dfferent t"pes of bac# Electromote /orce (EM/)' Refer

to the 2*hat s Bac# EM/54 secton for more nformaton'

As ther names ndcate$ the trapeodal motor ges a bac# EM/ n trapeodal

fashon and the snusodal motor.s bac# EM/ s snusodal$ as sho%n n /gure 5 and

/gure :' 0n addton to the bac# EM/$ the phase

current also has trapeodal and snusodal aratons n the respecte t"pes of motor'

Ths ma#es the tor&ue output b" a snusodal motor smoother than that of a

trapeodal motor' 3o%eer$ ths comes %th an e+tra cost$ as the snusodal motors

ta#e e+tra %ndng nterconnectons because of the cols dstrbuton on the stator

perpher"$ thereb" ncreasng the copper nta#e b" the stator %ndngs'

Dependng upon the control po%er suppl" capablt"$ the motor %th the

correct oltage ratng of the stator can be chosen' /ort"!eght olts$ or less oltage

rated motors are used n automote$ robotcs$ small arm moements and soon' Motors

%th 577 olts$ or hgher ratngs$ are used n applances$ automaton and n ndustral

applcatons'


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Roto"

The rotor s made of permanent magnet and can ar" from t%o to eght pole

pars %th alternate North (N) and 2outh (2) poles' Based on the re&ured magnetc

feld denst" n the rotor$ the proper magnetc materal s chosen to ma#e the rotor'

/errte magnets are tradtonall" used to ma#e permanent magnets' As the technolog"

adances$ rare earth allo" magnets are ganng populart"' The ferrte magnets are less

e+pense but the" hae the dsadantage of lo% flu+ denst" for a gen olume' 0n

contrast$ the allo" materal has hgh magnetc denst" per olume and enables the

rotor to compress further for the same tor&ue'


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Also$ these allo" magnets mproe the se!to!%eght rato and ge hgher

tor&ue for the same se motor usng ferrte magnets' Neod"mum (Nd)$ 2amarum

Cobalt (2mCo) and the allo" of Neod"mum$ /errte and Boron (Nd/eB) are some

e+amples of rare earth allo" magnets' Contnuous research s gong on to mproe the

flu+ denst" to compress the rotor further'

/gure > sho%s cross sectons of dfferent arrangements of magnets n a rotor'

Ha## S!n$o"$

9nl#e a brushed DC motor$ the commutaton of a BLDC motor s controlled

electroncall"' To rotate the BLDC motor$ the stator %ndngs should be energed n a

se&uence' 0t s mportant to #no% the rotor poston n order to understand %hch

%ndng %ll be energed follo%ng the energng se&uence' Rotor poston s sensed

usng 3all effect sensors embedded nto the stator'

Most BLDC motors hae three 3all sensors embedded nto the stator on the

non!drng end of the motor' *heneer the rotor magnetc poles pass near the 3all

sensors$ the" ge a hgh or lo% sgnal$ ndcatng the N or 2 pole s passng near the


14/90

sensors' Based on the combnaton of these three 3all sensor sgnals$ the e+act

se&uence of commutaton can be determned'

/gure 6 sho%s a transerse secton of a BLDC motor %th a rotor that has

alternate N and 2 permanent magnets' 3all sensors are embedded nto the statonar"

part of the motor' Embeddng the 3all sensors nto the stator s a comple+ process

because an" msalgnment n these 3all sensors$ %th respect to the rotor magnets$

%ll generate an error n determnaton of the rotor poston'

To smplf" the process of mountng the 3all sensors onto the stator$ some

motors ma" hae the 3all sensor magnets on the rotor$ n addton to the man rotor

magnets' These are a scaled do%n replca erson of the rotor' Therefore$ %heneer

the rotor rotates$ the 3all sensor magnets ge the same effect as the man magnets'

The 3all sensors are normall" mounted on a PC board and f+ed to the enclosure cap

on the non!drng end' Ths enables users to ad?ust the complete assembl" of 3all

sensors$ to algn %th the rotor magnets$ n order to achee the best performance'


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Based on the ph"scal poston of the 3all sensors$ there are t%o ersons of

output' The 3all sensors ma" be at F7I or 5:7I phase shft to each other' Based on

ths$ the motor manufacturer defnes the commutaton se&uence$ %hch should be

follo%ed %hen controllng the motor'

T-!o"& o6 Op!"ation

Each commutaton se&uence has one of the %ndngs energed to poste

po%er (current enters nto the %ndng)$ the second %ndng s negate (current e+ts

the %ndng) and the thrd s n a non!energed condton' Tor&ue s produced

because of the nteracton bet%een the magnetc feld generated b" the stator cols and

the permanent magnets' 0deall"$ the pea# tor&ue occurs %hen these t%o felds are at

J7I to each other and falls off as the felds moe together' 0n order to #eep the motor

runnng$ the magnetc feld produced b" the %ndngs should shft poston$ as the

rotor moes to catch up %th the stator feld' *hat s #no%n as G2+!2tep

CommutatonH defnes the se&uence of energng the %ndngs' 2ee the

GCommutaton 2e&uenceH secton for detaled nformaton and an e+ample on s+!step

commutaton

S!n$o" #!$$ Cont"o# o6 BLDC Moto"$

9ntl no% %e hae seen commutaton based on the rotor poston gen b"the 3all sensor' BLDC motors can be commutated b" montorng the bac# EM/

sgnals nstead of the 3all sensors' The relatonshp bet%een the 3all sensors and bac#

EM/$ %th respect to the phase oltage$ s sho%n n /gure K' As %e hae seen n

earler sectons$ eer" commutaton se&uence has one of the %ndngs energed

poste$ the second negate and the thrd left open' As sho%n n /gure K$ the 3all


16/90

sensor sgnal changes the state %hen the oltage polart" of bac# EM/ crosses from a

poste to negate or from negate to poste' 0n deal cases$ ths happens on ero!

crossng of bac# EM/$ but practcall"$ there %ll be a dela" due to the %ndng

characterstcs' Ths dela" should be compensated b" the mcrocontroller' /gure 57

sho%s a bloc# dagram for sensorless control of a BLDC motor'

Another aspect to be consdered s er" lo% speeds' Because bac# EM/ s

proportonal to the speed of rotaton$ at a er" lo% speed$ the bac# EM/ %ould be at a

er" lo% ampltude to detect ero!crossng' The motor has to be started n open loop$

from standstll and %hen suffcent bac# EM/ s bult to detect the ero!cross pont$

the control should be shfted to the bac# EM/ sensng' The mnmum speed at %hch

bac# EM/ can be sensed s calculated from the bac# EM/ constant of the motor'

*th ths method of commutaton$ the 3all sensors can be elmnated and n

some motors$ the magnets for 3all sensors also can be elmnated' Ths smplfes the

motor constructon and reduces the cost as %ell' Ths s adantageous f the motor s

operatng n dust" or ol" enronments$ %here occasonal cleanng s re&ured n

order for the 3all sensors to sense properl"' The same thng apples f the motor s

mounted n a less accessble locaton'

TYPICAL BLDC MOTOR APPLICATIONS

BLDC motors fnd applcatons n eer" segment of the mar#et' Automote$

applance$ ndustral controls$ automaton$ aaton and so on$ hae applcatons for

BLDC motors' @ut of these$ %e can categore the t"pe of BLDC motor control nto

three ma?or t"pes8

< Constant load

< 1ar"ng loads

< Postonng applcatons

App#ication$ it- Con$tant Loa+$


17/90

These are the t"pes of applcatons %here a arable speed s more mportant

than #eepng the accurac" of the speed at a set speed' 0n addton$ the acceleraton and

deceleraton rates are not d"namcall" changng' 0n these t"pes of applcatons$ the

load s drectl" coupled to the motor shaft' /or e+ample$ fans$ pumps and blo%ers

come under these t"pes of applcatons' These applcatons demand lo%!cost

controllers$ mostl" operatng n open!loop'

App#ication$ Wit- 7a"&in8 Loa+$

These are the t"pes of applcatons %here the load on the motor ares oer a

speed range' These applcatons ma" demand a hgh!speed control accurac" and good

d"namc responses' 0n home applances$ %ashers$ dr"ers and compressors are good

e+amples' 0n automote$ fuel pump control$ electronc steerng control$ engne

control and electrc ehcle control are good e+amples of these' 0n aerospace$ there are

a number of applcatons$ l#e centrfuges$ pumps$ robotc arm controls$ g"roscope

controls and so on'

These applcatons ma" use speed feedbac# deces and ma" run n sem!

closed loop or n total closed loop' These applcatons use adanced control

algorthms$ thus complcatng the controller' Also$ ths ncreases the prce of the

complete s"stem'

Positioning Applications

Most of the ndustral and automaton t"pes of applcaton come under ths

categor"' The applcatons n ths categor" hae some #nd of po%er transmsson$

%hch could be mechancal gears or tmer belts$ or a smple belt dren s"stem' 0n

these applcatons$ the d"namc response of speed and tor&ue are mportant' Also$

these applcatons ma" hae fre&uent reersal of rotaton drecton'

A t"pcal c"cle %ll hae an acceleratng phase$ a constant speed phase and a

deceleraton and postonng phase$ as sho%n n /gure 55' The load on the motor ma"


18/90

ar" durng all of these phases$ causng the controller to be comple+' These s"stems

mostl" operate n closed loop' There could be three control loops functonng

smultaneousl"8 Tor&ue Control Loop$ 2peed Control Loop and Poston Control

Loop' @ptcal encoder or s"nchronous resolers are used for measurng the actual

speed of the motor'

0n some cases$ the same sensors are used to get relate poston nformaton'

@ther%se$ separate poston sensors ma" be used to get absolute postons' Computer

Numerc Controlled (CNC) machnes are a good e+ample of ths' Process controls$

machner" controls and cone"er controls hae plent" of applcatons n ths categor"'

UNIPOLAR E9CITATION OF BLDC MOTORS

The motor under nestgaton s of the surface!mount permanent magnet

aret" %th concentrated stator %ndngs such that the nduced bac#!emfs %th

respect to rotor poston are trapeodal %th a flat!top %dth that s as %de aspossble' 2mooth tor&ue producton re&ures forcng a constant current through each

phase %ndng %hen ts bac#!emf s at ts pea# alue and turnng off the current %hen

the bac#!emf s changng'


19/90

/or bpolar e+ctaton$ poste current s n?ected %hen the bac#!emf s

poste$ and negate current %hen the bac#!emf s negate$ %th each conducton

perod lastng 5:7' Ths results n t%o phases conductng current and producng tor&ue

at an" nstant of tme as sho%n n the %aeforms of /g' F(a)' 9npolar current

conducton lmts the phases to onl" one drecton of current as sho%n n /g' F(b)'

Constant tor&ue producton s stll possble because one phase s conductng current at

an" nstant' 0t s of course possble to hae an oerlap n the phase conducton to hae

a smoother tor&ue producton' 0n an" case$ the motor %ndngs are poorl" utled

compared to the bpolar case'

Ths s reflected n the lo%er output tor&ue of the unpolar motor for the same

pea# phase currents' The prmar" motaton for choosng unpolar e+ctaton s that n

practce$ the nerter t"pcall" costs more than the motor and there s a great potental

for reducng ts cost and hence the oerall cost of the dre' 0n addton to cost

reducton$ unpolar e+ctaton offers the follo%ng adantages8

There s onl" one dece n seres %th each phase$ mnmng conducton

losses'

The rs# of shoot!through faults s elmnated'

2%tchng of deces connected to the suppl" rals$ %hch generall" re&ures

some solaton crcutr"$ can be aoded'

Another factor that has to be consdered before choosng unpolar e+ctaton s that the

motor neutral has to be aalable because the phase currents are no longer balanced'


20/90

/g'K' 2chematc of 2EP0C conerter based BLDC motor dre'

Cost mnmaton s the #e" to the large olume manufacture and applcaton

of brushless dc (BLDC) motors n arable speed dres' BLDC motors areconentonall" e+cted %th bpolar currents %hch re&ures a s+!s%tch nerter

TWO:PHASE MOTOR

*e frst consder a :!phase motor %th > slots as sho%n n /g' =' Ths s an

ntegral slots-pole motor and the col span s : slots' The currents of /g' l(a) are used

to e+cte ths motor' Ths dre re&ures onl" one poston sensor and a sngle current

sensor' The number of turns-phase s hgher than the reference for the same amount of

copper' Ths results n hgher pea# tor&ue$ but as Table 0 sho%s$ the tor&ue rpple of

ths motor %th unpolar currents s er" hgh' Ths s

:!phase >!slot BLDC motor

because commutaton bet%een the phases ta#es place durng the ero crossng of the

bac#!emf' Ths can be aoded b" ncreasng the number of phases'

'

7. THREE:PHASE MOTORS

Let us consder a =!phase$ 5:!slot motor as sho%n n /g' >(a) and use the

currents of /g l(c)' 0n ths case$ the commutaton ta#es place before the bac#!emf of


21/90

each phase reaches ero$ and the rpple s reduced as sho%n n /g' F(b)' *hen %e use

the currents n /g' l(d)$ the tor&ue rpple s %orse as sho%n n /g' F(c)' Ths s

because dfferent numbers of phases contrbute to the tor&ue at dfferent nstants of

tme' /rom the preous t%o cases$ %e reale that %e need a combnaton of 57

unpolar currents and small bac#!emf %dth to reduce the tor&ue pulsaton'

To nestgate ths case$ %e consder the F!slot motor sho%n n /g' >(h)' The

bac#!emf plots as a functon of rotor poston for both the =!phase motors are sho%n

n /g' 5. *e fnd that the F!slot motor has a hgher pea# and smaller bac# emf %dth

than the 5:!slot motor' Ths can be e+planed as follo%s'

The end!turns are shorter for the F!slot motor$ because of %hch the number of

turns per col s more for the same amount of copper' Ths ncreases the pea# alue of

the bac# emf' The ma+mum col span or %ndng ptch s determned b" ddng the

number of slots b" the number of poles and roundng off to the ne+t lo%est nteger '

/or the 5:!slot motor$ the slots-pole s =$ and th!$ cols are full!ptched$ %hch

ma+mes the %dth of the bac#!emf' 0n the F!slot motor$ the slots-pole s 5'6$ and the

col span used s 5 because of %hch the %dth of the bac#!emf %aeform s smaller'

Ths effect can also be acheed b" short!ptchng the cols n an ntegral

slots-pole desgn' The possble short!ptch col spans for the 5:!slot motor are 5 and :'

9sng a col span of 5 %ould ma#e the bac#!emf %dth too narro% and ncrease the

tor&ue rpple' A col span of : %ould be deal$ but %ould leae half the slots

unutled' 9sng a fractonal slots-pole motor has the addtonal adantage of

reducng the coggng tor&ue' 0f the number of slots s ncreased to :> or =:$ man"

more combnatons are possble for obtanng smaller bac#!emf %dth$ and the

desgner can then ma#e a choce based on other consderatons'

3o%eer$ n general$ the smallest number of slots ges the lo%est labor cost

n %ndng$ and a col span of 5 or : slots mnmes the end turns' Note that smlar

results could also be obtaned b" usng full!ptch stator cols and a magnet pole arc of

5:7 electrcal as dscussed'


22/90

/rom Table 55$ %e see that the =!ph F!slot motor e+cted %th 57

unpolar currents ges better performance n terms of tor&ue rpple$ %th some loss n

pea# and aerage tor&ue' Ths s e+planed as follo%s' 0n /g' l(d)$ %e hae one phase

conductng durng nterals :$> and F and t%o phases conductng durng nterals 5$=

and 5. 0n partcular$ %hen the bac#!emf of phase A reaches ts pea# durng nteral :$

onl" phase A s conductng'

*hen the bac#!emf of phase A starts decreasng n nteral =$ phase B also

comes nto conducton' The decreasng tor&ue contrbuton of phase A s compensated

b" the ncreasng contrbuton from phase B' The result s an almost constant tor&ue

oer the entre c"cle' The remanng case of the F!slot motor e+cted %th 5:F unpolar

currents results n hgh tor&ue rpple because the bac#!emf durng the commutaton

nstants s lo%'

Both the unpolar and the bpolar dres re&ure three hall!effect sensors$ %th

the second and thrd dsplaced b" 5:7 and 240" electrcal respectel" from the frst'

The bpolar dre re&ures s+ s%tches %hle the unpolar dre re&ures onl" three$

albet %th hgher current ratngs' The adantage of usng 5:7 currents s that %e

re&ure onl" one current sensor n the dc ln#' 3o%eer$ the tor&ue rpple s not lo%

enough' 0t can be reduced further b" ncreasng the

number of phases to four'


23/90

FOUR:PHASE MOTORS

T%o >!phase motors are consdered8 @ne %th I; slots$ and the other %th

$#ot$ as sho%n n /g' K' Both motors are ntegral slots-pole desgns' The 5F!slot

motor s short!ptched b" a factor of 2, %hle the slot motor s full!ptched$ %hch


24/90


25/90

/g'F' Tor&ue outputs of =!Phase Motor (a) 5:!slot motor %th 5:7 bpolar

current$ (b) 5:!slot motor %th 5:7 unpolar current$ (c) 5:!slot motor %th 57

unpolar current' (d) F!slot motor %th 5:7 unpol+ current$ (e) F!slot motor %th 57

unpolar current'

E+plans the dfference n the %dth of ther bac#!emf %aeforms sho%n n

/g' ' The number of turns-phase s more n the 5F!slot motor because of the shorter

end!turns$ %hch e+plans the hgher pea# of ts bac#!emf' These motors are e+cted

%th the current %aeforms of /g' l(e) and (0. The tor&ue outputs are sho%n n /g'

J and Table 555 ges the numercal alues' /or both motors$ usng J7 conducton

ges better results because the commutaton bet%een phases ta#es place %hen the

bac#!emfs are hgh'

0n addton$ t re&ures the use of onl" a sngle current sensor n the dc ln#' 0n

the 57 conducton scheme$ t%o phases conduct at all tmes$ and the bac#!emf of the

ncomng and outgong phases are lo%$ resultng n large tor&ue rpple' 0t also re&ures

the use of a current sensor n each phase'

POWER FACTOR


26/90

The electrcal energ" s almost e+clusel" generated$ transmtted and

dstrbuted n the form of alternatng current' Therefore$ the &ueston of po%er factor

mmedatel" comes nto pcture' Most of the loads (e'g' nducton motors$ arc lamps)

are nducte n nature and hence hae lo% laggng po%er factor' The lo% po%er

factor s hghl" undesrable as t causes an ncrease n current$ resultng n addtonal

losses of acte po%er n all the elements of po%er s"stem from po%er staton

generator do%n to the utlaton deces' 0n order to ensure most faorable condtons

for a suppl" s"stem from engneerng and economcal standpont$ t s mportant to

hae po%er factor as close to unt" as possble' 0n ths chapter$ %e shall dscuss the

arous methods of po%er factor mproement'

Po%er /actor

The cosne of angle bet%een oltage and current n an a'c' crcut s #no%n as

po%er factor.0n an a'c' crcut$ there s generall" a phase dfference bet%een oltage

and current' The term cos s called the po%er factor of the crcut' 0f the crcut s

nducte$ the current lags behnd the oltage and the po%er factor s referred to as

laggng' 3o%eer$ n a capacte crcut$ current leads the oltage and po%er factor s

sad to be leadng' Consder an nducte crcut ta#ng a laggng current 0 from suppl"

oltage 1 the angle of lag beng ' The phasor dagram of the crcut s sho%n n /g'

F'5'

The crcut current 0 can be resoled nto t%o perpendcular components$ namel"

a/ 0 cos n phase %th 1

%/ 0 sn J7o out of phase %th 1

The component 0 cos s #no%n as acte or %att full component$ %hereas component

0 sn s called the reacte or %att less component' The reacte component s a


27/90

measure of the po%er factor' 0f the reacte component s small$ the phase angle s

small and hence

po%er factor cos %ll be hgh' Therefore$ a crcut hang small reacte current ('e'$

0 sn ) %ll hae hgh po%er factor and ce!ersa' 0t ma" be noted that alue of

po%er factor can neer be more than unt"'

i/ 0t s a usual practce to attach the %ord ,laggng. or ,leadng. %th the numercal

alue of po%er factor to sgnf" %hether the current lags or leads the oltage' Thus f

the crcut has a p'f' of 7O6 and the current lags the oltage$ %e generall" %rte p'f' as

7O6 laggng'

ii/ 2ometmes po%er factor s e+pressed as a percentage' Thus 7O laggng po%er

factor ma" be e+pressed as 7 laggng'

P@*ER TR0ANLE

The anal"ss of po%er factor can also be made n terms of po%er dra%n b" the a'c'

crcut' 0f each sde of the current trangle oab of /g' F'5 s multpled b" oltage 1$

then %e get the po%er trangle @AB sho%n n /g' F': %here

1. @A Q 10 cos and represents the acte po%er n %atts or #*

AB Q 10 sn and represents the reacte po%er n 1AR or #1AR

@B Q 10 and represents the apparent po%er n 1A or #1A

The follo%ng ponts ma" be noted form the po%er trangle 8

i/ The apparent po%er n an a'c' crcut has t%o components '$ acte and reacte

po%er at rght angles to each other'


28/90

or (apparent po%er): Q (acte po%er): (reacte po%er):

or

ii/

Thus the po%er factor of a crcut ma" also be defned as the rato of acte po%er to

the apparent po%er' Ths s a perfectl" general defnton and can be appled to all

cases$ %hateer be the %aeform'

iii/ The laggngS reacte po%er s responsble for the lo% po%er factor' 0t s clear

from the po%er trangle that smaller the reacte po%er component$ the hgher s the

po%er factor of the crcut'

i'/ /or leadng currents$ the po%er trangle becomes reersed' Ths fact prodes a

#e" to the po%er factor mproement' 0f a dece ta#ng leadng reacte po%er (e'g'

capactor) s connected n parallel %th the load$ then the laggng reacte po%er of the

load %ll be partl" neutralsed$ thus mprong the po%er factor of the load'

'/ The po%er factor of a crcut can be defned n one of the follo%ng three %a"s8

'i/ The reacte po%er s nether consumed n the crcut nor t does an" useful %or#'

0t merel" flo%s bac# and forth n both drectons n the crcut' A %attmeter does not

measure reacte po%er'Dsadantages of Lo% Po%er /actor

The po%er factor pla"s an mportance role n a'c' crcuts snce po%er consumed

depends upon ths factor'


29/90

0t s clear from aboe that for f+ed po%er and oltage$ the load current s nersel"

proportonal to the po%er factor' Lo%er the po%er factor$ hgher s the load current

and ce!ersa' A po%er factor less than unt" results n the follo%ng dsadantages 8

i/ La"8! 7 A

%hch %ould be re&ured at unt" po%er factor'

iii/ La"8! copp!" #o$$!$. The large current at lo% po%er factor causes more 0:R

losses n all the elements of the suppl" s"stem' Ths results n poor effcenc"'


30/90

i'/ Poo" 'o#ta8! "!8)#ation. The large current at lo% laggng po%er factor causes

greater oltage drops n alternators$ transformers$ transmsson lnes and dstrbutors'

Ths results n the decreased oltage aalable at the suppl" end$ thus mparng the

performance of utlsaton deces' 0n order to #eep the receng end oltage %thn

permssble lmts$ e+tra e&upment ('e'$ oltage regulators) s re&ured'

'/ R!+)c!+ -an+#in8 capacit& o6 $&$t!m. The laggng po%er factor reduces the

handlng capact" of all the elements of the s"stem' 0t s because the reacte

component of current preents the full utlaton of nstalled capact"'

The aboe dscusson leads to the concluson that lo% po%er factor s an ob?ectonable

feature n the suppl" s"stem

CA92E2 @/ L@* P@*ER /ACT@R

Lo% po%er factor s undesrable from economc pont of e%' Normall"$ the

po%er factor of the %hole load on the suppl" s"stem n lo%er than 7O' The follo%ng

are the causes of lo% po%er factor8

i/ Most of the a'c' motors are of nducton t"pe (5 and = nducton motors) %hch

hae lo% laggng po%er factor' These motors %or# at a po%er factor %hch s

e+tremel" small on lght load (7O: to 7O=) and rses to 7O or 7OJ at full load'

ii/ Arc lamps$ electrc dscharge lamps and ndustral heatng furnaces operate at lo%

laggng po%er factor'

iii/ The load on the po%er s"stem s ar"ng beng hgh durng mornng and eenng

and lo% at other tmes' Durng lo% load perod$ suppl" oltage s ncreased %hch

ncreases the magnetaton current' Ths results n the decreased po%er factor'

P@*ER /ACT@R 0MPR@1EMENT

The lo% po%er factor s manl" due to the fact that most of the po%er loads are

nducte and$ therefore$ ta#e laggng currents' 0n order to mproe the po%er factor$

some dece ta#ng leadng po%er should be connected n parallel %th the load' @ne

of such deces can be a capactor' The capactor dra%s a leadng current and partl" or


31/90

completel" neutralses the laggng reacte component of load current' Ths rases the

po%er factor of the load'

P@*ER /ACT@R 0MPR@1EMENT E490PMENT

Normall"$ the po%er factor of the %hole load on a large generatng staton s n

the regon of 7O to 7OJ' 3o%eer$ sometmes t s lo%er and n such cases t s

generall" desrable to ta#e specal steps to mproe the po%er factor' Ths can be

acheed b" the follo%ng e&upment8

1. Static capacito". The po%er factor can be mproed b" connectng capactors n

parallel %th the e&upment operatng at laggng po%er factor' The capactor

(generall" #no%n as statc capactor) dra%s a leadng current and partl" or completel"

neutralses the laggng reacte component of load current' Ths rases the po%er

factor of the load' /or three!phase loads$ the capactors can be connected n delta or

star as sho%n n /g' F'>' 2tatc capactors are narabl" used for po%er factor

mproement n factores'


32/90

AD7ANTA3ES

i/ The" hae lo% losses'

ii/ The" re&ure lttle mantenance as there are no rotatng parts'

iii/ The" can be easl" nstalled as the" are lght and re&ure no foundaton'

i'/ The" can %or# under ordnar" atmospherc condtons'

DISAD7ANTA3ES

i/ The" hae short serce lfe rangng from to 57 "ears'

ii/ The" are easl" damaged f the oltage e+ceeds the rated alue'

iii/ @nce the capactors are damaged$ ther repar s uneconomcal'

>. S&nc-"ono)$ con+!n$!". A s"nchronous motor ta#es a leadng current %hen oer!

e+cted and$ therefore$ behaes as a capactor' An oer!e+cted s"nchronous motor

runnng on no load s #no%n as s"nchronous condenser' *hen such a machne s

connected n parallel %th the suppl"$ t ta#es a leadng current %hch partl"

neutralses the laggng reacte component of the load' Thus the po%er factor s

mproed' /g F'6 sho%s the po%er factor mproement b" s"nchronous condenser

method' The = load ta#es current 0L at lo% laggng po%er factor cos L' The

s"nchronous condenser ta#es a current 0m %hch leads the oltage b" an angle mS'


33/90

The resultant current 0 s the phasor sum of 0m and 0L and lags behnd the oltage b"

an angle ' 0t s clear that s less than L so that cos s greater than cos L' Thus

the po%er factor s ncreased from cos L to cos ' 2"nchronous condensers are

generall" used at ma?or bul# suppl" substatons for po%er factor mproement'

A+'anta8!$

i/ B" ar"ng the feld e+ctaton$ the magntude of current dra%n b" the motor can

be changed b" an" amount' Ths helps n acheng stepless control of po%er factor'

ii/ The motor %ndngs hae hgh thermal stablt" to short crcut currents'

iii/ The faults can be remoed easl"'

Di$a+'anta8!$

i/ There are consderable losses n the motor'

ii/ The mantenance cost s hgh'

iii/ 0t produces nose'

i'/ E+cept n ses aboe 677 #1A$ the cost s greater than that of statc capactors of

the same ratng'

'/ As a s"nchronous motor has no self!startng tor&ue$ therefore$ an au+lar"

e&upment has to be proded for ths purpose'

*. P-a$! a+'anc!"$. Phase adancers are used to mproe the po%er factor of

nducton motors' The lo% po%er factor of an nducton motor s due to the fact that

ts stator %ndng dra%s e+ctng current %hch lags behnd the suppl" oltage b" J7o'

0f the e+ctng ampere turns can be proded from some other a'c' source$ then the

stator %ndng %ll be releed of e+ctng current and the po%er factor of the motor

can be mproed' Ths ?ob s accomplshed b" the phase adancer %hch s smpl" an

a'c' e+cter' The phase adancer s mounted on the same shaft as the man motor and

s connected n the rotor crcut of the motor' 0t prodes e+ctng ampere turns to the

rotor crcut at slp fre&uenc"' B" prodng more ampere turns than re&ured$ the

nducton motor can be made to operate on leadng po%er factor l#e an oer!e+cted

s"nchronous motor'


34/90

Phase adancers hae t%o prncpal adantages' /rstl"$ as the e+ctng ampere

turns are suppled at slp fre&uenc"$ therefore$ laggng #1AR dra%n b" the motor are

consderabl" reduced' 2econdl"$ phase adancer can be conenentl" used %here the

use of s"nchronous motors s nadmssble' 3o%eer$ the ma?or dsadantage of phase

adancers s that the" are not economcal for motors belo% :77 3'P'

P@*ER /ACT@R C@RRECT0@N

An electrc utlt"s po%er load on an electrcal dstrbuton s"stem fall nto one

of three categores resste$ nducte or capacte' 0n most ndustral facltes$ the

most common po%er usages are nducte' E+amples of nducte loads nclude

transformers$ fluorescent lghtng and AC nducton motors' Most nducte loads use

a conducte col %ndng to produce an electromagnetc feld %hch permts the

motor to functon'

All nducte loads re&ure t%o dfferent t"pes of po%er for the motor to operate8

Acte po%er (measured n #* or #lo%atts) ! ths po%er produces the mote force

Reacte po%er (#ar) ! ths energes the magnetc feld of the motor'

The operatng po%er from the dstrbuton s"stem s composed of both acte

(%or#ng) and reacte (non!%or#ng) elements' The acte po%er does useful %or# ndrng the motor %hereas the reacte po%er onl" prodes the magnetc feld'

9nfortunatel"$ electrc utlt"s customers are charged for both acte and reacte

po%er'

E+ample8 A customers po%er factor drops$ the s"stem becomes less effcent'

A drop from 5'7 to 7'J results n 56 more current beng re&ured for the same load'

A po%er factor of 7'K re&ures appro+matel" >7 more current and a po%er factor of

7'6 re&ures appro+matel" 577 (t%ce as much) to handle the same load' The

ans%er to these problems s to reduce the reacte po%er dra%n from the suppl" b"

mprong the po%er factor'


35/90

0f an AC motor %ere 577 effcent t %ould consume onl" acte po%er'

3o%eer$ snce most AC motors are onl" K6 to 7 effcent$ the" operate at a

lo%er po%er factor' Ths means neffcent and een %asteful energ" usage and cost

effcenc" because most electrc utltes charge penaltes for poor$ neffcent po%er

factor' 2mpl" nstallng capactors %ll mproe a commercal or ndustral compan"s

po%er factor and %ll result n sangs on ther electrct" bll eer" monthU Addtonal

potental benefts for correctng poor po%er factor ncludes8

Reducton of heatng losses n transformers and dstrbuton e&upment Longer

e&upment lfe

2tabled oltage leels

0ncreased capact" of "our e+stng s"stem and e&upment

0mproed proftablt"

Lo%ered e+penses

DC:DC CON7ERTER BASICS

A DC!to!DC conerter s a dece that accepts a DC nput oltage and produces a DCoutput oltage' T"pcall" the output produced s at a dfferent oltage leel than the

nput' 0n addton$ DC!to!DC conerters are used to prode nose solaton$ po%er bus

regulaton$ etc' Ths s a summar" of some of the popular DC!to!DC conerter

topologes'

BUC? CON7ERTER STEP:DOWN CON7ERTER

0n ths crcut the transstor turnng @N %ll put oltage 1inon one end of the nductor'

Ths oltage %ll tend to cause the nductor current to rse' *hen the transstor s @//$

the current %ll contnue flo%ng through the nductor but no% flo%ng through the

dode'


36/90

*e ntall" assume that the current through the nductor does not reach ero$ thus the

oltage at 10 %ll no% be onl" the oltage across the conductng dode durng the full

@// tme' The aerage oltage at 10%ll depend on the aerage @N tme of the

transstor proded the nductor current s contnuous'

Buc# Conerter

1oltage and current changes

To anal"e the oltages of ths crcut let us consder the changes n the nductor

current oer one c"cle' /rom the relaton

VVVVVV' (5)

the change of current satsfes


37/90

VVV' (:)

/or stead" state operaton the current at the start and end of a perod T %ll not

change' To get a smple relaton bet%een oltages %e assume no oltage drop across

transstor or dode %hle @N and a perfect s%tch change' Thus durng the @N tme

1+Q1nand n the @// 1+Q7' Thus

VVVVVVV' (=)

*hch smplfes to

VVVVV (>)

or

VVVVV (6)

and defnng dut" rato as

VVV' (F)

the oltage relatonshp becomes 1oQD 1n 2nce the crcut s lossless and the nput

and output po%ers must match on the aerage 1oS 0oQ 1nS 0n' Thus the aerage nput

and output current must satsf" 0 nQD 0oThese relatons are based on the assumpton

that the nductor current does not reach ero'

T"an$ition %!t!!n contin)o)$ an+ +i$contin)o)$

*hen the current n the nductor L remans al%a"s poste then ether the transstor

T5 or the dode D5 must be conductng' /or contnuous conducton the oltage 1+s


38/90

ether 1nor 7' 0f the nductor current eer goes to ero then the output oltage %ll not

be forced to ether of these condtons' At ths transton pont the current ?ust reaches

ero as seen n /gure (buc# booster boundar")' Durng the @N tme 1 n!1outs across

the nductor thus

@/

The aerage current %hch must match the output current satsfes

/

Buc# Conerter at Boundar"

0f the nput oltage s constant the output current at the transton pont satsfes

(J)


39/90

7o#ta8! Ratio o6 B)c< Con'!"t!" Di$contin)o)$ Mo+!/

As for the contnuous conducton anal"ss %e use the fact that the ntegral of oltage

across the nductor s ero oer a c"cle of s%tchng T' The transstor @// tme s no%

dded nto segments of dode conducton ddT and ero conducton doT' The nductor

aerage oltage thus ges

(Vin- Vo) DT + (-Vo)dT = 0 (57)

Buc# Conerter ! Dscontnuous Conducton

(55)

for the case ' To resole the alue of consder the output current %hch s

half the pea# %hen aeraged oer the conducton tmes

(5:)

Consderng the change of current durng the dode conducton tme


40/90

(5=)

Thus from (F) and (K) %e can get

(5>)

usng the relatonshp n (6)

(56)

and solng for the dode conducton

(5F)

The output oltage s thus gen as

(5K)

Defnng #S Q :L-(1nT)$ %e can see the effect of dscontnuous current on the oltage

rato of the conerter'


41/90

The aboe fgure sho%s @utput 1oltage s Current

As seen n the fgure$ once the output current s hgh enough$ the oltage rato depends

onl" on the dut" rato d' At lo% currents the dscontnuous operaton tends to

ncrease the output oltage of the conerter to%ards 1n'

BOOST CON7ERTER STEP:UP CON7ERTER

The schematc n /g' F sho%s the basc boost conerter' Ths crcut s used %hen a

hgher output oltage than nput s re&ured'

Boost Conerter Crcut


42/90

*hle the transstor s @N 1+ Q1n$ and the @// state the nductor current flo%s

through the dode gng 1+ Q1o' /or ths anal"ss t s assumed that the nductor

current al%a"s remans flo%ng (contnuous conducton)' The oltage across the

nductor s sho%n n /g' K and the aerage must be ero for the aerage current to

reman n stead" state

VVVV (5)

Ths can be rearranged as

VVV' (5J)

and for a lossless crcut the po%er balance ensures

VVV'' (:7)

1oltage and current %aeforms (Boost Conerter)

2nce the dut" rato D s bet%een 7 and 5 the output oltage must al%a"s be hgher

than the nput oltage n magntude' The negate sgn ndcates a reersal of sense of

the output oltage'


43/90

BUC?:BOOST CON7ERTER

2chematc for buc#!boost conerter

*th contnuous conducton for the Buc#!Boost conerter 1+Q1n%hen the transstor

s @N and 1+Q1o%hen the transstor s @//' /or ero net current change oer a

perod the aerage oltage across the nductor s ero

*aeforms for buc#!boost conerter

VVVV'' (:5)

%hch ges the oltage rato

VVVV (::)

and the correspondng current


44/90

VVV'' (:=)

2nce the dut" rato D s bet%een 7 and 5 the output oltage can ar" bet%een lo%er

or hgher than the nput oltage n magntude' The negate sgn ndcates a reersal of

sense of the output oltage'

C@N1ERTER C@MPAR02@N

The oltage ratos acheable b" the DC!DC conerters s summared n /g' 57'

Notce that onl" the buc# conerter sho%s a lnear relatonshp bet%een the control

(dut" rato) and output oltage' The buc#!boost can reduce or ncrease the oltage

rato %th unt gan for a dut" rato of 67'

Comparson of 1oltage rato


45/90

CU? CON7ERTER

The buc#$ boost and buc#!boost conerters all transferred energ" bet%een nput andoutput usng the nductor$ anal"ss s based of oltage balance across the nductor' The

C9W conerter uses capacte energ" transfer and anal"ss s based on current

balance of the capactor' The crcut n /g' belo% (C9W conerter) s dered from

D9AL0TX prncple on the buc#!boost conerter'

C9W Conerter

0f %e assume that the current through the nductors s essentall" rpple free %e can

e+amne the charge balance for the capactor C5' /or the transstor @N the crcut

becomes

C9W @N!2TATE

and the current n C5 s 0L5' *hen the transstor s @//$ the dode conducts and the

current n C5 becomes 0L:'


46/90

C9W @//!2TATE

2nce the stead" state assumes no net capactor oltage rse $the net current s ero

VVVVV (:>)

%hch mples

VV'' (:6)

The nductor currents match the nput and output currents$ thus usng the po%er

conseraton rule

VVVV (:F)

Thus the oltage rato s the same as the buc#!boost conerter' The adantage of the

C9W conerter s that the nput and output nductors create a smooth current at both

sdes of the conerter %hle the buc#$ boost and buc#!boost hae at least one sde %th

pulsed current'

I$o#at!+ DC:DC Con'!"t!"$

0n man" DC!DC applcatons$ multple outputs are re&ured and output solaton ma"

need to be mplemented dependng on the applcaton' 0n addton$ nput to output


47/90

solaton ma" be re&ured to meet safet" standards and - or prode mpedance

matchng'

The aboe dscussed DC!DC topologes can be adapted to prode solaton bet%een

nput and output'

F#& %ac< Con'!"t!"

The fl" bac# conerter can be deeloped as an e+tenson of the Buc#!Boost conerter'

/g (a) sho%s the basc conerter /g (b)(replacng nductor b" transformer) replaces

the nductor b" a transformer' The buc#!boost conerter %or#s b" storng energ" n

the nductor durng the @N phase and releasng t to the output durng the @// phase'

*th the transformer the energ" storage s n the magnetaton of the transformer

core' To ncrease the stored energ" a gapped core s often used'

0n /g (c) the solated output s clarfed b" remoal of the common reference of the

nput and output crcuts'

(a) Buc#!Boost Conerter

(b) Replacng nductor b" transformer


48/90

(c) /l" bac# conerter re!confgured

Fo"a"+ Con'!"t!"

The concept behnd the for%ard conerter s that of the deal transformer conertng

the nput AC oltage to an solated secondar" output oltage' /or the crcut n /g'

(for%ard conerter)$ %hen the transstor s @N$ 1n appears across the prmar" and

then generates

VVVV (:K)

The dode D5 on the secondar" ensures that onl" poste oltages are appled to the

output crcut %hle D: prodes a crculatng path for nductor current f the

transformer oltage s ero or negate'


49/90

/or%ard Conerter

The problem %th the operaton of the crcut n /g aboe (for%ard conerter) s that

onl" poste oltage s appled across the core$ thus flu+ can onl" ncrease %th the

applcaton of the suppl"' The flu+ %ll ncrease untl the core saturates %hen the

magnetng current ncreases sgnfcantl" and crcut falure occurs' The transformer

can onl" sustan operaton %hen there s no sgnfcant DC component to the nput

oltage' *hle the s%tch s @N there s poste oltage across the core and the flu+

ncreases' *hen the s%tch turns @// %e need to suppl" negate oltage to reset the

core flu+' The crcut n /g' belo% sho%s a tertar" %ndng %th a dode connecton

to permt reerse current' Note that the dot conenton for the tertar" %ndng s

opposte those of the other %ndngs' *hen the s%tch turns @// current %as flo%ngn a dot termnal' The core nductance act to contnue current n a dotted termnal'

/or%ard conerter %th tertar" %ndng

Sin8#!:p-a$! 'o#ta8! $o)"c! in'!"t!"$

2ngle!phase oltage source nerters (120s) can be found as half!brdge and

full!brdge topologes' Although the po%er range the" coer s the lo% one$ the" are

%del" used n po%er supples$ sngle!phase 9P2s$ and currentl" to form elaborate

hgh!po%er statc po%er topologes$ such as for nstance$ the mult cell confguratons

that are ree%ed' The man features of both approaches are ree%ed and presented n

the follo%ng'


50/90

T&p!$ o6 7SI

Ha#6:B"i+8! 7SI

The po%er topolog" of a half!brdge 120$ %here t%o large capactors are

re&ured to prode a neutral pont N$ such that each capactor mantans a constant

oltageQ:' Because the current harmoncs n?ected b" the operaton of the nerter are

lo%!order harmoncs$ a set of large capactors (C' and CY) s re&ured' 0t s clear that

both s%tches 2' and 2Y cannot be on smultaneousl" because short crcut across the

dc ln# oltage source %ould be produced' There are t%o defned (states 5 and :)

and one undefned (state =) s%tch state as sho%n n Table 5>'5' 0n order to aod the

short crcut across the dc bus and the undefned ac output oltage condton$ the

modulatng techn&ue should al%a"s ensure that at an" nstant ether the top or the

bottom s%tch of the nerter leg s on'

sho%s the deal %aeforms assocated %th the half!brdge nerter sho%n n /g'

5>':' The states for the s%tches 2' and 2Y are defned b" the modulatng techn&ue$

%hch n ths case s a carrer!based P*M'

The Carrer!Based Pulse %dth Modulaton (P*M) Techn&ue8 As mentoned

earler$ t s desred that the ac output oltage' 1a N follo% a gen %aeform (e'g'$


51/90

snusodal) on a contnuous bass b" properl" s%tchng the po%er ales' The carrer!

based P*M techn&ue fulfls such a re&urement as t defnes the on and off states of

the s%tches of one leg of a 120 b" comparng a modulatng sgnal c (desred ac

output oltage) and a trangular %aeform D (carrer sgnal)' 0n practce$ %hen c Z

D the s%tch 2' s on and the s%tch s off smlarl"$ %hen c [ D the s%tch 2' s

off and the s%tch 2Y s on'

A specal case s %hen the modulatng sgnal c s a snusodal at fre&uenc" fc

and ampltude \c $ and the trangular sgnal D s at fre&uenc" fD and ampltude

\D' Ths s the snusodal P*M (2P*M) scheme' 0n ths case$ the modulaton nde+

ma (also #no%n as the ampltude!modulaton rato) s defned as

And the normaled carrer fre&uenc" mf (also #no%n as the fre&uenc"!modulaton

rato) s

' 1an s bascall" a snusodal %aeform plus harmoncs$ %hch features8 (a)

the ampltude of the fundamental component of the ac output oltage \o5 satsf"ng

the follo%ng e+presson8


52/90


53/90

%ll be dscussed later) (b) for odd alues of the normaled carrer fre&uenc" mf the

harmoncs n the ac output oltage appear at normaled fre&uences fh centered

around mf and ts multples$ specfcall"$

*here # ' : > F ' ' ' for l ' 5 = 6 ' ' ' and # ' 5 = 6 ' ' 'for l ' : > F ' ' '

(c) the ampltude of the ac output oltage harmoncs s a functon of the modulaton

nde+ ma and s ndependent of the normaled carrer fre&uenc" mf form f Z J (d)

the harmoncs n the dc ln# current (due to the modulaton) appear at normaled


54/90

fre&uences fp centered around the normaled carrer fre&uenc" mf and ts multples$

specfcall"$

*here # ' : > F ' ' ' for l ' 5 = 6 ' ' ' and # ' 5 = 6 ' 'for l ' : > F ' ' ' '

Addtonal mportant ssues are8 (a) for small alues of mf (mf [ :5)$ the carrer sgnal

D and the modulatng sgnal c should be s"nchroned to each other(mf nteger)$

%hch s re&ured to hold the preous features f ths s not the case$ sub harmoncs

%ll be present n the ac output oltage (b) for large alues of mf (mf Z :5)$ the sub

harmoncs are neglgble f an as"nchronous P*M

techn&ue s used$ ho%eer$ due to potental er" lo%!order sub harmoncs$ ts

use should be aoded fnall" (c) n the oer modulaton regon (ma Z 5) some

ntersectons bet%een the carrer and the modulatng sgnal are mssed$ %hch leads to

the generaton of lo%!order harmoncs but a hgher fundamental ac output oltage s

obtaned unfortunatel"$ the lneart" bet%een ma and \o5acheed n the lnear

regon does not hold n the oer modulaton regon$ moreoer$ a saturaton effect can

be obsered

The P*M techn&ue allo%s an ac output oltage to be generated that trac#s a

gen modulatng sgnal' A specal case s the 2P*M techn&ue (the modulatng


55/90

sgnal s a snusodal) that prodes n the lnear regon an ac output oltage that ares

lnearl" as a functon of the modulaton nde+ and the harmoncs are at %ell!defned

fre&uences and ampltudes'

These features smplf" the desgn of flterng components' 9nfortunatel"$ the

ma+mum ampltude of the fundamental ac oltage s Q: n ths operatng mode'

3gher oltages are obtaned b" usng the oer modulaton regon (ma Z 5) ho%eer$

lo%!order harmoncs appear n the ac output oltage'

S()a"!:Wa'! Mo+)#atin8 T!c-ni()!

Both s%tches 2' and 2Y are on for one!half c"cle of the ac output perod' Ths

s e&ualent to the 2P*M techn&ue %th an nfnte modulaton nde+ ma' /gure

5>'6 sho%s the follo%ng8 (a) the normaled ac output oltage harmoncs are at

fre&uences h ' = 6 K J ' ' ' $ and for a gen dc ln# oltage (b) the fundamental ac

output oltage features an ampltude gen b"

and the harmoncs feature an ampltude gen b"

S!#!cti'! Ha"monic E#imination

The man ob?ecte s to obtan a snusodal ac output oltage %aeform %here

the fundamental component can be ad?usted arbtrarl" %thn a range and the ntrnsc

harmoncs selectel" elmnated' Ths s acheed b" mathematcall" generatng the

e+act nstant of the turn!on and turn!off of the po%er ales'


56/90

The ac output oltage features odd half! and &uarter %ae s"mmetr"

therefore$ een harmoncs are not present(oh ' 7 h ' : > F ' ' ')' Moreoer$ the per!

phase oltage %aeform (o ' aN)$ should be chopped N tmes per half!c"cle n order

to ad?ust the fundamental and elmnate N Y 5 harmoncs n the ac output oltage

%aeform' /or nstance$ to elmnate the thrd and ffth harmoncs and to perform

fundamental magntude control (N' =)$ the e&uatons to be soled are the follo%ng8

%here the angles a5$ a:$ and a= are defned as sho%n' The angles are found b" means

of terate algorthms as no anal"tcal solutons can be dered' The angles a5$ a:$

and


57/90

are plotted for dfferent alues of n /g' 5>'Ka' The general e+pressons to

elmnate an een N Y 5'N Y 5 ' : > F ' ' ') number of harmoncs s

*here a5$ a: ' ' ' an should satsf" a5 [ a: [ ] ] ] [ aN [pQ:' 2mlarl"$ to

elmnate an odd number of harmoncs$ for nstance$ the thrd$ ffth and seenth$ and

to perform


58/90

/undamental magntude control (N Y 5 ' =)$ the e&uatons to be soled are8

*here the angles a5 a: a=$ and a> are defned as sho%n n /g'5>'Fb' The angles a5

a:$ a= and a> are plotted for dfferent alues of 'The general e+pressons to


59/90

elmnate an odd N !5 (N Y 5 ' = 6 K ' ' ') number of harmoncs are gen b"

F)##:B"i+8! 7SI

The po%er topolog" of a full!brdge 120' Ths nerter s smlar to the half!brdge nerter ho%eer$ a second leg prodes the neutral pont to the load' As

e+pected$ both s%tches 25' and 25Y (or 2:' and 2:Y) cannot be on smultaneousl"

because a short crcut across the dc ln# oltage source %ould be produced'

The undefned condton should be aoded so as to be al%a"s capable of defnng the

ac output oltage' 0n order to aod the short crcut across the dc bus and the


60/90

undefned ac output oltage condton$ the modulatng techn&ue should ensure that

ether the top or the bottom s%tch of each leg s on at an" nstant' 0t can be obsered

that the ac output oltage can ta#e alues up to the dc ln# alue $ %hch s t%ce

that obtaned %th half!brdge 120 topologes'

2eeral modulatng techn&ues hae been deeloped that are applcable to full!brdge

120s' Among them are the P*M (bpolar and unpolar) techn&ues'

Bipo#a" PWM T!c-ni()!

2tates 5 and : (Table 5>':) are used to generate the ac output oltage n ths

approach' Thus$ the ac output oltage %aeform features onl" t%o alues$ %hch are

and Y' To generate the states$ a carrer!based techn&ue can be used a sne half!

brdge confguratons %here onl" one snusodal modulatng sgnal has been used' 0t

should be noted that the on state n s%tch 2' n the half!brdge corresponds to both

s%tches 25' and 2:Y beng n the on state n the full!brdge confguraton'

2mlarl"$ 2Y n the on state n the half!brdge corresponds to both s%tches

25Y and2:' beng n the on state n the full!brdge confguraton' Ths s called bpolar

carrer!based 2P*M' The ac output oltage %aeform n a full!brdge 120 s

bascall" a snusodal %aeform that features a fundamental component of ampltude

\o5that satsfes the e+presson


61/90

0n the lnear regon of the modulatng techn&ue (ma 5)$%hch s t%ce that

obtaned n the half!brdge 120' 0dentcal conclusons can be dra%n for the

fre&uences and ampltudes of the harmoncs n the ac output oltage and dc ln#

current$ and for operatons at smaller and larger alues of odd mf(ncludng the oer

modulaton regon (ma Z 5))$ than n half brdge 120s$ but consderng that the

ma+mum ac output oltage s the dc ln# oltage '

Thus$ n the oer modulaton regon the fundamental component of ampltude

\o5 satsfes the e+presson

0n contrast to the bpolar approach$ the unpolar P*M techn&ue uses the

states 5$ :$ =$ and to generate the ac output oltage' Thus$ the ac output oltage

%aeform can nstantaneousl" ta#e one of three alues$ namel" The sgnal c

s used to generate an$ and s used to generate bn '@n the other

hand$ thus Ths s called unpolar

carrer!based P*M'

0dentcal conclusons can be dra%n for the ampltude of the fundamental

component and harmoncs n the ac output oltage and dc ln# current$ and for

operatons at smaller and larger alues of mf $ (ncludng the oer modulaton regon

(ma Z 5))$ than n full!brdge 120s modulated b" the bpolar 2P*M' 3o%eer$

because the phase oltages are dentcal but 57] out of phase$ the

output oltage %ll not contan een harmoncs' Thus$ f mf sta#en een$ the harmoncs n the ac output oltage appear at normaled odd

fre&uences fh centered around t%ce the normaled carrer fre&uenc" mf and ts

multples' 2pecfcall"$

*here # ' 5 = 6 ' ' ' and the harmoncs n the dc ln#


62/90

current appear at normaled fre&uences fp centered around t%ce the normaled

carrer fre&uenc" mf and ts multples' 2pecfcall"$

*here #' 5 = 6 ' ' '' Ths feature s consdered to be an adantage because t allo%sthe use of smaller flterng components to obtan hgh!&ualt" oltage and current

%aeforms %hle usng the same s%tchng fre&uenc" as n 120s modulated b" the

bpolar approach'

S!#!cti'! Ha"monic E#imination

0n contrast to half!brdge 120s$ ths approach s appled n a per!lne fashon

for full!brdge 120s' The ac output oltage features odd half! and &uarter!%ae

s"mmetr" therefore$ een harmoncs are not present

Moreoer$ the ac output oltage %aeform n /g' 5>')$ should feature Npulses per half!c"cle n order to ad?ust the fundamental component and elmnate N Y

5 harmoncs' /or nstance$ to elmnate the thrd$ ffth and seenth harmoncs and to

perform fundamental magntude control (N ' >)$ the e&uatons to be soled are8

The general e+pressons to elmnate an arbtrar" N number of

harmoncs are gen b"


63/90

2ho%s a specal case %here onl" the fundamental ac output oltage s controlled' Ths

s #no%n as output control b" oltage cancellaton$ %hch deres from the fact that ts

mplementaton s easl" attanable b" usng t%o phase!shfted s&uare!%ae s%tchng

sgnals as sho%n n


64/90


65/90


66/90

C-oppin8 an8#!$ 6o" SHE an+ 6)n+am!nta# 'o#ta8! cont"o# in -a#6:%"i+8! 7SI$

a/ 6)n+am!nta# cont"o# an+ t-i"+ 6i6t- an+ $!'!nt- -a"monic !#imination %/

6)n+am!nta# cont"o#.

Thus$ the ampltude of the fundamental component and harmoncs n the ac output

oltage are gen b"

0t can also be obsered n /g' 5>'5:c that for a5 ' 7 s&uare %ae operaton s

acheed' 0n ths case$ the fundamental a output oltage s gen b"


67/90

*here the fundamental load oltage can be controlled b" the manpulaton of the dc

ln# oltage'

TOTAL HARMONIC DISTORTION

3armonc problems are almost al%a"s ntroduced b" the consumers.

e&upment and nstallaton practces' 3armonc dstorton s caused b" the hgh use of

non!lnear load e&upment such as computer po%er supples$ electronc ballasts$

compact fluorescent lamps and arable speed dres etc$ %hch create hgh current

flo% %th harmonc fre&uenc" components'

The lmtng ratng for most electrcal crcut elements s determned b" the

amount of heat that can be dsspated to aod oerheatng of bus bars$ crcutbrea#ers$ neutral conductors$ transformer %ndngs or generator alternators'

DEFINITION

T3D s defned as the RM2 alue of the %aeform remanng %hen the

fundamental s remoed' A perfect sne %ae s 577$ the fundamental s the s"stem

fre&uenc" of 67 or F73' 3armonc dstorton s caused b" the ntroducton of

%aeforms at fre&uences n multples of the fundamental 'e'8 =rd harmonc s =+ the

fundamental fre&uenc" - 5673' Total harmonc dstorton s a measurement of the

sum alue of the %aeform that s dstorted'


68/90

POWER MEASUREMENT

Despte the use of good &ualt" test meter nstrumentaton$ hgh current flo%

can often reman undetected or under estmated b" as much >7' Ths seere

underestmaton causes oerl" hgh runnng temperatures of e&upment and nusance

trppng' Ths s smpl" because the aerage readng test meters commonl" used b"

mantenance techncans$ are not desgned to accuratel" measure dstorted currents$

and can onl" prode ndcaton of the condton of the suppl" at the tme of chec#ng'

Po%er &ualt" condtons change contnuousl"$ and onl" nstruments offerng true

RM2 measurement of dstorted %aeforms and neutral currents can prode the

correct measurements to accuratel" determne the ratngs of cables$ bus bars and

crcut brea#ers'

NEUTRAL CURRENTS

3gh harmonc enronments can produce une+pected and dangerous neutral

currents' 0n a balanced s"stem$ the fundamental currents %ll cancel out$ but$ trple!

N.s %ll add$ so harmonc currents at the =rd$ Jth$ 56th etc' %ll flo% n the neutral'

Tradtonal = phase s"stem meters are onl" able to calculate the ector of lne to

neutral current measurements$ %hch ma" not regster the true readng' 0ntegra 56=7$


69/90

56F7 and 567 offer a = phase > %re erson %th a neutral >th CT allo%ng true

neutral current measurement and protecton n hgh harmonc enronments'

HARMONIC PROFILES

There s much dscusson oer the practcal harmonc range of a measurement

nstrument$ ho%eer stud" of the harmonc profles of t"pcall" nstalled e&upment

can gude the s"stem desgner to the practcal soluton' A t"pcal harmonc profle

graph %ll sho% a logarthmc deca" as the harmonc fre&uenc" ncreases' 0t s

necessar" to establsh the upper leel at %hch the harmonc content s neglgble'

MODELLIN3 OF CASE STUDY

PROPOSED SPEED CONTROL SCHEME OF PMBLDC MOTOR FOR AIR

CONDITIONER


70/90

/gure5' Control schematc of Proposed Brdge!buc# P/C conerter fed PMBLDCM dre

The proposed speed control scheme (as sho%n n /g' 5) controls reference oltage at

DC ln# as an e&ualent reference speed$ thereb" replaces the conentonal control of

the motor speed and a stator current nolng arous sensors for oltage and current

sgnals' Moreoer$ the rotor poston sgnals are used to generate the s%tchng

se&uence for the 120 as an electronc commutator of the PMBLDC motor' Therefore$

rotor!poston nformaton s re&ured onl" at the commutaton ponts$ e'g'$ eer"

F7Ielectrcal n the three phase' The rotor poston of PMBLDCM s sensed usng hall

effect poston sensors and used to generate s%tchng se&uence for the 120 as sho%n

n Table!0'

TABLE I

7SI SWITCHIN3 SEQUENCE BASED ON THE HALL EFFECT

SENSOR SI3NALS


71/90

The DC ln# oltage s controlled b" a half!brdge buc# DC!DC conerter

based on the dut" rato (D) of the conerter' /or a fast and effecte control %th

reduced se of magnetc and flters$ a hgh s%tchng fre&uenc" s used ho%eer$ the

s%tchng fre&uenc" (fs) s lmted b" the s%tchng dece used$ operatng po%er

leel and s%tchng losses of the dece' Metal o+de feld effect transstors

(M@2/ETs) are used as the s%tchng dece for hgh s%tchng fre&uenc" n the

proposed P/C conerter' 3o%eer$ nsulated gate bpolar transstors (0BTs) are used

n 120 brdge feedng PMBLDCM$ to reduce the s%tchng stress$ as t operates at

lo%er fre&uenc" compared to P/C s%tches'

The P/C control scheme uses a current control loop nsde the speed control

loop %th current multpler approach %hch operates n contnuous conducton mode

(CCM) %th aerage current control' The control loop begns %th the comparson of

sensed DC ln# oltage %th a oltage e&ualent to the reference speed' The resultant

oltage error s passed through a proportonal!ntegral (P0) controller to ge the

modulatng current sgnal' Ths sgnal s multpled %th a unt template of nput AC

oltage and compared %th DC current sensed after the DBR'

The resultant current error s amplfed and compared %th sa%!tooth carrer

%ae of f+ed fre&uenc" (fs) n unpolar scheme (as sho%n n /g':) to generate the

P*M pulses for the half!brdge conerter' /or the current control of the PMBLDCM

durng step change of the reference oltage due to the change n the reference speed$ a

oltage gradent less than 77 1-s s ntroduced for the change of DC ln# oltage$

%hch ensures the stator current of the PMBLDCM %thn the specfed lmts ('e'

double the rated current)'


72/90

/gure :' P*M control of the buc# half!brdge conerter

DESI3N OF PFC BUC? HALF:BRID3E CON7ERTER BASED PMBLDCM

DRI7E

The proposed P/C buc# half!brdge conerter s desgned for a PMBLDCM dre

%th man consderatons on P4 constrants at AC mans and allo%able rpple n DC

ln# oltage' The DC ln# oltage of the P/C conerter s gen as$

VVVVVVV' (5)

*here N5$ N:5$ N:: are number of turns n prmar"$ secondar" upper and lo%er

%ndngs of the hgh fre&uenc" (3/) solaton transformer$ respectel"

1n s the aerage output of the DBR for a gen AC nput oltage (1s) related

as$

VVVVVV' (:)

A rpple flter s desgned to reduce the rpples ntroduced n the output oltage

due to hgh s%tchng fre&uenc" for constant of the buc# half!brdge conerter'

The nductance (Lo) of the rpple flter restrcts the nductor pea# to pea#

rpple current (^0Lo) %thn specfed alue for the gen s%tchng fre&uenc" (fs)$

%hereas$ the capactance (Cd) s calculated for a specfed rpple n the output oltage

(^1Cd)' The output flter nductor and capactor are gen as$

VVVVVV' (=)

VVVVVVVV (>)

The P/C conerter s desgned for a base DC ln# oltage of 1dc Q >77 1 at

1n Q 5J 1 from 1s Q ::7 1rms' The turn.s rato of the hgh fre&uenc" transformer

(N:-N5) s ta#en as F85 to mantan the desred DC ln# oltage at lo% nput AC

oltages t"pcall" at 5K71' @ther desgn data are fs Q >7 #3$ 0o Q > A$ ^1CdQ > 1


73/90

(5 of 1dc)$ ^0LoQ 7' A (:7 of 0o)' The desgn parameters are calculated as

LoQ:'7 m3$ CdQ5F77 _/'

MODELIN3 OF THE PROPOSED PMBLDCM DRI7E

The man components of the proposed PMBLDCM dre are the P/C

conerter and PMBLDCM dre$ %hch are modeled b" mathematcal e&uatons and

the complete dre s represented as a combnaton of these models'

A. PFC Con'!"t!"

The modelng of the P/C conerter conssts of the modelng of a speed

controller$ a reference current generator and a P*M controller as gen belo%'

1) Speed Controller: The speed controller$ the prme component of ths

control scheme$ s a proportonal!ntegral (P0) controller %hch closel" trac#s the

reference speed as an e&ualent reference oltage' 0f at #th nstant of tme$ 1Sdc(#) s

reference DC ln# oltage$ 1dc(#) s sensed DC ln# oltage then the oltage error

1e(#) s calculated as$

VVVVV'' (6)

The P0 controller ges desred control sgnal after processng ths oltage error' The

output of the controller 0c(#) at nstant s gen as$

VVVVV' (F)

*here Wp and W are the proportonal and ntegral gans of the P0 controller'

2) Reference Current Generator: The reference nput current of the P/C

conerter s denoted b" dcS and gen as$

VVVVV' (K)

*here u1s s the unt template of the oltage at nput AC mans$ calculated as$

a voltage controlled adjustable speed pmbldcm drive using a single

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