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Und
erstanding
Power
Concep
tsPart3
•Motors
–AC
Indu
ctionMotors
–Motor
EfficiencyandAssessmen
t–
Motor
Controllers(VFD
andHa
rmon
ics)
–Ap
plicationCo
nsiderations
•Transformers
•Substatio
ns•
Switchgear
•Pane
ls(LightingandPo
wer)
•MCC
•DC
/UPS
System
s
Applicationconsiderations
•AC
lineha
rmon
ics
•Whileharm
onicsinthePW
Mou
tput
caneasilybe
filteredby
carrier
freq
uencyrelatedfilterind
uctanceto
supp
lyne
arsin
usoidal
currentsto
themotor
load,
•theVF
D'sd
iode
bridge
rectifier
convertsAC
linevolta
geto
DCvolta
geou
tput
bysupe
rim
posin
gno
nlinearh
alfp
hase
current
pulse
sthu
screatingharm
oniccurrent d
istortio
n,andhe
ncevolta
gedistortio
n,of
theAC
lineinpu
t.
•Whe
ntheVF
Dloadsa
rerelativelysm
allincomparison
tothelarge,
stiff
power
system
availablefrom
theelectricpo
wer
company,the
effectso
fVFD
harm
onicdistortio
nof
theAC
gridcanoftenbe
with
inacceptablelim
its.
Applicationconsiderations
•AC
lineha
rmon
ics
•Furthe
rmore,inlowvolta
gene
tworks,harmon
icsc
aused
bysin
gleph
aseeq
uipm
entsuchas
compu
tersandTVsa
repartially
cancelledby
threeph
asediod
ebridge
harm
onics
becausetheir5
thand7thharm
onicsa
reincoun
terphase.
•Ho
wever,w
hentheprop
ortio
nof
VFDandothe
rnon
linear
load
comparedto
totalloador
ofno
nlinearloadcompared
tothestiffne
ssat
theAC
power
supp
ly,or
both,isrelatively
largeen
ough,the
load
canhave
ane
gativeim
pacton
the
ACpo
wer
waveform
availableto
othe
rpow
ercompany
custom
ersinthesamegrid.
Applicationconsiderations
•Tw
oothe
rharmon
icsm
itigatio
ntechniqu
esexploitu
seof
passiveor
activefilters
conn
ectedto
acommon
busw
ithat
leasto
neVF
Dbranch
load
onthebu
s.Passivefilters
involvethede
signof
oneor
morelow
pass
LCfiltertraps,eachtrap
beingtune
das
requ
ired
toaharm
onicfreq
uency(5th,7th,11th,13
th,
...kq+
/1,whe
rek=integer,q=
pulse
numbe
rof
converter).
Applicationconsiderations
•Whe
nthepo
wer
company'svolta
gebe
comes
distorteddu
eto
harm
onics,losses
inothe
rloads
such
asno
rmalfixed
speedAC
motorsa
reincreased.Thismay
lead
tooverhe
atingand
shortero
peratin
glife.
•Also
substatio
ntransformersa
ndcompe
nsation
capacitorsareaffected
negatively.Inparticular,
capacitorscancauseresonancecond
ition
sthat
canun
acceptablymagnifyharm
oniclevels.
Applicationconsiderations
•Inorde
rtolim
itthevolta
gedistortio
n,ow
nersof
VFDload
may
berequ
iredto
installfilteringeq
uipm
enttoredu
ceharm
onicdistortio
nbe
lowacceptablelim
its.
•Alternatively,theutility
may
adop
tasolutio
nby
installing
filterin
geq
uipm
ento
fitsow
nat
substatio
nsaffected
bythelargeam
ount
ofVF
Deq
uipm
entb
eing
used
.
•Inhigh
power
installatio
nsharm
onicdistortio
ncanbe
redu
cedby
supp
lyingmultipu
lserectifier
bridge
VFDs
from
transformersw
ithmultip
leph
aseshifted
windings.
Applicationconsiderations
•Itisalso
possibleto
replacethestandard
diod
ebridge
rectifier
with
abi
directionalIGB
Tsw
itching
device
bridge
mirroringthestandard
inverter
which
uses
IGBT
switching
device
output
tothemotor.
•Such
rectifiersa
rereferred
toby
vario
usde
signatio
nsinclud
ingactiveinfeed
converter(AIC),activerectifier,
IGBT
supp
lyun
it(ISU),activefron
tend
(AFE)o
rfou
rqu
adrant
operation.
Applicationconsiderations
•Itisalso
possibleto
replacethestandard
diod
ebridge
rectifier
with
abi
directionalIGB
Tsw
itching
device
bridge
mirroringthestandard
inverter
which
uses
IGBT
switching
device
output
tothemotor.
•Such
rectifiersa
rereferred
toby
vario
usde
signatio
nsinclud
ing
activeinfeed
converter(AIC),activerectifier,IGB
Tsupp
lyun
it(ISU),
activefron
tend
(AFE)o
rfou
rqu
adrant
operation.
•With
PWM
controland
suita
bleinpu
treactor,A
FE'sAC
linecurrent
waveform
canbe
nearlysin
usoidal.AF
Einhe
rentlyregene
rates
energy
infour
quadrant
mod
efrom
theDC
sideto
theAC
grid.
•Thus
nobrakingresistorisn
eede
dandtheefficiencyof
thedriveis
improved
ifthedriveisfreq
uentlyrequ
iredto
brakethemotor.
Applicationconsiderations
Long
lead
effects
•Thecarrierfrequ
ency
pulse
dou
tput
volta
geof
aPW
MVF
Dcauses
rapidrisetim
esinthesepu
lses,
thetransm
issionlineeffectso
fwhich
mustb
econsidered
.
•Sincethetransm
issionlineim
pedanceof
the
cableandmotor
arediffe
rent,pulsesten
dto
refle
ctback
from
themotor
term
inalsintothe
cable.
Applicationconsiderations
Long
lead
effects
•Theresulting
volta
gesc
anprod
uce
overvolta
gese
qualto
twicetheDC
busv
oltage
orup
to3.1tim
estheratedlinevolta
geforlon
gcableruns,puttin
ghigh
stresson
thecableand
motor
windingsa
ndeventualinsulatio
nfailure.
•Notethat
standardsfor
threeph
asemotorsrated
230Vor
lessadeq
uatelyprotecta
gainstsuch
long
lead
overvolta
ges.
Applicationconsiderations
Long
lead
effects
•On46
0or
575Vsystem
sand
invertersw
ith3rdgene
ratio
n0.1
microsecond
risetim
eIGBTs,themaxim
umrecommen
dedcable
distance
betw
eenVF
Dandmotor
isabou
t50m
or15
0feet.
•Solutio
nsto
overvolta
gesc
ausedby
long
lead
lengthsinclude
–minim
izing
cabledistance,
–loweringcarrierfrequ
ency,
–installingdV
/dtfilters,
–usinginverter
dutyratedmotors(that
arerated60
0Vto
with
stand
pulse
trains
with
risetim
eless
than
oreq
ualto0.1microsecond
,of
1,60
0Vpe
akmagnitude
),and
–installingLCRlow
pass
sinewavefilters.
Applicationconsiderations
Long
lead
effects
•Re
gardingloweringof
carrierfrequ
ency,notethat
audibleno
iseis
noticeablyincreasedforcarrie
rfrequ
encies
less
than
abou
t6kH
zand
ismostn
oticeableat
abou
t3kH
z.
•Notealso
that
selectionof
optim
umPW
Mcarrierfrequ
ency
forA
Cdrives
involves
balancing
–no
ise,
–he
at,
–motor
insulatio
nstress,
–common
mod
evolta
geindu
cedmotor
bearingcurren
tdam
age,
–sm
ooth
motor
operation,andothe
rfactors.
Furthe
rharmon
icsa
tten
uatio
ncanbe
obtained
byusingan
LCRlow
passsin
ewavefiltero
rdV/dt
filter.
Applicationconsiderations
Motor
bearingcurren
ts•PW
Mdrives
areinhe
rentlyassociated
with
high
freq
uencycommon
mod
evolta
gesa
ndcurrentswhich
may
causetrou
blewith
motor
bearings.
•Whe
nthesehigh
freq
uencyvolta
gesfindapath
toearththroughabe
aring,transfer
ofmetalor
electrical
dischargemachining
(EDM
)sparkingoccursbe
tween
thebe
aring'sb
alland
thebe
aring'srace.
•Overtim
eED
Mbasedsparking
causes
erosioninthe
bearingrace
that
canbe
seen
asaflu
tingpattern.
BearingCu
rrents
•Be
causethevolta
geou
tput
oftheVF
Disa
resultof
high
speedsw
itching,the
waveform
morecloselyresembles
asquare
waveinstead
ofasin
ewave.
•As
aconseq
uence,an
imbalanceispresent
betw
eenthethreeph
ases
that
areenterin
gthemotor
leads,referred
toas
common
mod
evolta
ge,
BearingCu
rrents
•As
soon
asthemotor
isstartedwith
aVF
D,the
common
mod
evolta
geim
balancesearches
fora
path
togrou
nd,and
aparasiticcoup
lingof
therotora
ndstator
windingsd
evelop
s.
•Oncetheoilfilm
inthedielectricbreaks
down,volta
gelevelsincrease
onthemotor
shaft,andthelowest
impe
dancepath
forthe
currentimbalancebe
comes
themotor
bearings
them
selves.
•Vo
ltage
begins
toarcacrossthebe
aringat
high
freq
uencies,leadingto
electricaldischargemachining
(EDM
).
Applicationconsiderations
Motor
bearingcurren
ts
•Inlargemotors,thestraycapacitanceof
the
windingsp
rovide
spaths
forh
ighfreq
uency
currentsthat
passthroughthemotor
shafte
nds,
leadingto
acirculatingtype
ofbe
aringcurrent.
•Po
orgrou
ndingof
motor
statorsc
anlead
toshaft
grou
ndbe
aringcurrents.Smallm
otorsw
ithpo
orlygrou
nded
driven
equipm
enta
resuscep
tibleto
high
freq
uencybe
aringcurrents.
Applicationconsiderations
Motor
bearingcurren
ts•Preventio
nof
high
freq
uencybe
aringcurrent
damageuses
threeapproaches:
–good
cablingandgrou
ndingpractices,
–interrup
tionof
bearingcurrents,and
–filterin
gor
damping
ofcommon
mod
ecurrents.
Applicationconsiderations
Motor
bearingcurren
ts
Good
cablingandgrou
ndingpractices
caninclud
euseof
shielded
,symmetricalgeom
etry
power
cableto
supp
lythemotor,
installatio
nof
shaftg
roun
ding
brushe
s,andcond
uctivebe
aring
grease.
Bearingcurrentscanbe
interrup
tedby:
–installatio
nof
insulatedbe
arings
–andspecially
desig
nedelectrostatic
shielded
indu
ctionmotors.
Filte
ringanddamping
high
freq
uencybe
aring,or,instead
ofusing
standard
2levelinverterd
rives,usin
geither
3levelinverterd
rives
ormatrix
converters.
Applicationconsiderations
Motor
bearingcurren
ts•Sinceinverter
fedmotor
cables'high
freq
uencycurrentspikesc
aninterfe
rewith
othe
rcablinginfacilities,such
inverter
fed
motor
cables
shou
ldno
tonlybe
ofshielded
,symmetricalgeom
etry
desig
nbu
tsho
uldalso
berouted
atleast5
0cm
away
from
signal
cables.
Applicationconsiderations
•Motor
fram
esize—
Motor
bearingcurrentshave
adirectrelatio
nshipto
motor
size.Specifically,
motorsa
bove
450hp
have
been
foun
dto
bethe
greatestcand
idates
forb
earin
gcurrents,due
tothehigher
likelihoo
dof
magne
ticasym
metry
intheconstructio
nof
themotor
windings.While
largemotorsw
illexpe
riencesomeam
ount
ofbe
aringcurrents,m
otorsp
owered
from
VFDs
introd
uceadditio
nalm
agne
ticasym
metry
from
common
mod
evolta
ge.
Applicationconsiderations
•Ro
ughha
ndling—
•Motorsa
reat
anincreasedriskof
shaftvoltage
ifthey
were
hand
ledroughlydu
ringshipment.
•Ofte
ntim
es,dam
agecanoccurinthefactory,intransit,or
whe
nthemotor
isbe
ingliftedon
toits
pede
stal.
•Minor
motor
damagedu
ringshipmentm
ayno
tcause
noticeableprob
lemsw
ithmotor
perfo
rmance;h
owever,it
increasesthe
likelihoo
dof
magne
ticasym
metry
leadingto
bearingcurrentd
evelop
ment.
Measurin
gforB
earin
gCu
rrents
•Afterthe
ambientn
oise
measuremen
thas
been
completed
,the
carbon
brushisplaced
directlyto
thespinning
motor
shaft.
•Severalm
easuremen
tsshou
ldbe
recorded
,with
effortstoen
sure
theen
tirewaveform
canbe
view
edon
theoscilloscop
e.
•Carefulobservatio
nof
thewaveform
shou
ldrevealifasharpspike
involta
geispresen
t,which
wou
ldbe
indicativeof
ashaftvoltage
discharge.
•Vo
ltage
inexcessof
0.5m
V(peakto
peak)iso
ftentim
escausefor
furthe
rinvestigation.Ho
wever,inVF
Ds,the
nominalvaluewill
almosta
lwaysb
einexcessof
0.5m
V.Theexam
ples
discussedbe
low
indicate
vario
usexam
ples
ofshaftvoltage
tests.
Measurin
gforB
earin
gCu
rrents
•Theoscilloscop
escreen
shot
show
ninpresents
cleare
vide
nceof
shaftvoltage,w
ithape
akto
peak
valueof
74.3V.
•Thisiswellabo
vetheIEEE
112allowablevalueof
0.5m
V,as
wellasthe
manufacturer’s
allowable
volta
geof
1Vto
2VforV
FDs.
•Ape
akvolta
geof
74.3Vim
posedup
ontheshaft
likelymeans
that
thebe
aringwillno
tsurvive
muchlonger
unlessremed
ied.
Compare
SimilarM
otors
•Whe
ntestingamotor,itiso
ftenusefulto
testthe“B”
motor
tocompare
values.
•Thescreen
shot
show
ninisthe“B”motor
oftheacross
the
linemotor
that
was
drop
peddu
ringshipment.
•Clearly,the
reisno
eviden
ceof
shaftvoltage
ordamageto
thebe
aringinthiswaveform.
•Ofte
ntim
es,com
parin
gtw
osim
ilarm
otorsc
anhe
lppinp
oint
iftheprob
lem
liesinthemotor
itself,with
transie
ntvolta
gepo
wer
supp
lies,or
inafaulty
grou
nding
system
.
Applicationconsiderations
Dyna
micbraking
•Torque
gene
ratedby
thedrivecauses
the
indu
ctionmotor
torunat
synchron
ousspe
edless
theslip.
•Ifload
inertia
energy
isgreaterthantheen
ergy
delivered
tothemotor
shaft,motor
speed
decreasesa
snegativetorque
isde
velope
dinthe
motor
andthemotor
actsas
agene
rator,
convertin
gou
tput
shaftm
echanicalpow
erback
toelectricalen
ergy.
Applicationconsiderations
Dyna
micbraking
•Thispo
wer
isreturned
tothedrive'sD
Clinkelem
ent(capacitoro
rreactor).
•ADC
linkconn
ectedelectron
icpo
wer
switchor
brakingDC
chop
per(either
built
inor
externalto
thedrive)
transfersthise
nergyto
externalresistorsto
dissipatetheen
ergy
ashe
at.
•Co
olingfans
may
beused
topreventresistor
overhe
ating.
•Dy
namicbrakingwastesb
raking
energy
bytransformingitto
heat.B
ycontrast,regen
erativedrives
recoverb
raking
energy
byinjectingthis
energy
ontheAC
line.
•Thecapitalcosto
fregen
erativedrives
isho
wever
relativelyhigh.
Applicationconsiderations
•Re
gene
rativ
edrives
•Re
gene
rativeAC
drives
have
thecapacityto
recover
thebrakingen
ergy
ofaload
movingfaster
than
the
desig
natedmotor
speed(anoverha
ulingload)and
return
itto
thepo
wer
system
.
•Cu
rrentSou
rce(Cycloconverter,Sche
rbius,matrix,CSI
andLCI)drives
inhe
rentlyallowreturn
ofen
ergy
from
theload
totheline
•whilevolta
gesource
invertersreq
uire
anadditio
nal
convertertoreturn
energy
tothesupp
ly.
Applicationconsiderations
Regene
rativ
edrives
•Re
gene
ratio
nison
lyusefulinVF
Dswhe
rethevalueof
the
recovereden
ergy
islargecomparedto
theextracostof
aregene
rativesystem
,and
ifthesystem
requ
iresfrequ
ent
brakingandstartin
g.Re
gene
rativeVF
Dsarewidelyused
whe
respeedcontrolofo
verhaulingloadsisreq
uired.
•Someexam
ples:
•Co
nveyor
beltdrives
form
anufacturin
g,which
stop
every
fewminutes.W
hilestop
ped,partsa
reassembled
correctly
;on
cethat
isdo
ne,the
beltmoves
on.
•Acrane,whe
retheho
istmotor
stop
sand
reverses
freq
uently,
andbrakingisrequ
iredto
slowtheload
durin
glowering.
Recommended