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.

BearingCu

rrents

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

BearingCu

rrents

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.

Motor

A

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

.

Motor

B

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

•Dy

namicbraking

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.