Werner Leonhard Control of Electrical Drives

221

'2'20

11. Power Supplies for

Adjustable

Speed AC Drives

iRef

(I)

F

i(.!;.

11.

4.

Current

waveform

produced

by a single-phase

transistor

converter

with

illdnclive load

impedance

and

synchronised

On

-Off controller

()(TlIf

only

at

equidistant

sampling

instants

t = v

To

defined by a fixed clock

Cn:qlwncy

to =

l/To

[B45]

. While Fig. 9.16 was derived with a simple free

l'llllllillg On/Off-controller,

the

effect of synchronization is sketched in Fig.

I

1,'1

, again for a single-phase converter with inductive load;

the

amplitude

(lI'

LIli:

ripple superimposed on

the

load current is now increased, because

the

::

w;

I.l'hillg operations are delayed until

the

next sampling instants,

thus

cre-

:,",,;II/,

; waiting intervals ranging between zero

and

To.

If

synchronised On/Off-

{'O

lIl.r()III'I'~

a.r

e employed,

the

hysteresis

band

Ll

can

be

omitted

because

there

,:'

"

"W

:\.

IlIillimum time interval

To

between subsequent switching operations.

W

hl

'

ll

a very

low

ripple content of

the

currents is specified, as on machine

I.,"I!

CI

"

,tI

drives, the use of a pulse-width

modulator

in combination

with

i'

1""';

1,

1'

I"llrr

ent controUer is preferable

to

On/Off

current

control. This is

d

l

'IIIIH,

~

I.],

;

lt

e

d

in Fig. 11.5 where measured

current

waveforms obtained with

III<'

::

;1.111

"

thr

ee-phase

transistor

converter are compared. Fig, 11.5 a depicts

III<'

('

IIIT"Jlt

produced by a synchronised controUer having a sampling period

'I

II

'22'1

,s,

whereas

the

curves in Fig. 11.5 b show

the

result of linear

current

1;

0111

,

1',,1

with a pulse-width

modulator

operating

at

5 kHz [H20].The audible

1

IIIi

:;I

' is !llo

rc

pronounced with

PWM

control unless a higher clock frequency

i::

d()

~I:

JI,

but

the

current waveform is much more acceptable.

The

reason for

1,

111'

illcn-:ased ripple current is

the

quantisation

of

the

voltage-time

area

with

1.1)('

s

Yll

ch

ro

nised On/ Off - control.

1'lIbl:-w

idth

modulators are now available in a variety of integrated cir-

"

II

i

!.

:;

, wltich

gn

'a

tly silllplifies

the

design

of

PWM

CO

llverters,

Thcr

e is also

thc

p

nHs

i

liíliCy

01'

so[l.w

an:-based mo(lulatioll HH

ill/

'; fasl, s

i!!:J1

al proccsso

l'

S which

"II

;'

I'

III11illlil.('«

1I1'xihili

I.y

hy colllhilliJlI

.r

I'W

M

wi

I.h

H0l'his

LÍc,ÜI

:d,

I'

udl

as

11.1

Pulse

width

modulated

Volt age Source

Transistor

Converter

*j

No

,,,d

;f

b

MM

Load

20

' 200 '

7~;ef

7

~;ef

)~,

-20~

"""---'

-20,

I

5ms

Stator current references

I-

-I

'"

-20

i

A

F

~cc,n'

200 ,

is

-200~

i

Ã

200

-200

~

"

b) Currents with linear contrai, PWM

at

5 kHz

Fig.

11.5.

Comparison of c

urrent

waveforms

with

different

types

of controllersi

(a)

Current

waveform

with

synchronised On/Off-control,

To

=

22J.Ls

(h)

Curr

e

nt

waveform

with

linear

current

controller

and

PWM

,

operating

at

5 kHz

predictive or

time-optimal

current

control;

this

is of interest with high power

converters switching

at

a lower frequency [H67, H71],

The

assumption

of

virtual

current sources for

the

stator

windings is, of

course, only valid as long as

the

ceiling volt age of

the

converter is

not

reached,

This

is

demonstrated

in

Fig. 11.6 for a free

running

On

/ Off - controUer when

the

frequency of

the

current reference is gradually increased. Due

to

the

indu

ctive

load

impedance,

the

necessary

fundamental

c

omponent

of

the

volt-

age rises with frequency according

to

UI

= h

JR2

+ (wIL)2,

It

is seen

that,

beginning

with

Fig

, 11,6 b,

there

are

signs of

saturation

as indicated by pro-

longed intervals where no switching occurs, which is due

to

insufficient supply

voltage, Clearly, this results

in

a rising control

error

between

the

sinusoidal

current

reference

and

the

piecewise exponential feedback signal

and

must

be

taken

into

account when designing a drive control system,

A ve

ry

effective

method

of pulse-width-modulation

that

is

particularly

slIitcu for fast switching converters is called vectorial

PWM

because

it

repre-

s('IILs an a

tt

e

mpt

to

reproduce in a given time interval vTo < t

:=:;

(v +

1)

To

a

v

()l

t

a ~

t:

v

(~ct

or

'U

s

II

d (t)

demanded

by the

current

controUers [P29], For this

iI.

is illJp"rl.alll. 1,

(1

rellle

m!>

!:

r

t.lt

a

l.

tll(:

COll

v

erter

circnit of

Fi

g_ 11.3, which in

·

.!

22

11. Power Supplies

for

Adjustable Speed

AC

Drives

.m.~;

;/

iReI

(t), i(t)

u(t)

OOllPl~i

/)

(.'

i(t)

u(t)

,

.,

,.,'

"

iRe/t)

10'

1

/\.

I

I.

H.

S;LI

.llration effects of single-phase switched transistor converter with

(

.,,

/(

Hr

cml't:1I1.

control supplying a passive R-L load impedance

I",!" . I

I.

7 a is

represented

by a switching model,

must

be

so

operated

that

11t0l11'

0('

I.h(~

Icgs is

short-

circuiting

the

link

voltage

UD

and

that

each

motor

1.('J'llIill

;d

<Lssumes

a

potential

defined

by

the

control. Hence one

transistor

in

('

;

1('11

I(

'

I~

lllust

be

blocked while

the

other

is conductive,

except

for

the

short

1'1'<

"

,

( ~

cl.i

vc iutervals, where

both

transistors

are

blocked

and

the

motor

cur-

1'('

/11.

is

l"I

owing

through

one

of

the

shunting

diodes

(The

protective

interval

ICi'i

C

lllhks

t.he

red-light-overlap

at

a signalled

road

crossing).

To

prevent

short

(·

il'(·i1it.

couditions, each

half

bridge i

is

modelled by a reversing

switch

indi-

f ;I.i."tI

h'y

a.

IJilmry variable Si =

I,

-

I,

depending

on

whcther

the

switch

is in

1.

11<'

IIpp(

~

r

01' lowcr j>osition.

Tlw

switching

:;t

,

at(~

(8

1

,

,'!h,

Sa)

of

the

complete

"4IIIVl'I'l

.

t'l'

i ii

!.Il(

i

li

dcscribed hy

a.

tlm'c

hit.

biuary word h:wiug

eight

diffc

rcnt

V:

dlWi

l,

ill(

~

llIdilll':

(I,

I)

aud

(

I,

-

I,

I)

, (·"III,tI

Y,

I'I'O

v(

~

dor

sl,

at.(

·s

,

wll(~rc

t.11<'

1/101.01

1.1

'

IIII

·

ill

ll.

h

111'(

'

li

hol't.

(~ir(,IIi1.('d

""

.

1.111'

111'111'1

"I'

I"wlil'

'ne

IIml.

Tht:

11.1 Pulse width modulated Voltage Source Transistor Converter

223

protective

interval

which

lasts

only a few rnicroseconds

when

transistors

are

used,

can

be

assigned

to

a finite switching

time

in

the

converter

model.

When

writing

the

volt age vector 1f.s(t)

in

terms

of

line- to-line

voltages

(10,38) which

can

assume

the

values UD, O

and

-UD

,

"2

-'

J

1f.s(t) =

USI

+

US2

eJ"Y

+

US3

e

"'(

=

U12

-

U23

e

J"'(

(11.1)

six

distinct

voltage

vectors

and

two zero vectors result, as seen

in

Fig. 11.7

b, Clearly, when going from one

comer

of

the

hexagon

to

the

next,

only

one

leg of

the

converter

needs

to

change

its

state,

S2

1l.2

"Zero vectors'

2x

,

, U

S3

S1

~

-SRef

Uo

-1

(

'.

()ff

\

.' U

l

-I

I

Available

U23

[

voltage vectors

"51 [ "'2"52 [

a

uS3

b

Fig.

11.1.

Switching model of converter in Fig. 11.3 and generated

output

voltage

vectors

It

is

assumed

that

the

converter

is

switched

at

a basic clock frequency

lo

=

l/To,

which

may

either

be

fixed

or

correspond

to

a

multiple

of

the

variable

fundamental

stator

frequency,

and

that

the

current

controllers have

determined

a

command

value

1f.SRef,

valid

in

the

To;

1f.SRef

is

assumed

to

lie inside

the

first sector, as shown

in

Fig. 11. 7 b.

Since

this

commanded

voltage

vector is

normally

not

coinciding

with

one

of

the

available,voltage vectors,

it

is to

be

composed

by a switching sequence

comprising

the

adjacent

vectors

1f.l

=

1f.S(Sl'

8

2

,

8

3

)

= 1f.s(l,

-1,

-1)

= UD

and

1f.2

= 1f.s(1, 1, - 1) = UD e

j1T

/

3

,

while filling

up

the

rest

of

the

time

interval

with

zero-vectors.

The

sub-intervals

tI

and

t2

for

the

two

adjacent

vectors

are

to

be

computed

from

the

following equivalence

')

h

t2

U (

tl

j

1T

/3

t2

)

1f.s

Ref

=

(

USa

+J

USb

Ref

=1f.l

To

+

1f.2

To

= D

To

+e

To

(11.2)

III

addil.ioll /, I I 1.

1.

1- 2to

To

holds, where to is

the

zero vector interval.

224

11. Power Supplies for

Adjustable

Speed AC Drives

Samplíng

of

current Samplíng instants

!.-

T.

t

T.

-.I

of

current

• O

O.

controllerT

O

1

..

---

,:",:---.

.-'

t I I

I

......

1

i+-

.

to

I-

:

-!t

o

I-!

-

I

~

t

2

I

I I

t

I

I/S1

~S2

\A

I

- 1

-

.

_.

.-=.-:-1.._.J

....

_.

0_'

_

._.-.

Voltage vectors

º1

~

1 1

!:

21

ºI

~21

~

1 Iº

~

Convener 1

switching period

2T

o

I."ig.

11.8.

Vectorial

PWM

with

symmetrical

switching sequence

Solving for

tI,

t

2

results

in

tI

USaRe!

1

USbRe!

To

-

u;;-

-

v'3-U;;-

t

2

USbRe!

(11.3)

To

=

v'3-U;;-

~

ln

= 1 _

USaRe!

_

~

USbRe!

> O .

'

/:1

UD

v'3

UD

-

WlwlI

()lIlittillg

zerO

vectors,

to

=

O,

the

resulting equivalent voltage vec-

1."1

l'II<l

S

()II

Lhe

straight

line connecting

the

two

adjacent

switching vectors

;

IC

'c"ldill/(

to

tI

'//':

I,,"f

= (1lI - 1l2)

To

+

112

;

(11.4)

!.II<'

lilllitói for tI = O

and

tI =

To

confirm this resulto Hence,

by

inclusion

of

l(J

ally

dcsired voltage vector inside

the

hexagon defined by

the

six switching

v('c/.ors

lIlay

be

realised.

Of

course,

the

adjoining switching vectors

must

1)('

choscn according

to

the

sectorial position of

1ls

ReC(t).

If

the

switching

rn'(/lI(~llcy

io

is high enough, fluctuations

of

the

link volt age

may

be

taken

ill/.n

accoullt by

substituting

a recent

measurement

of

UD(t)

for U

D

in

the

I))()I!

1l1al.i.llg

equations

(11.3).

T() relllove

the

ambiguity

with

respect

to

which

of

the

two adjoining

Hwil.chillg sUItes should

be

applied first,

it

is

advantagcous

to

repeat

tbe

H

11

,

11

1('

switchillg

cyck

with rcverse sequencc

50

that

a syllundrÍCal switchilll.

';

p;d

.

1.c1'll

of

kll/

'.

!.h

2'/'0

is

('.n~at(~d.

Salllplillg

l.ll(~

1I10!.O!'

cllrrcIlI.!-i,

ill

tll<~

C<~Jlt.(:r

"r

1.11('

:i

!im!.

rircllil.

illl.(

'rv

lIlc

l

l.(I

dilllilli

::

!iI'

M

1.111'

n

fl

,

llIpJill,~

lIoi:

·

;("

;u,

willlw

di:

.;

11.2 Voltage Source

PWM

Thyristor

Converter 225

cussed

later.

The

switching sequence

pertaining

to

the

example

in

Fig. 11. 7

b is

plotted

in Fig. 11.8,

depicting

the

potentials

of

the

motor

terminaIs.

Eq. (11.2) describes

an

idealised

situation,

where

the

protective intervals

and

the

inherent

delays of

the

switching devices

are

neglected. For

the

actual

design

of

modulators,

these

effects

must

be

taken

into

account,

particularly

the

differences between

tum

-

On

and

tum

- Off times, which

can

cause con-

siderable

distortion

of

the

converter

characteristics

at

low

output

volt age

and

frequency.

Hardware-

as well as software-

based

methods

have

been

proposed

for solving this

problem

[K35,S81,WlO].

11.2

Volt

age

Source

PWM

Thyristor

Converter

The

use

of

transistors

(MOSFET,

Bipolar) is

presently

restricted

to

lower

and

medium

power applications,

with

lGBT

's

up

to

about

1

MW.

With

thyristors

and

GTO's,

the

rating

of

semiconductor

converters

extends

to

even

higher

power. A proven circuit, resembling

the

transistor

converter

in Fig. 11.3 is

the

thyristor

converter

shown

in

Fig. 11.9. Because

of

the

constant

link volt age

and

the

low AC

impedance

of

the

DC link

it

is also called a volt age source

converter.

ln

view

of

the

higher power

and

the

fact

that

the

motor

may

have

to

operate

for longer periods

of

time

in

the

generating

region,

the

line-side

converter is

usually

an

active front-end rectifier,

able

to

feed power

back

to

the

line

by

reverse

direct

current

ID.

When

employing

gate

tum

off

(GTO)

thyristors,

which

are

available

in

kV, kA-

ratings,

the

converter

circuit

corresponds

directly

to

the

one

in

Fig.

11.3,

with

the

transistors

replaced by

GTO's

but

with

normal

thyristors

that

cannot

be

blocked by

gate

signals,

additional

components

are needed for

implementing

forced

commutation

of

the

machine-side converter;

this

is so

because

the

stator

windings

of

an

induction

motor

do

not

contain

current-

independent

volt age sources as

in

the

case

of

a line-connected

transformer

or

a synchronous

machine

with

an

internal

induced

voltage. Hence,

natural

commutation

of

the

machine-side converter feeding

an

induction

motor

is

not

feasible.

This

is

understood

when

remembering

that

a converter

with

delayed

firing calls for lagging reactive power which a

normal

induction

machine

is

unable

to

supply.

The

converter

depicted

in

Fig. 11.9 is

an

example

of

a variety of different

circuitsj

it

is considerably simplified, showing only

the

main

components.

Instead

of a

pair

of

transistors

as in Fig. 11.3, each

phase

contains

a force-

commutated

thyristor

switch, comprising two

main

thyristors

T,

T',

with

ant.iparallel diodes D, D', two

auxiliary

thyristors

AT,

AT',

a

commutation

capacitor

C

and

an air cored

inductor

L.

The

resonant

circuits

in

Fig. 11.9

have

the

ti\.sk

of

creating

zero

currentjnegative

voltage intervals for

the

main

Lby

risl.o/'s '/',1"

hy

lirillg allxiliary

thyrisl

ors

Al',

AT'; briefly, a

commutating

JlI'()('I':

ic

: l.iI

k"

!i

pla(',

'

;I:

i

f'()llows:

226

11. Power Supplies for

Adjustable

Speed AC Drives

Assume

that

the

main

thyristor

T initially carries

the

load

current

iSl

and

that

the

commutating

capacitor

C

is

positively charged,

Ue

>

Oj

then

firing

of

the

auxiliary

thyristor

AT

causes T

to

be blocked so

that

the

load

current

flows

temporarily

through

AT

and

C thus recharging

the

capacitor. At

the

sarne

time

a resonant circuit

is

formed consisting of

AT,

C, L

and

the

diode

Dj

when

about

a

half

period of

the

oscillation has elapsed

and

the

charging

current

tends

to

reverse its sign,

AT

and

D

are

blocked, which leaves

the

capacitor

with opposite voltage, ready for

the

transient

when

TI

has

to

be extinguished.

The

load

current

is

now fiowing

through

DI

and

the

motor

terminal 1

is

connected to

the

lower DC bus.

If

the

current

pulse

through

the

resonant circuit

is

of

sufficient

magnitude

as compared

to

the

load current,

the

commutation

is almost independent of

the

load so

that

the

voltage

UlN

can

be approximated by a

rectangular

wave ±UD/2

with

short

but

continuous

commutating

transients.

The

load

current

is

l

,

Supply

----.---

Ü

o

uo

Inverter Rectifier

Line-side converter

iO

Generator

\

U

o

Motor

-------r-----..

0

Machine-side converter

AT T

AT'

T'

lU

7N

o

Pulse width modulator

Fig.

11.9.

Thyristor

converter

with

constant

DC-link

voltage (Mc

Murray

circuit)

hving

co

utinuous due

to

the

motor

leakage reactance,

can

in principie

flow

in

citJwr dimction, even though alI four thyristors may be blocked temporarily,

hCC<LI\

SC

Lh

erc is always a p

at

h open through one of the diodes D, DI. During

J'(·gvl1(

\r;d.ioll,

the

cOllllucting periods of

Lh

e

dio<!

cs

are

incn\a

.s

ing, evcntually

C;

UI

S

illl':

t.!1f \

lll('

~

1I1

of

!.lw

lilllc curre

llL,

r;;

,

Lo

challl

~

('

sigu 'which illdic

al.(·s

1"

'VVl

:w

d P

"W

l ' l lI"w .

11.2 Voltage Source

PWM

Thyristor

Converter

227

It

is realised

that

the

initial

transient,

when

iS

l

is

commutated

from T

to

AT,

is

of course also continuous because smalI inductances,

not

shown in

Fig. 11.9, limit

the

rate

of

change of

the

thyristor

currents. Likewise

there

are paralIel

RC

snubber

circuits connected

to

each

thyristor

to

limit

the

rate

of change of

the

voltages. A detailed analysis of converter circuits,

taking

alI these effects into account, is

best

performed by digital simulatioll'j

this

is

particularly

true

for

transient

operation, when

the

initial conditions, for

instance

the

charge on

the

capacitor when

AT

is fired, are changing. As

with

alI converter circuits,

the

function of

the

converter is eventualIy endangered

when

the

required recovery

time

of

the

thyristors

cannot

be

maintained,

for example due

to

excessive load current or insuilicient initial charge on

the

commutating

capacitor. Rence

the

accurate analysis

of

the

commutating

transients

is of

major

concem

to

the

circuit designer. Special types of thyris-

tors

with

short

recovery

time

«

20f,Ls)

are usualIy specified for pulse-width

modulated

converters, which limits

the

power

rating

to

some MW. AIso,

the

occurrence of unacceptable load currents

must

be

prevented by fast control

action.

The

firing

instants

for

the

main-

and

auxiliary

thyristors

are

determined

by volt age

and/or

current

controlIers.

ln

view of

the

limited switching fre-

quency, which is usualIy below 1 kRz for

thyristor

converters of higher power,

the

load

currents

can

no longer be considered close

to

sinusoidal as was pos-

sible with

transistor

converters

operating

at

a higher switching frequency.

Rence

PWM

-

thyristor

converters of

the

type

shown in Fig. 11.9 are

nat-

uralIy

suited

to

act as controlIable voltage sources, presenting

to

the

load

pulse-width

modulated

rectangular

voltages,

the

fundamental components

of which are prescribed by volt age

command

signals. Because of

the

large

superimposed harmonics, caused by

the

chosen switching strategy, closed

loop control of

the

fundamental currents

at

variable frequency

is

normalIy

not

practicalj it was different with

transistor

converters, where

the

switch-

ing frequency

is

higher

and

the

currents are suiliciently filtered by

the

load

impedances. Therefore, open loop voltage control

through

a pulse-width mod-

ulator

is

the

standard

solution, while

the

current

loops are normally closed

on

the

12

,

The

pulse-width

modulators

may again be

of

a variety

of

designs. One

frequently used scheme with converters

of

higher power (Fig. 11.10) employs

a

triangular

sampling signal [J8, M42, S29],

the

frequency

to

of which

is

a

multiple n of

the

stator

fundamental frequency ft

to

avoid subharmonics;

n is reduced in stages as

the

stator

frequency

and

volt age rise,

to

limit

the

switching frequency

and

to

fulIy utilise

the

voltage

capab

ility

ofthe

converter.

A drawback of

this

otherwise simple scheme is

the

appearance

of large

har

-

monics ai

the

frequencies

to

±

2ft

.

The

load

currents

are

smoothed

by

the

leakage illductance of

the

motor

which should not be chosen too smalI with

this

tyP('

01'

COllv(·rl.(\r.

Ali

tripkll

harlllolli

c;

a.re climinated from

th

e cur-

l'('

uL

H

iII

:;

1.,':'

01.

1'

::

l.:t.I

.,.

dup

1.0

I.lw i

:;

ola.t.,:d

IlPIlLral

<lI'

t.lw

s

l

,

;~1.()r

wiJHlill

~\

.

Tlll'l'<:

22

8

11. Power Supplies for

Adjustable

Speed AC Drives

are

still

noticeable

additionallosses

as well as sometimes

objectionable

noise

c

mitted

by

the

converter

and

the

motor.

A typical

current

waveform

with

a

PWM-converter

and

this

type

of

"synchronous

modulation"

is shown

in

Fig.

11.12 b.

Ar

v

I t

r;-;::::::-,

T

UI

Ref

----.ó---.i

!1T> Tc

I'

,_o

I

!

r

AT

u

UI

[Jo

2

,

I

,

I

,

I,

I

I I

()

II

I,

"

\

"

/ 1

"

I"j,~.

11.10.

Pulse-width

modulator

with

a

triangular

sampling

signal

'I'his un

sa

tisfactory

situation

has

given rise

to

the

development

of

a mul-

f.if.II<I(~

of advanced

modulation

schemes such as

digital

pattern

generators

w

h

cr<

~

binary

switching sequences for different

amplitudes

of

the

fundamen-

lal

voHage

are

kept in a microelectronic

memory

to

be

called

up

in

real

liUl(~

for

small increments

of

angle T = Jw1dt.

This

is depicted in Fig.

I

1.1

I IU85, D6, P5,

P42,

T36].

The

patterns

are

comput

ed off-Iine

with

the

oh.iccl.ivc

of

reducing voltage-

and

current-harmonics

or power losses,

torque

plll

sa

t:ions

01'

lloise

und

er

steady

state

CülluitiollS.

Cl(

~

arly,

the

more

oftcn

th

c

vII

II.a.I';('

is

r(:v(

~

rs<,d

per period,

the

mor<'

si.!(' coudil.ioJls

Ci:\.1l

be

sa

t.isfi

ed, givcll

fi.

pJ'<

'::ni

lwd

r'IJI<I

<

UlI(

~

lIl.al

volt.ag(' <;011111011('111. ,

'1'11<'

,,

'::t

ridioll

l:

01'

/.II('

(·oJlv('

rl.('1'

wll.l,

"'·I

I.

II

'c!

1.11

f.tt('

IJlinilJ"I'"

tilllc

II<'

I.

\V"

""

/.WIJ

:."1,

:.

,,,1'''''''.

::

wil,('hiul',

"1)("

'/1.1,

111"

11

111

11/:

1,

"r ,'

''"

!'ti('

III'

"llill'l'I

l('

d 1.0

Triangular clock signal v(t)

11.2 Voltage Source

PWM

Thyristor

Converter

229

Ü

l1

n

2

UO

m=1

m=2

Numberof

reversals

per

quarter-period

m=4

m=

12

a I

.

I

"wt=1:

*

n 1

2

(1:)

U

IN

W t =

1:

-1:

1

1°

Tt

l

1:

2

1:

3

1:

4

1:

5

1:

6

n 1

U

o

2

Voltage for m = 6

-2

b

Fig.

11.11.

Optimised

pulse-width

modulation

with

precalculat

ed

switching

patterns

allow

the

commutation

to

be

completed. AIso,

the

losses

in

the

converter

caused

by

each

commutation

transient

should be considered, which

means

that

there

is

an

upper

limit

for

the

switching frequency.

At

the

maximum

fundamental

frequency, which corresponds

to

full

output

voltage

of

the

con-

verter

when feeding

an

AC

motor,

the

optimal

volt age waveform is a

square

wave

with

one reversal

per

half

periodj

this

is included

in

the

switching

pat-

te

rn

shown

in

Fig. 11.11.

When

realising a

suitable

modulator

it

is

of

course desirable

to

keep

the

req

uired

volume

of

memory

smallj

this

can

be

achieved by

storing

only one

quarter

cycle O

~

T

~

of

the

binary

pattern

for one

phase

and

obtaining

t.

lte

reruaining informa

tion

by

transposing

and

inverting

this

pattern.

On

the

oLheI'

hand

, consi<lcrable

angular

resolution is needed

to

satisfy

the

various

(:oudif.ious

wiLh

ad(~qllate

ac

curacy.

Assllmill~

for

exa

mple

a resolution in

i

l.lllplit.lld,

' 0

1'

1.

111'

rlllld

alll<'lll.:t1

VO!t.Hj(t'

01'

2

7

= 128

kvd

s

a.

nd an

angular

!'<

·lI

pl"I.i,,"

(Ir

2'0

lO

)"

! HI.f'\IH

pt

'"

qll

a

dr

'

ll

lIl., t.

hi

H

l

·

t· l

lII

ll.~

iII

a

IllclIlory

(Ir

11i

·.no

11. Power Supplies for

Adjustable

Speed AC Drives

II

I

!

yt.(

~

which could easily

be

ac

commodated

by one microelectronic chip

of

11'

:

1<1

only memory (ROM). At

the

very low end

of

the

frequency range some

(ll.bel'

means

of

modulation

must

be employed because even 4 000 steps

of

:1.lIg111ar

resolution per period will eventually prove

to

be insufficient.

ln order

to

avoid continued fluctuations

of

the

volt age amplitude, which

would

disturb

the

switching sequences and render

the

notion

of

"harmonics"

1IJ(~allingless,

the

superimposed control should contain a hysteresis

band

in

1.11('

amplitude

channel; fine

and

rapid speed control are still possible

through

i.l1(~

frequency

input

of

the

modulator

but

the

voltage would be changed more

~

:Imvly

and

temporarily in somewhat coarser steps. Some results

of

the

current

w:Lvdorms

and

spectra

obtained

with

different

modulators

are shown

in

Fig.

1 1.12.

The

Curves were recorded on a

22

kw drive supplied by a voltage source

I.hyristor converter [P44].

U

IN

-I

11111

III

11111

IRlm

IL;

~

11111

Wulli±illu~

III11

~U~

t

•

i

SI

i

Slv

I I

__

~II~I~I

__

~I~L\~~~M~o

___

,

~,

17

29

fifI

1

13,

29

43

fifI

; /

b

17

I>'il-(_

11.12_

Current

waveforms

and

spectra

achieved

with

different

methods

of

I'lIls(·

,

widl.h

modulation.

(a)

Optimised

switching

patternj

(")

SI! hharmonic

modulation

with

triangular

sampling signal

fi

particular feature

of

the

pulse-width

modulator

with optimised switch-

1II1',

patt

crn is

the

fact

that

the

pattern

can easily be changed for different

:1

pplical:iolls, placing more emphasis for instance on losses, torque pulsations

<lI

llois(!.

With tlte rapid advances

of

micro-electronics even on-line optimi-

na

t.iOll

oi'

!.hc

individual switching

instanl

s hits bccII showII

to

be feasible

wllidl

OjWllS

lhe way 1.0 opl.imal modulat.iofl

1I0d('.1'

dynamic conditioll

S,

80

I.Ii:d.

( ~

V('1l

on

..

Jitw

<:llfr<'Ilt.

(

~

ollt.rol

('ould '

IH'(:OIlH

' pra,ct.icnl

wit.h

higll

pow(~r

' ·

"IIVCI'I.('l'ii

111(iIJ

, II(i71 .

11.2 Voltage Source

PWM

Thyristor

Converter

231

The

load-independent

commutation,

resulting in low impedance

motor

volt ages makes

the

PWM

converter

an

excellent choice for drives

with

high

dynamic performance

at

power ratings where high frequency switching is

not

feasible;

an

example are high power

traction

drives with GTO-converters

which require good dynamic performance

to

damp

oscillations

in

the

drive

train

and

prevent wheel slippage during

starting

and

braking.

1

~L~

~

+jQL

U

LI

machine-side

PWM

converter

i

Ll

U

o

Une-side

PWM

converter

Fig.

11.13.

Symmetrical voltage source

GTO-converter

with

forced

commutation

at

machine-

and

line-side

With

increased use

of

controlled drives the

problem

of

line-side interfer-

ence by power electronic converters is becoming

an

important

issue for

the

utilities. A line-commutated converter supplying

the

DC-link is

particularly

notorious because

it

not

only produces distorted line currents

with

low orders

of

harmonics

but

draws also

substantial

reactive

currents

of

line frequency.

The

first mentioned effect can

be

alleviated by increasing

the

pulse

number

of

the

converter

but

the

other

requires capacitive compensation or, combining

both

aspects, filters

with

capacitive fundamental

input

currents.

Another

possibility

that

is likely

to

be

of

strong

future interest, is

to

e

xtend

the

principie

of

forced

commutation

also

to

the line-side converter.

An example is seen

in

Fig. 11.13 where a

PWM

volt age source converter

with

G

TO-thyri

sto

l's

at

both

ends is shown

in

simplified

formo

The

line-side

PWM

t;

OI1V

Ü

l't

.t:

J'

c.olllrl

llC

cOl1troll

ed

::;u

t:

h

that

the

line currents

iN

are not only

a

ppJ'

()a('hill

i\

Ni

llll

li

()

ldld Wav({oflu

ImL

LhaL

Lh

eir fundamental components are

n.I

I:

"

iII

1'1111.

:;1'

\\'

l

l.h

!.II<'

lillC'

v()IL

a.

l'

i'

H, LlIIIH t,jilllill

fl

t.ill

({

l't'aC'l.iv

u

CIIJTtmLR.

Singll'

'2:\2

11. Power Supplies for

Adjustable

Speed AC Drives

phase converters of

this

type

are

common

on

large

AC-traction

drives, where

1.11(:

line

impedance

caused by

the

single

phase

catenary

is

quite

large so

that

voltage fluctuations

and

distortions

would

be

particularly

severe, Fig. 9.17.

The

line side converter

may

also

be

used as a

static

compensator

by

producing controllable, usually capacitive reactive line

current,

whereas

the

;I.cl.iv

e

component

of

the

line

current

serves for

maintaining

a

commanded

('OlI

st.ant

or

variable link volt age U

Dj

this

is discussed in Sect. 13.2.

I 1

.3

Current

Source

Thyristor

Converters

'1'1((:

converter circuit shown

in

Fig. 11.14 comprises once

again

a line-side

11IId

a machine-side converter

part

connected

through

a DC-linkj

the

two

co"verters

operating

at

different frequencies

are

now decoupled by a

smooth-

illg

reactor

L

D

which, in

combination

with

a

current

control loop, serves

Lo

lllaintain a

direct

link

current

iD

as prescribed by

the

current

refer-

"IlCC.

Thus

the

machine-side converter is effectively supplied from a

current

SOllrce [F2, K56, P32].

The

circuit offers

the

following simplifications:

•

Th

e unidirectional link

current

iD

allows

the

use

of

a

two-quadrant

line

(,()l\verter, where reverse power flow is achieved by inverting

the

mean

of

1.11(

: link volt age

UD

through

delayed firing.

• 'I'hc link

reactor

permits

the

volt age

UD

to

be

temporarily

raised

or

low-

crcd

during

commutation

of

the

machine-side converter,

thus

avoiding

the

;wxiliary thyristorsj

commutation

is now performed

with

capacitors

and

d("'ollpling diodes.

• 1\

II.,

'I;d.her

there

are

12

thyristors,

the

sarne

number

as would

be

needed for

II. l'l'vcrsible DC-drivej also,

the

thyristors

for

the

machine-side converter

I"

'"d llot be

of

the

more expensive fast recovery

type

necessary for voltage

S'lIlrcc converters.

Tire DC-link could also

be

supplied from a force

commutated

converter

:;llch

as

a

chopper

with

current

controlj

this

is of

interest

on

subway

trains

f('d wi til

constant

direct voltage.

'I'lw lllode

of

operation

of

the

machine-side converter in Fig. 11.14 differs

"ollsiderably from

that

of

the

volt age source converter, Fig. 11.9.

While

the

1I",Lo!'

cmrent

supplied from a

PWM

voltage

saurce

converter was

at

least

I'I'lId<'iy

sillusoidal, it has now a

three

step

waveform,

assuming

the

values

'i/l,

O,

ij)

as seen in Fig. 11.15 a;

the

corners

are

rounded-off

due

to

finite

(,olllllllll.alillg time.

The

vector of

the

stator

current

follows the

trajectory

of

;1.

::

1" poillted

star;

this is shown in Fig. 11.15 b which was recorded

duriug

a

:

:J",,

'd

l'I

'v

(,rsal

of

a.

22

kW

drive

[G5].

T"(~

cOllllllllI.aLillg t.ransiCllts

<ln!

lIJOr

e

'illvolv('d

1I0W

1)(,(,;lI1:

:<

!

Lhe

lllOtor

ill1p(,dIUI('(

' iii pnrl.

,,('

I.hl:

COIIII

II

11

Lal'.ioll

circllil.

wlii!'''

1

111';111

::

I.Il

i

ll.

UI<'

""11111l1l1

.

H

.

I.iI,,

~

l.i"I<'

d"I"'II"

!!

""

!.III'.!un

,

cI

I"

/!:

o('!.ll<'

Il)ol,u!'

1

111<1

1

""1"

11

::"

11

,ii. 111"

.111,

I"

I,HI.

233

11.3

Current

Source

Thyristor

Converters

Supp/y

LO

io

iNl

T

s

U

o

~---lll

t omitted

with

GTO

Os

°2

T

2

iS3

iO

is!

i

S2

i

ORef

a

U

o

Motor

io

Generator

b

Fig.

11.14

.

Thyristor

converter

with

dire

ct

current

link

(a)

Circuit

(b)

Operating

regions of

DC

link

The

commutating

transient

may

be

briefly described as follows:

With

no

commutation

in progress, two

thyristors,

for

example

TI

and

T

2

carry

the

direct

current,

iSl

=

-iS3

=

iD,

and

the

capacitor

C

l3

is positively

charged

as a

result

of

the

UCl3

>

O.

If

thyristor

T

3

is now fired,

T is

extinguished

in a

rapid

transient

and

T

3

assumes

the

direct

currentj

l

this

is

the

starting

condition

of

the

commutating

transient

proper.

While

the

current

iS

l

is now reduced towards zero,

iS

2

is rising

towards

iDj

during

this

interval

phase

1

of

the

motor

is fed

through

C

l3

as well as

through

the

series-connected

capacitors

C

35

, C

5l

,

Eventually

diode

DI

is blocked

and

the

commutation

is

completed

with

T

2

and

T3

conducting,

UCl3

<

O,

UC35

>

O.

Clcarly the

motor

transient

impedance

between

terminal

1

and

2

is

part

of

lhe

CQmmutation circuit, as

mentioned

before.

The

diodes

are

required

for

dtcol.lplillg

t,o

prcve

nt

the

capacitors

from losing

their

charge, necessary for

L11(~

lt(

!:{(. cOlllllllll.al.ioll.

The

cOlI\lHlltatill[';

inl.(~rval

ca

ll

be

reduced

by

choosing

It

IlIo l,o!' wi

I.lt

low

l<

'll

k:tr

~e

n,a,cl.allcl:.

U I II l 1 (, I I

I'

f\

1

234

11. Power Supplies for

Adjustable

Speed AC Drives

15{t}

\,/

a

-~\

...

I o

b

Fig.

11.15.

Current

and

volt age waveforms of a

current

source converter

(li) Wave forms;

(b)

Stator

current

vector

Beginning

and

end

of

the

current steps are manifested by volt age spikes

superimposed on

the

otherwise sinusoidal terminal voltages

of

the

motor; this

ix

(ln obvious consequence of

the

changing

stator

currents, Outside commu-

l.aLiOIl,

where

dis

l

/

=

O,

IJ =

1,2,3

(11.5)

dt

h()lds,

Lhe

terminal volt age

of

the

motor

is practically sinusoidal since the

III;ll~lId.i

s illg

current

is filtered by

the

main

time constant of

the

motor.

IkpclIding

on

the

phase of

the

voltage surges due

to

commutation,

as

rd;i.I,cd

to

the

sinusoidal volt age component,

it

can

be

recognised whether

the

""I

,

('hill(~

is

operating

as a

motor

or

in

the

generating region, Fig. 11.15 aj

on

1.1(('

1)(

: si

de

this is reflected by

the

sign of the

mean

volt age

UD.

The

direction

(II'

\,o(.a(.ion

is solely determined by

the

firing sequence

of

the

machine-side

(,()lIvcrt.cr.

The

firing signals

may

for instance

be

derived from a ring counter

wil.li

(j states, which is

stepped

forward

or

backwards as commanded by the

:;I)('c<l

controller.

With a suitable control

structure,

this converter is also capable

of

sup-

plyinr;

high

dynamic performance drives,

operating

in four

quadrants

over a

\Vide

speed range [G5, G 15]. Besides its simplicity and, considering

the

link

rC;ldor,

the

ease of protection in case of

commutation

failure ,

it

has

the

;Idv;wt,age of little audible noise caused by

the

absence of "high frequency"

IIH)(lulatioll.

On

the

other

hand,

there

are

substantial

additionallosses caused

1'y

Lhe

brge

ha.rrnonic content of

the

currents. AIso

the

torque pulsations

d1l(~

1.0

t.IH~

illl.cract.ions

of

the

stepp

ed

stator

currenls

;wd

the

sinusoidal flux

wav

(

~

•

;lIl

I)(~

()h.i(~dioIl;lbl<:

aI;

low

SP(~c(1.

lu

ordcr

Lo

alkvink

this

('ollditioli,

t1w

1I

11

1<,hin,'

::icl.

\ ('()llv"I'I,

"r

IJlny

1)('

COllllllllt

.

nl.('c\

h

lL

<'k

/

wd

.forLh

by

jlllhi(

!-

widLh

1110'

lid.n

l.l

rt[', I.h" d

lll

', ('(

IIIIlI.('I

' , L

('

, :i

lli'

('r!

lIq,u

fil r

llf

o

/

~

II

n.it."r'Il

l1.

l.il

ll

( ' ·

(lIll1)(lIIC'oIlI.

11.3

Current

Source

Thyristor

Converters

235

on

the

slow progression of

the

state

of the counter; this is discussed in Sect.

12.4.

Of

course, due

to

the

relatively slow

commutation

at

light load,

the

switching frequency

is

bound

to

be much lower

than

with a voltage source

PWM

converter [W9].

It

is for this reason

that

this

converter is

not

suited

for

drives requiring continuous position control such as machine tool feed driveso

Apart

from this special restriction,

the

current-source converter

may

be

controlled nearly as rapidly as a voltage source converter, because

the

firing

instants

of

the

machine-side converter can

be

quickly shifted; changing

the

magnitude

of

the

link current involves some delay due

to

the

large

smoothing

reactor

but

with

adequate

ceiling volt age

the

equivalent lag can be reduced

to

about

10 ms

with

a 50 Hz supply, as was discussed in Sect. 8.5.

Machine-side

PWM converter

G"

,

LO

io

U

o

'1

· - Iue'

D

Pwu,;~;~:rle,

'

:.:

I

I~,

l :

~

'"

~

/~

,,'

US11

Fig.

11.16.

Symmetrical

current

source

PWM

converter

with

GTO-thyristors

As mentioned before,

the

current vector defined in Eq. (10.5) assumes

wilh this converter a

rather

unique shape; because

of

isv =

±iD

and

with

0\1<'

of 1,

lIc

rnotor

currents

zero (except for

commutation

intervals), the vector

roHows

a rq';\I1ar

si

x-pointed

star

with

thc

[adiu::;

V3iD,

'i

s

:

1

('

:i Z

J:\-í

/l

( oJ

(I

' '( /

'h

I

rr

/Ii)

(lUi)

·i

,<:

(t.)

'

':

:: 1 1::;

'.

(

..i

~n6

11. Power Supplies for

Adjustable

Speed AC Drives

Some

measured

traces

are

shown

in

Sect. 12.4.

When

assessing

the

relative

merits

of

the

different converter schemes,

it

is

of course necessary

to

compare

not

only

the

number

of

components

but

;Liso

their

rating

and

how

this

is reflected in

the

total

cost.

The

circuitry

of

the

current

source converter is

again

considerably sim-

plilied

when

GTO-thyristors

are

employed;

this

is

indicated

in Fig. 11.14

:lIId

shown

more

clearly in Fig. 11.16 for a

symmetrical

converter, where

the

I.

';1.<\

-

and

the

line-

currents

are

pulse-width-modulated

and

may

approach

:,i 1\ IIsoidal waveforms.

The

link

current

iD

is now switched between

the

motor

terminais

and

the

1II(,Lor

voltages

and

currents

are

filtered by capacitors.

It

is also possible

to

iIlLroduce zero

current

vectors, bypassing

the

load

through

a

short

circuited

I(~g

of

the converter which is similar

to

the

zero volt age vector

with

a voltage

Silllrce converterj

thus,

the

circuit offers possibilities for high

dynamic

per-

ronllance drives. However, in view of

the

increased complexity

of

the

control

pl:\ut,

caused

by

the

capacitors,

sophisticated control

algorithms

and

a pow-

nflll

microcomputer

are

neededj

in

particular,

the

question of L-C resonances

J(

~

qllires

detailed

study

[A18, A19, N17, N18, N19, N21].

11.4

Converter

Without

DC

Link

(Cycloconverter)

TIi(~

(:ollverters discussed so far

had

in

common

that

line-side

and

machine-

::id('

("()llv

c

rters

operated

at

different frequencies

and

were decoupled by a

1)(:

lil\k

colltaining energy

storage

devices in

the

form of low-pass filter com-

1")\I(')II,s.

'l'his twofold conversion provides a degree

of

independence in con-

l."I)lIilll':

Lbe

converter ends,

but

at

the

sarne

time

causes

additional

power

I",,

::

.,::,

Til,'

double conversion

may

be

avoided

with

direct

AC/AC-converters

w

lii('1i

I'crforrn

the

conversion from

the

constant

frequency,

constant

line volt-

;"',

" I

,()

variaLle frequency, variable

motor

voltage in one

stage

and

without

1I'::ml.illg to

intermediate

energy

storage

[49,58].

TJlI

~

rcversible

line-commutated

AC/DC

converter

without

circulating

('III\'(')lt shown in Fig. 9.4 is

capable

of

operating

in

ali 4

quadrants

of

the

'/I,,,I'/.

..

pl

ane supplying

an

inductive

load

(to assure continuous

current

flow).

I

kll(,(~,

by providing

it

with

closed loop control of

the

output

current

and

il.pply

illg

(l

low frequency

current

reference,

the

converter will

act

as a low

i'\'(''lIJ(~ll('.y

current

source. Clearly,

4-quadrant

capability

is necessary when

::lIl'plyillf';

a motor because,

when

a sinusoidal

current

ia is fed

to

a

R-L-

I,,;ul illlpedance,

the

operating

point

in

the

voltage/current-plane

describes

ali

nllíplical

path

around

the

origin, passing

tluough

ali

quadrants.

When

lI

'v(

'l'iii)l!~

t-he

sequellce

of

the

cnrrent

rderell(,(~s,

t;Jw

sequcncc

of

th

e

thre(~

pll;I.:;(' Olltpllt

syst(~lll

is

inverteu.

To

avoi<\

IIl1d(':;ir;Lhi<'

iJlt(~ractiI)IIR

UelW(

'(,

1l

1.11('

1.111'1

'<.'

<.'111'1'('111. ('

OIlI,l'IllI.

'

I'S

(OIH-

~

(J[ wiIich

i:

; /'('dlllldil,ll!.

III:(',n

,

ll.s(~

of

UI('

i',(,ro

:1

11111

"i'

1.1

1\"

("111'1'/'111.:;

),

i.lwy

IlIlI.y

1)(' r11":i(,\1 ,

.,

1

I'

ili

li

l./'

i

lll

pr

1'1 (,oJlI,

roll('r

n I,

hwi

oI

l

lI

wiul'.

Il

fl

1II"

IJl'Nil,il

li

,

\,

11.4 Converter

Without

DC

Link (Cycloconverter)

237

By

supplying

each

phase

of

the

motor

winding from a reversible converter,

a low frequency

AC

drive

system

is formed, as seen in Fig. 11.17. Again,

the

neutral

of

the

stator

winding

remains

isolated

to

exclude zero sequence,

such

as

triplen

harmonic,

currents,

The

main

feature

of

this circuit is

that

only

standard

line-commutated

thyristor-converters

are

required which

have

been

in use for

many

years

with

DC

drives

and

HVDC

systems

up

to

the

highest power

ratings.

AIso,

the

cost

of

the

thyristors

is reasonable since

no

particular

specifications

with

regard

to

turn-off

time

are

necessary

with

line-commutated

converters.

Netz

Neutral

01

converter

- - - - - -

~1

~

I I I I I

Converter\

IConverter

phase phase

2 3

UIN

iS!

tiS!

__

..J

" "

/

iS!

Ref

Fig.

11.17.

Six-pulse cycloconverter for a three-phase

motor

load

On

the

other

hand,

the

large

number

of

thyristors

seems

at

first sight

staggeringj for a six-pulse

converter

with

three-phase

output

a

minimum

of

36

thyristor

branches

is required as well as a

transformer

with

three

complete

se

ts

of

three-phase

secondary windings.

This

indicates

that

cycloconverters

are

mainly

of

interest

for

large

drives, where parallel

thyristor

branches

would

I>e

necessary

with

other

converter circuits.

An

important

restriction

is

the

li1l1itation of

output

frequency which is caused

by

the

discrete

nature

of

the

collirol process

and

the

presence

of

a carrier frequency, since

the

output

volt-

af~e:)

ar('

:tss(~lllhlc(1

frorn

scctiOllS of

thc

lin(

~

volta.[1;(~s.

As

the

output

frequency

l'i

s(,:-i

, t.lw olli.pll!.

CIlITI~llt.S

;tI'C

t.ra('.killl~

t,h t' sinllsoidal

rderenees

with

incn

~

as

illl~

"'

ITo

m

fl.

11I1

('(JIIS('qll('n!.

di

H

(.OItjoll

, 'f'11I'

freqll('II('Y

l'a

ll

I',I ,

:J.:lK

11. Power Supplies for

Adjustable

Speed AC Drives

ph

O

:::::

h

:::::

hmax

~

15

(11.

7)

is

usually

considered for

operation

of

a cycloconverter, where h is

the

line

frequency

and

p is

the

pulse

number.

With

a 50 Hz

grid

and

a

three-phase

bridge

circuit

(p

=

6)

this

results in h

ma",

~

20 Hz.

At

this

frequency, a

period

of

the

output

volt age would

on

the

average consists

of

15 sections

of

line voltages.

Of

course,

if

a three-phase

supply

of

higher frequency is

available,

the

range

of

output

frequency is

extended

accordinglYi

this

may

lJe

the

case on vehicles,

aircraft

or

ships, when a diesel- or

turbine-driven

gcnerator

provides

on-board

power for variable frequency AC drives. AIso, a

so

mewhat

wider frequency

range

could

be

achieved

by

cycloconverters

with

circulating

current,

as discussed in Sect. 9.1 [H28].

_

ult)

_

~'02

~~~~~

H1U

jjH~~~

MHII,

jj~~~1

~ml'1I11.1m"'1lm~_rr",

;

iR~/i(t)

~

/

'C7

"\.J

I

~ef(\

1\ i(t)

f\

(\

/ '---:l / / /

v 'V V

\J

V;

Fig.

11.18.

Waveforms

of

six-pulse cycloconverter

supplying

ii,

single

phase-load

at

different

output

frequencies (simulation)

Tt

is

of

interest

to

note

that

the

nature

of

the

load supplied

by

the

cyclo-

COllverter is

unimportant

as long as

it

contains a sufficiently large

inductive

illl(w(lance

to

assure continuous

current

flow.

ln

particular,

there

is no differ-

ellC(:

whd.her

til<:

load is active

or

passive, i.c. wltcthcr

thc

HlachiIl

(

~

operat(

:s

ii:;

<t

J:;(~II('ntl.()r

()r

III()tor

hec

iL

lIHe

operal.iolI

iII

ali

1<)111"

quadnlJlLH

is

[lossihl(:.

Wtl('lI c'''Jlllidc'IiIIJ';

1.11('

d\"(

!

<'.I

,H

or

a

cyC'lO(

'

(lIIV"I

'

t.t'l

·

"II

Ul(' liJl(,o

li

icle (".UI"[(\III.I;

11.

Iti

11..1.111

'

111

I .

..

" '

''"

\

1111)('1"

l."

~b

t.

II,

11

.l'IIIIIII"(

,r ic'lI

ll

.

hl

'

C'"

1'11111

1"

11

.1':

:

1."111

"r

l4

i

llll

~l

"i

dHI

11.4

Converter

Without

DC

Link (Cycloconverter)

239

Fig.

11.19.

Measured

output

current

and

volt age

of

a six-pulse cycloconverter

with

three-phase

inductive

load

at

10 Hz

volt ages

and

currents

results

in

constant

total

power. Since

the

cycloconverter

contains

only switches

but

no

storage

devi ces

(apart

from

the

unavoidable

leakage

inductances,

snubber

circuits, etc.),

the

total

three-phase

input

power

corresponds

to

the

output

power. Hence

it

is

mainly

the

effects

of

harmonics

that

have

to

be

considered.

ln

addition

there

will

be

reactive power

at

the

line side which is

inherent

in

the

control

of

line-commutated

converters by

delayed firing.

ln

Fig. 11.18

the

results

of

a

computer

simulation

are

shown, where a

reversible six-pulse converter

with

current

control,

operating

on

the

50 Hz

grid, supplies a

predominantly

inductive single-phase

impedance

with

cur-

rent

of

10 Hz

and

20 Hz.

At

these

frequencies

there

is still a

reasonable

agreement

between

reference

current

and

feedback signal, even

though

some

phase

shift becomes

noticeable

in

the

20 Hz-trace,

but

it

might

be

corrected

by a

lead-term

in

the

reference channel.

The

2 ms-zero

current

interval, which

is

necessary for safe

operation

when

activating

the

opposite

converter bridge,

is clearly seen.

With

a

three-phase

output

and

symmetricalload,

all

triplen

harmonics

would

be

removed from

the

load

current.

Due

to

the

inductive

load

the

volt age rises

with

increasing frequency.

This

will eventually

lead

to

addi-

tional

distortion

of

the

volt ages

and

currents

caused

by

the

ceiling voltage

of

the

converters.

Finally

in

Fig. 11.19 oscillograms of

the

measured

output

current

and

tine-to-neutral

volt age

of

a cycloconverter

are

shownj

the

converter is

supplied

from

the

50 Hz

three-phase

line

and

feeds a

symmetrical

three-phase

inductive

load

at

10

Hz [AI3].

For

large drives, synchronous

motors

are

often preferred ove r

induction

motors

since

there

the

reactive power does

not

have

to

be

supplied

through

I.!t

e converter

and

because a larger

airgap

is desirable for mechanical reasons.

Cydo

co

nverters

are

very

suitable

for

this

duty,

provided

the

stator

frequency

is s

uffi

cic

ntly

low, which is often

the

case

with

large low speed

motors

such

as rc

quir

cd for rolling mill

and

mine

hoist drivesj synchronous drives will

be

disclIssed

iII

Clw.p. 14.

Ás

:

\.11

a.11.(

:rruüiV('

t,o

DC

Iink

PWM

converters, force

commutated

cy-

C"\O('OIlV

('

I'

I.\'I

'

fI

,

(".I

dkcl

HllI.l.ri

x

eOllvert('J's

f1'~R,

S;J7],

JIIay bccome

attractive

at

lc/wl'\"

!,"IV"\"

1

11,1.1

111'"

1"" ';1.

11:-

...

1.1",

(

'III'I

',(

,Y

:ll.o

l'

lI,

l',"

"'IJIIJl0J\('lIts in

t,h

e

nC

·

li

llk

Werner Leonhard Control of Electrical Drives

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