Werner Leonhard Control of Electrical Drives

81

I)()

7. Contrai of a Separately Excited

DC

Machine

1------------

I

R·

L·

II

I I I

ia

Ra

I~

c:=Jt--I_----

Y

a

I

a

u

a

-1

: I

La

~el

e

J

00-----

~-----

______

I

Power

supp/y

(actuator)

F i

g.

7.3.

DC

motor with armature power supply

eaO

La

+Li

i"o = Ra +

Ri'

Ta

= Ra

+Ri

Witlt

the

results

of

8ect. 5.3

and

b =

I,

the

block diagram

of

the

motor

a S

SlllllCS

the

form shown in Fig. 7.4 a, where

the

controllable volt age source

('" is represented by a first order lag element having

the

voltage gaín G

as

and

Lh(' tiUle

constant

Tas, which strongly depends on

the

type

of power supply.

Whell

a

rotating

generator

is

used,

Tas

corresponds

to

its field time cons

tant

;uld may be in

the

range of 0.1

to

1.0 s, whereas

the

corresponding value

WOJdd

\)e

1

to

5 ms, if

the

residual delay of a líne-commutated converter is

1.

(1

III:

Jll()(klled. Naturally

this

difference

greatly

affects

the

dynamics of

the

I ' ,

111I

,

rol

systern;

it must be

taken

into account, when designing

the

controllers.

I"i,. .. '(,/1 b is ;LIl equivalent

structure,

redrawn for noninteracting access

to

the

ÓI

,

rlll

i d ,

III

T c

urr

cllt.

/'

(Iwnr

mL

:al/'/'/Y

mo

OJ

ia

mM

V

II

~I;;;=mo

~I

_____

-

U

aO

n

m

I..

I-

mo

(I)

;a

mM

~t;;;;=

mo

V

,,

-

I,

[

k

~

-

I"

i

~

(

•

,.1, 1'

'llli

'l! d

l"

nl

l1

J1l

f'

ct

t

1)(:

1111

ai

.

II

I' "

..

rl

l,

1.

1I

111

1lI

11

1I

1'"

\'\1

1' 1

II

l'pl

Y,

'I

',

'/'

,

II

7.2 Cascade Contrai

of

DC

Motor

in

the Armature Contrai Region

The

armature

time

constant

Ta

usually has values between 10 ms

and

100 ms;

it

is determined by

the

impedance of

the

complete

armature

circuit

including possibly a

smoothing

choke

that

may be needed for reducing

the

current

ripple caused by switching convertersj with small servo

motors

hav-

ing

an

iron-free

rotor

the

armature

time

constant

could be less

than

1 ms.

The

mechanical

time

constant

T

mn

comprises

the

total

inertia

of

the

drive

(assuming rigid couplíng) j therefore it covers a wide range from a

few

mil-

liseconds (servo

motors

and

reversing rolling

miU

dri ves)

to

several seconds

(paper

míll drives

and

mine hoists).

There

is general agreement

that

the

most

effective control scheme for

drives is a cascaded

or

nested

structure

with a fast inner

controlloop,

límiting

torque

or, on a

De

motor,

armature

current,

to

which

an

outer

speed control

loop

is

superimposed [K24, L79, 885].

This

multi-Ioop system proves very

flexible when

the

control

plant

possesses a chain like

structure

j for example, in

order

to

control position, it can be extended by a position loop superimposed

on

the

existing speed control loop. AIso, if acceleration

is

of

importance,

a

corresponding inner

controlloop

may

be added.

E

ú)

Supp/y

Actu-

ator

Fig.

7.5.

Cascade contrai structure

of

drive including position-, speed-, and current

contrai with feed-forward from a reference model

The

idea

of cascade control

is

exemplífied by Fig. 7.5, where a

lumped

inertia

drive

is

cquipped with most of

the

controlloops

mentionedj it is seen

t,h

at the

SCqUCll

CC:

torque-acceleration-speed-position is a

natural

one as it

cO

llforIJlS

to

Lltr.

strn

d nre of

the

plaJlt.

Tll(

~

to

rqu

e (or

annatur

e c

urr

e

nt.)

loop,

f'ur

;

llIilll

( I,!il;

illlW)'Illo

st

(:

oul.ro[

fllllcl.i

o

ll

l\

my lw re

gankd

as

appro

K

i-

Ill

a

l.r

'

ly

nl

'l

d,

i

ll

,~

:UI

illl[)J

'(':;

:a'<!

CIII')'(

'II1. S

Olln

'( '

for

tllc

il

l'llIHI,IIIT

, TIl(' C

III'I'('/I(,

('

(lIll.lolln

11:1

::

1.(,

<1"

11

.1

pr

i

lll

n

rily

w'il.lt

1,111'

<I

'y

11 11

II 1"':1 nl'

1.111'

pow\'r

li

lll'l'l

,v

Il.Ild

83

K:l

7.

Control

of

a

Separately

Excited

DC

Machine

Lhc

armature,

cancelling

the

effect

of

the

induced

armature

voltage;

thus

it

provides

the

fastest available control action.

By

limiting

the

reference

mM

ReJ

(or

'ia

ReJ),

the

inner

controlloop

also assumes a protective function.

An

ac-

("d(~ration

controlloop

which is

not

shown in Fig. 7.5 could be desirable

with

I!

iglt

dynamic

performance drives, such as servo

or

elevator drives;

its

main

I"l('pose would

be

to

counteract

the

effects

of

load torque

mL

as well as

cl!;lllges

of

inertia, by

generating

- within

the

admissible range - a

suitable

("llrn~nt

reference ia ReJ.

The

hierarchical

structure

is

then

continued

with

;1,

spccd controller which controls

the

closed inner loop in conjunction

with

LI[('

subsequent integrator;

the

same

idea is once

more

extended when going

Lo

;l

position control loop

with

the

position controller generating

the

speed

,'('I(~r(,Ilce.

()f

course, this multi-Ioop control

structure

can

only function

under

the

;I,s:mmption

that

the

bandwidth

of

the

control increases towards

the

inner

loops

with

the

current

loop being

the

fastest

and

the

position loop

the

slowest;

("\('arly,

the

position controller can only perform well if

the

speed loop is

(jllickly executing

the

commands

it

is given, etc. Following this reasoning,

the

dcsign of

the

controllers proceeds in

the

direction from

the

lower

to

the

higher

('()lIl.rol

leveis,

at

every

stage

approximating

by a simple model whatever

has

I

)(~(~n

a.chieved before. Obviously,

the

design

of

the

control system is

greatly

:lÍlllplincd by such

an

iterative

procedure, where only a

part

of

the

complete

pLtlIl,

is

dcalt

with

at

a

time.

The

fact

that

each feedback variable

can

be

lirllil.(~d

by

damping

the

pertinent

reference is a

major

advantage

of

cascade

1'11111,]'01;

a.l~o)

ficld work is considerably simplified, since

the

controlloops

can

I,,'

plll.

ird,o

op<,ration

and

tested

one after

another.

II,

rll:t,y

IJc

aq,;ued

that

a control system containing several inner loops is

I

"",Iy

1.11

IT:;pOlld

slower

to

changes

of

the

reference

than

an

equivalent single

1,,"1'

("olll.rol

(provided

that

it

can

be

stabilised). Therefore, when

dynamic

1"',lmIJI;\lIc,;

is

illlportant, e.g.

with

position controlled servo drives,

the

inner

1""1'::

,':lllIldd be activated in parallel by feed-forward reference signals derived

1111111

:\.11

"xt,cmal reference

generator

as indicated in Fig. 7.5; as long as

the

I('I"I('II('C

I\(~llcrator

is

outside

the

feedback loops,

the

stability

of

the

control

:;y::L"1I1

('clIlains unaffected.

At

the

sarne

time

ali disadvantages with regard

to

dYll<l.lIlic

n~sponse

are

avoided. For load disturbances entering

the

controlled

plalll., cascade control

is

superior

to

any

single loop control because

already

LlJ('

IICX.I. controller will detect

the

disturbance

and

counteract it. Cascade

(,()lIl.ro\

involving torque, speed, position etc.

may

be

considered as a special

('"111

01"

sbtc

variable control, making use

of

ali the available, physically

illlporl.nnt, q\lantities.

II.

s!tollld be rncntioned

that

cascade (',outrol

rcpl"(~s(;nts

a very powerful

I'III!I,

t'

''\

1.,~rlllÜq\W

that

is

llot

rcstriet(~d

to

(!r

i,

v,'h;

iII

fnct lfwrc is harclly

fWy

('

ouLro]

~

'yS

I.('ll

~

]

IlII.

inl,o

0lwrnl.ioll

tOd

iLy

I.h

a

l.

dlll

·t-l

lIol.

cOlll.ain illw:r loops.

or

('(Illlhl',

Ll

IÍ

l'l

p

l"Í

lI(" i

p\(',

I

mo

wll for

1'0

1111

'

1,

1

11

1('"

,',,"ld

olJly

'1)('

widdy

nppli('d

7.2

Cascade

Control

of

DC

Motor

in

the

Armature

Control

Region

after

the

cost

of

control amplifiers was

dramatically

reduced by

transistor

operational

amplifiers

and

electronic

integrated

circuitry.

When

comparing

the

structure

of

the

plant

in Fig.7.5

with

the

block

diagram

of

a

DC

machine

in

Fig. 7.4 a

it

is seen

that

the

machine-internal

feedback

of

the

induced volt age is

not

immediately

compatible

with

cascade

control because

it

represents a deviation from

the

nested loop

structure

of

the

plant.

However, by redrawing Fig. 7.4 a in

the

form

of

Fig. 7.4 b, a

non-interacting

structure

is obtained;

the

fact

that

the

load

torque

is now

entering

the

plant

in two places is no problem since

mL

is

assumed

to

be

an

independent

disturbance.

The

variables

not

specified in Fig. 7.4 b have no

immediate

physical significance.

The

part

ofthe

plant

that

lies between ea/uaO

and

ia/iao

may

be

described

by

the

transfer

function

~(8)

= T

m

8

(7.1)

Ea T m

Ta

8

2

+ T m 8 + 1

corresponding

to

F3

(8)

in Eq. (5.18) for b =

1.

The

differentiating effect

vanishes

with

speed

dependent

load, as was seen from Eq. (5.24).

The

design

of

the

current

controller is explained

with

the

help

of

Fig. 7.6

assuming no

inherent

damping

by

the

load.

The

feedback signal derived from

the

current

transducer

is passed

through

a low-pass filter

to

reduce possible

ripple; a filter

time

constant

of

2

to

5

ms

is usually sufficient.

mL

mo

Current

controller

Actuator

Tms

T

m

T

a

8

2

tT

m

8t1

-----1

Motor I

I

I.

'

-f'ª--

= mM

{aO

mo

Sensor

Fig.

7.6.

Design

of

the

current

controller,

Pc

=

p.o

If

a

POW(T

dcctronic

converter is

clllploy(~(!

as

armature

supply, a pro

..

port.ioualilll.(·I':ral cO!ll;rollcr (PI)

i::;

t\<kqllal,('

(J

S will

h(~

discusscd lal.er.

1"0)'

a roLal.illl',

1

~

('

II

t!

r

H

L()!'

"av

i

IlI

~

a IIlIlC!t

1II

.l

'lç

(·

r

Iii!",

1.1,,'

(,\I!T""1.

,'oIll.rolkr

1l1

:L.'y

l)(~

('XLI'

l

ld,'"

ii.\'

11.

1

<'

(1

<1

1",'.

1.1"

'

111

(PD

T)

Lo

f"

J'

llJ

11

, 1'1

1)

I ,,"L!',,1I1",

Wh('II

IJ'('il

l

l!1Í

III~

85

1\

1

7. Control of a

Separately

Excited

DC

Machine

Zener díodes

Clamping

circuit

Feedback

circuit

7]C

4

~

Rs

Reference

RI

signal

X1i

1

Actuating

Y signal

wíth

X21

Rs «

R4

«R3'

C

3

« C

4

F

ee

dback

y

(T

I

s+1)(T

2

s+1)

F

(s)=-(s)""G

signal

c

XI

c T

I

s(T

'2

s

+

1

)

R3 ,

G

c

""-,

T

I

",

R

3

C

3

,

T

2

""R

4

C

4

,

""

R

S

RI

T

2

C

4

Fig.

7.7.

Circuit

of analogue PID-controller employing an

operational

amplifier

tlw

controller function by

an

analog operational amplifier, this

is

a minor

ildditi()\l to

the

RC-feedback network of

the

amplifier. A typical example of

/1.11

all;tl()

~

\l

e

PID-controller

is

shown

in

Fig. 7.

7.

Wil

.h tbese assumptions

the

open loop transfer function of

the

current

('

())

tI

,]"

()

I bccomes

PI

PDT

r---A--.

, T

l

S

+ 1 T

2

S

+ 1 G

as

1'

11/'

,;

(8) = GCi S

T~

S + 1

Tas

S + 1T

l

'-------.,,,...------'

'-v--'

PID-controller,

T~

~

T2

Supply

Tms

1

(7.2

)'

T

m

T

a

s2

+Tms

+ 1

Tfs

+ 1

, v

'''-v--'

Armature

Sensor

lkcau:ic of the factor s in

the

numerator

of

the

armature

transfer func-

l.i()lJ,

a jJroportional-acting cJosed loop results, lim

Fods)

= FodO) finite,

s

-+

O

(,

V

(

~

II

I.h()lIgh

the

contro

ll

cr itseJf comprises

an

integral term; as a result the

c!()s(,d ('.IIITI,

'I11.

r ()lIl,rol loup will exhilJit a steady

state

erro

r.

Th

e r

ea

son is

III'

f'(l\

u

r ~

(\

I.h

l' ,('

.11

I.

f.

r(,,'dhadc which rcmaim,

df

c

ctiv

e

also

ir;

t.h(

~

l,r: t

ll

li

f

;

)l'IIJ(

~

d

I,l"

d!,

cl

l(l

.

,~

f

·'~

1

11

"r

I"

I

~

'(,

II h.

C

l

{~

arly,

wlll'lI

:;

lIpplyill!,:

(·()II

H

l.al1

l.

1' (

11'1"

'

\11.

I.()

UI('

I

lH

' lld.

l\

II

'

1,1

'

II

UI

:

111

(11.

... '

11.

1.

""

l

Clfv

l,

1.1

....

"",1."1'

/1

)""'11,

1.1i.

,

illdl

WI'cI

VI

.!!.;II',"

7.2 Cascade Control

of

DC

Motor

in

the

Armature

Control Region

and

the

applied voltage rise linearly with time; with

other

words, generation

of

a

ramp

function with a controller containing a single integration calls for

a

constant

control error.

For

the

practical

application

it

is

normally

of no consequence

whether

the

current

controller

admits

a

steady

state

control

error

during acceleration,

because

current

control represents

an

inner (auxiliary) function, being

part

of

the

superimposed speed loop.

The

main

purpose

of

the

current controller is to

linearise

the

plant

and

prevent overloading

the

motor;

at

constant

speed, this

is accomplished by a controller

with

a single integration.

It

would

in

principIe

be possible

to

use a current controller performing double integration, e.g. a

dual

PI-controller having

the

transfer function

Fb,i

(S)

= GC,i T

l

S + 1 T

2

S + 1

(7.3)

T

l

S

T

2

S '

for avoiding a

steady

state

control error during acceleration. However this is

not

normally done in view

of

the reduced margin

of

stability.

If

maintaining

a prescribed acceleration is a necessary condition, as

may

be

the

case with

machine tool- or elevator-drives,

it

would

be

easier

to

include a

separate

acceleration

controlloop

as will be shown

in

Fig

. 15.9.

The

choice

of

parameters

for

the

current

controller according

to

Eq. (7.2)

foJlows

the

usual

lines for

the

design of cascade control systems [K25,

38]

.

ln

the

case of a Ward Leonard scheme containing a

rotating

generator and

with

aperiodic

damping

of

the

drive

(T

m > 4

Ta)

the

lead time constants T

l

,

T

2

of

the

PID-controller could be

tuned

to

Ta

and

one

of

the

emerging lag

time

constants

of

the

drive, while

the

rest can be

dealt

with

by

approximation

on

the

basis of

the

principie

of

"equivalent lag" , e.g. [38].

The

closed

current

controlloop

is

now inserted

into

the

speed loop form-

ing

the

is

shown

in

Fig. 7.8. Provided

the

current

loop is well

damped,

it

is acceptable

to

replace its closed loop trans-

fer function by a

proportionallag

element having

the

gain

Gc

L,i

and

the

time

constant

Tc

L,i' Since

this

equivalent lag, being a first order, low frequency

approximation

of a much more complicated transfer function,

cannot

be

sim-

plified by lead time constants, a PI-controller for speed

is

the

appropriate

choice. Hence

the

transfer function of

the

open speed loop is

approximated

by

Tc,w

s + 1

FOL,w(

S)

~

G

ow

T S

c ,w

GCL,i

TCL

iS

+ 1

,

T

m

S

1

(7.4)

'-v-'~

PI·

Controller

Curr

e

nt

control

The

pa

ramet

ers Gc , w ,

Tc,

w

may

be

chosen in accordance

with

the

"symmetri-

cal

Opt.illIHIlI

"

IX

2:i,L33,38], which is a

standard

design procedure for transfer

fllJldioll

s

cU

Ilt.ai II iIIg a do

ubl

e integratio

ll

(indllding

the

controller).

'1'1",

ll

l,i

ll

iel"

II

, ln

I.()

c11()W

,

(~

t.h(

~

cro

ss-

()vc'r

frc'CjIl('IW

Y

at

t\w

gC'o

rnetric

IIl

ca

n

III'

1.11<'

I.w"

,'"","

(',' r

...

'

'I

""l

l('i('H,

1.

0

ohl

.;I.ill

111

11

Si

11111111

ph

lls,'

IIIi1rl

~ llI

'l/I,{

whi

ch

iII

87

mL

.!!!1..

mo mo

Speed

control/er

r((\

7,

Control

of a

Separately

Excited

DC

Machine

Limiter

(O

llol

("0

Current

controlloop

ctJ

e

=

ctJ

eO

(Approximation)

"'i

~

.

7.8.

Speed

control

with

approximate

representation

of

current

c

ontrolloop

I' " I

úJ

d

aW

d

I........

W

1

jlm

1 '

""--2

I

- 1

I

Re

'P

I)

I Cross-over frequency

/"

I

1 (J)

1

FoOw)

I

II

~

Phase margin

1/

b

"'ll\·

7.!.1.

Bo

de-diagram

(a)

and

frequency response locus

(b)

n""ord

illg to "

symmetrical

o

ptimum"

t

lll"ll

wiU

result

ín

optimum

damping

of

the

speed loop.

The

name

stems

1'1"1111

ch

c Dode-dia

gram

that

shows

symmetry

with

respect

to

the

cross-over

Ilt'qll

( ~

ll

c

y,

Fig. 7.9.

Il

f

ll

orrnalising

with

'/:.)(

1)

(1,'2

'

1(

,'

I" i ,

a >

1,

(7.5)

Ll1I'

('ro

S~H

)V(

'

r

f'y

qlll'llry

is

/";1

(7.1

i)

)

'1'

I , i "

'I'

f ' I}, i

n

'

/~

A

'

l , . ,

7.2

Cascade

Control

of

DC

Motor

in

the

Armature

Control

Region

From

the

cross-over

condition

!FOL,w(j

wd)1

= 1

(7.7)

th

e

gain

of

the

speed controller is

obtained

with Eqs.(7.5),(7.6)

1

T

m

(7.8)

G c

,w

=

aGcL

,i

TC

L,i

The

poles

of

the

resulting

transfer

function

of

the

closed speed

control

loop,

i.e.

the

eigenvalues

of

the

speed

control

loop,

can

be

ca1culated explicitely.

They

are,

assuming

a

conjugate

complex

pair,

positioned

on

a circle in

the

s-plane

with

the

radius

Wd,

"

~

-w"

".'

~

w,

[-

a;1 ± j J1 -

(a;

1

)'

1

(7.9)

Hence

/

D =

a-I

(7.10)

2

is

the

damping

ratio

of

the

oscillatory

part

of

the

response. Assuming a

slightly

underdamped

response, D =

ljV2

, we have a = 1 +

V2

::::,;

2.4l.

When

aperiodic

transients

are specified, D = 1, leading

to

a = 3, a

triple

real

pole

at

s =

-W

d results.

Due

to

the

double

integrating

term

in

the

open

loop

transfer

function,

the

closed

speed

loop

exhibits

zero control

area

or, synonymously, zero velocity

error

[38,88].

This

means

that

the

step

response is c

haracteúsed

by

consider-

able

overshoot

eventhough

the

transients

ar

e well

damped.

To

eliminate

this

effect which is

caused

by

the

lead

term

of

the

Pl-controller,

a

corresponding

lag

term

1

T

c

,w

s

+

1

may

be

inserted

into

the

reference channel

of

the

speed controller.

ln

practice

this

can

be

done by

connecting

a

suitable

capacitor

Cl

between a

center

tap

of

the

input

resistor

and

ground,

as

is

indicated

in Fig. 7.7.

The

s

peed

controller

shown

in

Fig. 7.8

contains

a nonlinear feedback block

which Jimits

the

current

reference

to

the

range

-ia

ma:c

:S

iRe!

:S

ia

ma:c'

This

serves as an effective

protection

for

the

power

suppl

y

and

the

drive, pro-

dUeillg

tllP de

sir

ed s

te

a

dy

state

torque-speed

characteristics

shown in

Fig

.

7,1

Ü;

of

(,()lIr

s

(

~ ,

a :

;)Iort

ove

rshoot

of

th

e

current

, for

example

following a

load

disl.llrhall(,(·, C l

lI

llPI.

h(~

rnled

o

ut

hy

dalllpi,'1

!;

t11C

eurr

e

nt.

rd

crenee.

If

the

l'It'('j,f(lIti. · cOlrl,I'

"II,,!'

i,:

lI'

a

li

s(·d wit.ll

t!te

IJI'lp

/)1'

nll

opt'r:üiollal

a

'llplifi(

~

r

,

1.11(

:

89

HH

7.

Control

of

a

Separately

Excited

DC

Machine

O)

Speed

control/er

0)0

clamped

_TI

O)Ref

0)0

o

I"

fII

tJ

X

iam~ICD

ia

Ref

('

o

iaO

"'ig.

7.10.

Steady-state

torque-

speed

characteristics

of

speed

control loop

with

1.1)"'1I1'

~

lilllit

..

1;\lIlpiug can

be

achieved by placing Zener-diodes in

the

feedback

branch,

I"il~·

7.7. Limiting

the

current

reference removes

in

principIe alI restrictions

"II

I.he

rate

of

change of

the

speed reference, which is

important

if

it

is pro-

dllC(~d

lllanually. Whenever

the

speed controller,

due

to

a rapidly changing

"')(lIlllal1<1,

reaches clamped condition,

the

speed loop is disconnected while

Uu

' Cllrrent control loop

remains

operative

with

clamped

current

reference.

fi:

; S

()OI1

as

the speed has

caught

up

with

the

command,

the

speed controller

I"I'I'I'I.S

1.0

linear

operation

and

resumes controI.

Ilow('ver,

with

larger

motors

under

manual

control, a rate-of-change lim-

it

"

·,

, is

()rt.(

~

1l

illserted in

the

reference channel

ofthe

speed controller

to

ensure

1.1,

:

11,

1.11<'

(,111J'(~llt

limit, which

may

correspond

to

twice nominal current, is

not

"'./1111'.1

1IIIIICn~ssa.rily

.

A block

diagram

ofthis

device,

together

with

typical

td/ W.f/u l

úJRef

1rJ

1

)

úJo

r

l~

:I

iIITorqet

ü)o

\ úJRef

\ /

úJ

o

~

I'

"'

1,.,;

. 7. i I .

't

ll

l.,

'

,01

d"

"

il

v'

',,"

l

l.n

7.2 Cascade Control

of

DC

Motor

in

the

Armature

Control Region

transients,

is seen

in

Fig. 7.11;

it

restricts

the

occurrence

of

current

limit

to

load

surges, for

instance

when a reversing rolling mill is mechanically

jammed

because

of

a wrong screw- down

setting

calling for

an

excessive

reduction

of

the

billet thickness.

On

the

other

hand,

current

limit

is

not

reached

just

be-

cause

an

impatient

or

careless

operator

rapidly

changes

the

manual

speed

command.

dJ.L

J.L+T

a

~

,I

"

'aO

dt

,I

e

a

0,5

,I

--

e

a

/~

1 I uao

I I

I'

_______

_

--

//.

0,4

I

I,

-

I

UaO/

/

,

I '"

I

...

r--;~--F--

0,3

I

...

úJRef

II

--/

'

,I

úJo

I , /

0,2

cP

e

=

cP

eO

=

0,

°

1

V

UaO

~

1,0 1,5

°

0,5

. Start

at

no load

Load applied

s

·

'.

i

amax

L

Imlt.

- .

-=0,2

0,2

'aO

___

~

I'

J.L

iao "

"

ia

Ref.J

0,1

lo'

a ,

,

I

II

°I

'"

._

0,5\1

1,0 1,5

s

°

Fig.

7.12.

Computed

transients

of

DC

motor

with

currentjspeed

cascade control

Computed

transients

of

a DC drive

with

currentjspeed

cascade control

are

seen in Fig. 7.12.

The

parameters

assumed correspond

to

those

of

a

thyristor-

fed drive

with

a six-pulse-converter, Chapo 8.

The

controllers have

been

de-

signed as

outlined

in

the

preceding

paragraph;

if

a

rotating

motor-generator

were used

instead

of

the

power electronic converter,

the

response would

nat-

urally lw llllldl slower, particular1y in

the

current

loop.

AI.

Jirsl., a sLarl,illg (,ransient is displayed, caused

by

a

step

of

the

speed ref-

("ICll('

,

(~

1'J'll/1I

'/,!'l'n

Lo

0.:\

Wo.

Tltc

p{r(

~

d

of

CIl1T<'lIt,

limit dllring

the

acccle

ration

ph;I::" ;,::

\\".Ii

/Iii

/.I", dyll:l.Illic

("olll.!'()l

erro!' are

c1(~ar-ly

:;('(~II.

'1'11

i:;

I.rall::icIlL

91

!l()

7.

Control

of

a SeparateIy

Excited

DC

Machine

corresponds

roughly

to

the

transition

of

the

operating

point

in Fig. 7.10 from

()

lo

3

via

1,

2.

At

time

t = 1 s

the

drive,

having

attained

the

no-load

point

(

:1)

, is

loaded

with

nominal

torque.

This

causes a

transient

speed

drop

but

the

drive

remains

in

the

linear control regime,

eventually

reaching

the

operating

poillt 4.

Prom Fig. 7.12

it

is seen

that

the

source voltage e

a

rises

with

the

induced

vo

ltage, which is

proportional

to

speed; in

addition

there

is

the

resistive

and

luductive voltage

drop

in

the

armature

circuito

The

principie

of

cascade control has proven its usefulness over

the

years

III

ill11umerable applications;

as

mentioned

before,

the

two leveis

of

control

III

ay

be

extended

in

both

directions, for

example

by

adding

acce

leration

or

position controi. Since cascade control is by

no

means

restricted

to

DC

drives,

W(

~

will come across

more

examples where this exceedingly fl.exible

method

of

mlltrol

can

be

used

to

great

advantage

, see also Chapo 12.

7

.

~

Cascade

Control

of

DC

Motor

iII

the

Field-weakening

Region

III

S(~ct..

5.3

it

was shown

that

by reducing

the

main

field, a

DC

motor

can

I

I("

()p(~rated

above base speed. For good

utilisation

it

is

important

that

the

111"tor

should

be supplied

either

with

maximum

field-

and

reduced

armature

voll,

ag<'

or

with

maximum

armature

voltage

and

reduced field voltage; as

111('lll.iOIl(~d

uefore

there

can

be

some overlap for convenience of controi.

Si

11('('

Lhe

motor

may

alternately

operate

in

either

one

of

these regions

it

is

:"l'proJlrial.c

to

choose a control

strategy

which fulfills

both

requirements

and

I

"'1"111

i

I.

s a continuous

and

automatic

transition

from one

operating

regime

to

1.11<'

"l.IwJ'.

/\

(

',

nll.rol scheme

that

has proved useful in practice, is shown in Fig. 7.13

III

,:

illlplificd form [G20, V9].

It

contains

the

currentjspeed

cascade

control

11

<1.

iI

1/'

;

through

the

armature

voltage, as discussed before; in

addition,

there

I

II

11

,

11

;

l.Il

xiliary

controlloop

with

the

field voltage as

actuating

variable, which

Ims

I.IIC

~

purpose

of

limiting

the

induced

armature

voltage as

the

speed

rises

I

)(,

y

olld

base speed.

The

adjustable

voltage source supplying

the

field winding,

i

ei

I

IS

II

;dly r

ea

lísed

by

a

thyristor

converter, which is represe

nted

in Fig. 7.13

I

'y

a la

!,!;

cl(~lnent.

The

nonlinear model of

the

magn

ctic circ

uit

corresponds

!.o

I

o'i/(

. ;).4.

/\

r(~cdklcl

c

signal for

the

induced volt

age

CilU

II('

n!c()l1structed from

the

111(

';I.

S

lIl'

cd

annature

vo

lt

age

and

curr

ent accoJ'(

lill

t.(

1

,<)

1,

11<

: volta,gc equa

tion

d

i"

( '

11.

" H

..

i"

" " -

ll

;

(7.1 I)

/

..

III

.'I.

II II

,

I"

!'

,II'

· (.

,,

[('1

III

I,/

II/"

"il'('flit,

1101.

fl

ho

W

II

I

II

FI

'"

./ 1:1,

1I

111.'y

1)('

11I

1I'

!!

foI'

1.1

Ii

I-<

1'

(11

1'"

rJ

". "I'ld

tl

fl

l

l'.

tHd

"II

I

,'.'V

I·I

'/d

ll/,

.

dl

l Vtl

I'I

l

ill.

l

'I

11

1,

11

1111,(,

1,011

I

ii

l'

IIIII

JllIl'

,'d

7.3 Cascade

Control

of

DC

Motor

in

the

FieId-weakening Region

with

a

constant

reference, e.g.

lema",

I

~

0.9

Uao;

the

e-controller

then

attempts

to

remove

the

difference

by

adjusting

the

field voltage. Since, however,

the

reference

volt

age is

out

of

reach

as

long as

the

motor

operates

below

base

speed,

the

controller

containing

an

integral

term

will be

saturated

and

the

field power

supply

produces

maximum

field volt

age

which is

the

aim

in

the

armature

control

range.

When

the

speed is rising,

the

induced

voltage

lei

approaches

the

reference value

lema",l;

after

a

balance

has been reached,

the

e-controller leaves

saturation

and

begins

to

lower

the

field voltage

to

limit

the

induced

voltage,

thus

automatically

weakening

the

field as desired.

Of

course,

the

initiative

for a change in field voltage

must

always come from

the

armature

because this is

the

direct channel

of

the

speed controller, working

through

the

armature

current

loop.

The

arrangement

drawn

in

Fig. 7.13

represents

a complex

and

highly

nonlinear

control system, confounding all

general

design

methods;

however,

considerable simplification

results

if

one

succeeds in speeding

up

the

e-controI

loop,

active

in

the

field weakening

range

only, so

that

e

may

be

approximated

in

that

operating

region by a

constant

impressed voltage. Such

an

approxima-

tion

is

not

entirely unrealistic

if

the

field power is supplied from a converter

having

a sufficiently large ceiling voltage (only one

polarity

of

the

field cur-

rent

is required).

If

the

stator

and

the

field poles

are

laminated,

the

"control

plant"

may

be

represented

by a simple lag element, having a

speed-dependent

gain,

and

the

fast

dynamics

of

the

actuator;

no

serious

stability

problems

are

mL

mo

Fi/ter

ú)

%

Ú)Re'

Wõ

Limiting

1el

Ua

O

Iemr1.1

--o-..t

u

llo

- control/er.

1"I

K. 7 . 1:1. ( :

,,111

,,'

,"

"i'

I

)(:

II

"'Lo

I'

iII

(.lU

!

IIfIlLl.LlIre

a.

lld

fidd

wP',d{(:ning r

eg

ioll

s

93

~)

'2

7.

Control

of a

Separately

Excited

DC

Machine

1.0

be

expected even

at

high controUer gain. Neither is

the

nonlinearity of

I.he

magnetic

circuit of

great

concern since

the

e-controUer operates predomi-

lI

all

tly

at

reduced flux where

saturation

is

not

pronounced. For these reasons

a PI-controUer is quite adequatej

its

lead

time

may

be

tuned

to

the

field

I.i

Ille

constant

of

the

motor

while

the

residual delays are taken care of by

the

r<'llI

aining

I-term

of

the

controUer.

Assuming fast e-control in

the

field weakening range,

the

simplified equiv-

al(

'llt block diagram in Fig.7.14 results.

There

the

effect of

the

e-controlloop

(:

; :l)lproximated by a

constant

volt age

le

max

Iacting like an

external

distur-

h;l.Il

ce

and

the

flux approximately becomes a nonlinear algebraic function

of

:

:p('('d,

e

~

_

1>

W

-I

1:01>

1,

'[>

eO

Wo

I l

for

allccLing

the

mechanical

time

constant.

ln

principie, this undesirable change

"r pa

rameter

could be compensated by dividing

the

speed error by

Pe/PeO

:d.

I.he

input

of

the

speed controUer, as is indicated in Fig. 7.14j however,

in

III

()S(.

cas

es

this is

omitted

and

practical results prove

that

the

correction is

II

s

llaIJy

not necessary. Some consideration

must

of

course

be

given

that

the

:,pe('

c!

COlltroUer yields acceptable results in

the

armature

as

weU

as

the

field

rl/lIl.rol regime.

As

an

example, Fig. 7.15 shows a simulated no-Ioad reversing

I.r;

LIIs

i(~J\t

between

±1.5

Wo

that

was computed on

the

basis of

the

system

::

I!" WII

in

Fig. 7.13.

The

effect

of

the

e-control loop is clearly recognisable

1',

()1I1

III<'

">

,.-

and

the

e-traces.

1

I.

h

;

I

,,~

IJC(m

mentioned

that

the

use

of

a fast e-control loop

with

high

r"r"

il

lj

':

voll

.age across

the

field winding calls for a

laminated

motor

frame.

W

11

.

1,

b"I':'

~

Illotors

it

may

be

necessary

to

employ a more accurate model

of

LI

...

li,'ld ,'jrclli

t.

for

the

design

ofthe

e-controlloop,

taking

eddy currents

in

the

II

""

iII!."

:ICCOllut

[G20]

. AIso

the

high voltages induced by transformer action

III

LlI<'

;1.1"I1I:t!.nre

winding affecting

the

commutator

must

be considered. To

u.

M()

~

,:\

~}

{1

1,

r,.

,

(110

«/'"

, . <1

11

/

1

(

1./111

/

Ílllil

O!

~

.!!§...

i

ao

l

uao

"'o

1\

emex

l .

(ü)

- 'slgn -

,

!!!.b..

mo

((uela ui II/ui

cI

cO

l/lrol

III

/lul<

I

WII.

1/"

"

li

"

li

I"U;OIl

I

't'

l

ll

1('

11

<.

1/'

I"IH. 7 . 1'

1.

Sl

lIq

d

W"

d " "

1>1

"'/('

11

1.

'

1.1.;11

11

"l' nl',\(,

,1

t'" "

(.

If

.I

I,

,,

,1'

ill l

i"

loI

W

I\lI

l1l'

" I'

''I

',

"/l

1l1

1"

7.4

Supplying

a

DC

Motor

from

a

Rotating

Generator

.!!!...j

mM

úJo

mo

~W

1,010,~

~

Wõ

r-

I'

i'';;';/J

WRef

/

Wa

L

I

mo

__

_____

>

1

~--

lP

e

lP

ea

ia

lP

e

-ç

1,0 0,5

~eo

'r-

:,

- -

ia'

-

--

!I-

--

-ç

~Reversal

of

: speed reference

e

a

e

uao'

UaO

1,0

--,

~

~:-"'~uao

, :-...

,

:-...

;::...

~

;::...

uao

;::...

:-...

Fig.

7.15.

Computed

reversing

transient

of

speed

control

system

in

Fig. 7.13

eliminate

the

effects of winding

temperature

and

supply voltage fluctuations

on

the

main

flux in the

bas

e speed region,

an

inner field current

controlloop

is often included which

maintains

a prescribed

maximum

field

current

in

the

armature

control range independent

of

a changing resistance of

the

field

winding

and

helps

to

speed up flux contro!.

7.4

Supplying

a De

Motor

from

a

Rotating

Generator

lu

HlI~

prpc(~c(lilll~

sections was mentioned

that

any adjustable voltage source

(".ol1ld

:,\

('\'V(

' ; \

:;

p(Jw(

~

r

:->npply

for

th

e

armatllre

of

the

De

motor.

The

classical

::('11('1111'

(Ir

I )(

~

III

'

d.or

('o\l!.rol

(kvdopcd

hy

W:I

,

rd

T.(~ollard

[L21, 1,22, Y

4,

Y51

,

95

H,I

7.

Control

of

a Separately

Excited

DC

Machine

1'111

pi

oys for

this

purpose a

rotating

D C generator,

the

armature

volt age

of

wllidl

is

contro11ed

through

the

field. Combined

with

up-to-date

control,

d

ri

VI~S

of

this

type

are

still

in

use today.

The

basic circuit is seen in Fig.

'/

.1

(;;

Lbe

separately

excited DC

motor

(2) driving

the

load, for

instance

a

rullillg mill

or

an

elevator, is fed from a

separately

excited DC generator,

J"lllIlIillg

at

approximately

constant

speed. While a rolling mill

or

a gear-Iess

"I"vaLor drive would norma11y

operate

at

very low speed, for example between

I

100 .L/min, resulting in high

torque

and

a relatively large frame size,

the

1'.'·llI'raLor being

of

similar power

rating

as

the

motor

can

run

at

any conve-

Uí'

·

lll.ly

higher speed

Wl,

with

the

aim

ofreducing

its

size

and

cost.

lnstead

of

:1.

I

l(;

gCllerator,

an

AC

generator

with rectifier could

be

used for supplying

Lili'

I )C-link;

of

course, this would rule

out

regeneration.

'I

'lte

prime

mover

of

the

rotating

converter is

most

frequently

an

AC induc-

Lil

III

I.llotor

(1)

with

cage

or

wound rotor, Chapo

10,

requiring no continuous

C"IlIII

.rol.

The

power

fiow

of

the

drive is

then

reversible

with

the

contro11ed

Illut,or being able to

operate

in

a11

quadrants

of

the

torque/speed

plane.

ln

("as.

~

of

a ship-

or

vehicle-propulsion scheme a

turbine

or

a Diesel engine could

bc

sllhstituted

as

prime

mover

(1)

(Turbo-

or

Diesel-electric drive), while

the

drive lllotor

(2)

would

most

likely be a series

type

motor. Resistive

braking

cUllld

he employed

instead

of

regeneration which is

of

course

not

possible

wil.lt

a llllidirectional

prime

mover.

Wi

Lll

largc rolling mill-

or

mine hoist-drives

the

total

drive is often split

I I I I i I I

LI

I several machines; for instance, on a large reversing mill, two twin

111()l.ur~

Illay drive

the

upper

and

lower

ro11s

without

direct mechanical con-

11

...

·

l.iUII

:l.p<l1'1.

I"rom

the

frictional forces

through

the

material

being

ro11ed.

A11

1I1

;

,<"iliIW:

;

("oll;-;I

.il.llting the

rotating

converter, such as AC motors, DC gener-

01.<11

11

1

\11d

, possibly, secondary machines are normally coupled

to

form a long

,J

I I

,"

" ::Ilitl"l., s(]lIlctimes

50

m long, which is located

in

a

ha11

adjacent

to

the

,

.,

'111.1",11.-.1

drive

and

the load.

A

II

:111

V;\.JI

La.ge

of

the

rotating

motor-generator-set is

the

storage

of

kinetic

"II"IT,'y

iii

LlI(~

lllasses

rotating

with

Wl,

which

may

be

accentuated

by connect-

ilq',

JI.\'wlil·I~ls

to

this shaft

and

increasing the slip

ofthe

wound

rotor

driving

I I

II

di

J/(

I

);

a COllverter of this

type

is

then

ca11ed

an

Ilgner-set. For large drives

(rvl

W)

ii

Ili'cia.\ schemes have been developed to

recuperate

slip power in

the

I "l.ur

1·

.

ircllit"

Sect. 13.3.

The

kinetic energy

of

the

rotating

masses provides

11

C'Ullvellil~llt

l1leans

of

dynamically decoupling

the

AC

supply from

the

load;

1.

11I'lC'

IU<l.y

be

different motives for this, depending

on

the

application.

With

II

'v"rsitlg roJling mills

there

has

always been

the

desire

to

smooth

the

load

Illlc:l.lIl\.l

.iolls

of

20

MW

or

more lasting a

few

seconds;

this

point

of

view

has

1)1"'11

01'

(>nrLiC1llar

interest

with

weak supply systems

but

its

importance

re-

cl'dceI

;,IS

LlII~

slIpplips !lave become stronger

anel

more

closely interconnected.

(:ollvI·

rsl'i.v,

;1,

J'()l.nl.illl~

COllvcrtc~r

can

servI: for

protl~(".t.ion

of

sensitivc loads,

i ii Ir"

;1::

[l

il.

pl'l'

IllilLdriVI':i, agaillSI. Short-

l.IIIlC

lillO disturhallccs. Silllilar argll-

1111

\

111

..

'1 I

lrn

vl

tI,,1

wlll'll

1I111111.1'ITllpl.ildl·

[lOWI'J'

rol'

hllplirl.alll loat!s

is

1'(~'111i['(

.

'd,

7.4

Supplying

a

DC

Motor

from a

Rotating

Generator

for

example

computers,

elevators

or

operating

rooms in hospitals;

with

an

intermediate

motor-generator-set

supplying

these

loads,

there

is

more

time

for switching

to

an

alternative

supply

in

case

of

a line disturbance.

The

main

drawback

of

the

Ward-Leonard-drive is of course

that,

besides

the

adjustable

speed drive, two

additional

machines

of

about

the

sarne power

rating

are

needed, Even

though

they

may

be

smaller

in

size because

of

higher

speed, large machines will still require heavy foundations

and

continuous

service

of

bearings

and

commutators.

It

is for these reasons

that

static

power

converters,

particularly

of

the

modern

semiconductor

type

have

supplanted

rotating

motor-generator-sets

in new insta11ations.

If

the

field

of

a

rotating

generator

is fed from a

rotating

exciter,

both

responding relatively slowly,

it

is

with

some care

quite

possible

to

control

the

drive

motor

manually

and

without

any

automatic

equipment. However,

the

machine

cannot

be

fu11y

utilised because

of

the

danger

of overcurrent; feed-

back control

with

its much faster response

then

offers considerable advantages

making

it

possible

to

take

various side conditions

into

account so

that

the

AG

supp/y

Rotating

converter

set

•

I!

'

DG-

generator

DG-motor

Load

<!lo

e

max

Induction

I

III

Motor

1

Starting

resistor

~~

-

---t

.o~.~~,--------------

Fi,l.

7.111.

(~I)III.I

'

1)1

li

l'ill'llIl:

()[

WMel

L(~OIlIl.rd

·

drivc:

!II'

7.

ContraI

of

a SeparateIy

Excited

DC

Machine

drive assumes well defined

operating

characteristics

and

may be inserted into

n,lIl.olllatic

production

processes.

By

exciting

the

generator

and

the

drive mo-

1,,1

I.ltrough

static

converters with high ceiling volt age

(2

to

3 times

steady

::Iill.('

voltage)

the

speed of response of

the

drive

may

be sufficiently improved

1m Illeding most dynamic specifications.

'l'lw control

structure

shown in Fig. 7.16

is

essentially in line with

that

in

I"il':

· 7.13.

The

only difference

is

the

possible introduction of

an

inner control

loop

ror

the

armature

voltage U

a

of

the

generator [876]. 8ince

the

pertinent

1'()Jlioll of

the

control

plant

comprises only

the

field

time

constant of

the

1',('ll<'ral,or

and

the

residual delay of

the

electronic field power supply,

the

re-

::pOIISC

of this control loop

can

be

made

quite

fasto

Its advantages are

the

lill('ari,sation of

the

inner

part

of

the

plant,

the

reduction of

the

effective ar-

111:\llIr('

impedance of

the

generator

and

the

possibility of precisely limiting

1.11<'

;tIInature voltage.

This

is

important

with large machines where

the

per-

III

i

ssi

ble limits of

commutation

must

be fully used without running

the

risk

01"

cx('cssively sparking brushes. For

the

superimposed

current

controller

the

c!os('d voltage

controlloop

looks like a linear

actuator

with

constant

gain

and

(·oll.si(lcrably reduced

lago

8.

Static

Converter

as a

Power

Actuator

for

DC

Drives

8.1

Electronic

Switching

Devices

A characteristic feature

of

the

rotating

converter discussed in 8ect. 7.4 is

the

consecutive power conversion from electrical (constant AC line volt age)

to

mechanical (speed of motor-generator)

and

back

to

electrical form (variable

direct voltage) from which it is eventually transformed by

the

drive

motor

into

controlled mechanical power. These conversions

constitute

the

advantages

of

the

control scheme (separate electrical circuits, decoupling by

rotating

masses) as well as its draw backs (cost of machines, foundations, power losses,

servicing, limited speed

of

response).

This

naturally

gave rise

to

early ideas for eliminating

the

intermediate

mechanical stage by supplying

the

DC drive

motor

directly from

the

AC grid

through

static

electronic devices.

That

such possibilities exist in

the

form of

static

switching converters has been known for a long

time

[A15,W29,46].

ln

fact, switching devices are

the

only available means for high power converters

because

any

other

circuit components would result in prohibitive power losses.

The

basic element of a power converter is

an

electronic switch having

the

properties

of

a controllable rectifierj

the

idealised

steady

state

characteristics

of different switches are seen in Fig. 8.1. An uncontrolled diode rectifier, Fig.

8.1 a, has two distinct states:

With

reverse volt age

(u

<

O),

there

is only

negligible leakage current, whereas in forward direction (i >

O)

a low forward

resistance combined with a threshold voltage exists. Controlled electronic

switches have

at

least

three

different states: A

thyristor,

Fig. 8.1 b, is blocking

reverse

current

like a diode,

but

in forward direction

(u,

i >

O)

two possible

states

exist:

the

device may either be "conducting"

01'

"blocking".

The

first

state

is again associated with a low conducting resistance, typically

10-

3

[l,

while in blocking condition

the

device exhibits a very high blocking resistance,

for example

10

5

[l,

similar

to

that

in

the

reverse direction. Hence

the

thyristor

valve may be approximately described by a mechanical switch in series with

an

uncontrolled rectifier (diode).

The

main difference between a

thyristor

and

a diodc

is

tlw existence of

the

forward blocking

state.

'flw

I.rall:iitioll frorn "blocking" to "conducting" may be

initiated

by a

slIlall

1'::\,1<'

('111'1'<:111.

plllse

-ii"

while the inverse

transition

fl"om

"conducing"

lo "lllo('liilll',"

('

i

l,II:

.i

for

a

:/'('1"0

inl.c:rscci.ioll of

UI('

1I1aill

cnl"n:llt, "Tnl"ll-0fF

aI.

('

111'1',

'

111

'/,(\

("

""',

I.

h

yr

l::I

,

llr:

, a

r,

' wid('l y

11:

; ('<1 ror ('olil,rollillJ': U

(:

<I"iv(':;,

99

'

!U

H. ,':iUtLic Converter

as

a Power

Actuator

for

DC

Dríves

fi

11101('

n~cent

development

are

Gate-Turn-Off thyristors

(GTO),

where

1',:11,,'

'''Idrolled

electronic blocking is also possible, Fig.8.1

Cj

they

are

mainly

Il

ro

, -c!

III

rorcc-commutated converter circuits with a DC link, to be discussed

III

( :lld,

P:

;.

~).2

and

11.

Other

types of electronic switches which are of

interest

',,1

::,,,,ci;d applications are reverse conducting thyristors or voltage controlled

1.1

1I.

1I

:IIK

l.ors

with internal diodes, where

the

two forward

states

are

combined

IV

iI

,

11

I('V

" ['sc conductionj

an

example is

the

"lnsulated

gate

bipolar transis-

1.,

'I"

(I ( : I

~T),

seen in Fig. 8.1 dj there

are

also applications for

symmetrical

11

11'

1

1."1

il'

S,

l"ig. 8.1

e,

as

wiU

be

shown in Chapo 11.4.

forward

conducting

!./

",

electranic

/

'"

electronic

, \ control

\ control

u

IUverse

rückwarts

I forward u

reverse

blocking

~

Iilucking

sperrend blocking

blocking

b)

!ig

c)

,I)

",

.

!ig

I

u

o ! f>r o

o •

f>(

o

Oiode

-u

Control/ed rectifier

Gate tum·of( Thyristor

Thyristor

(GTO)

conducting

, conducting

/

~

.

electronic

~

"

electronic

\ control

, \ contraI

blocking

:. .

...

.'. .

..

.

JI ,

VO

/

!J

t l

forWârdU

blocking u

blocking electronic \

control"-..,

conducting

" " '/Iludiu!}

e)

"J

u

-1

\f.

1

LfJt

U

g2

L

i

--.J

\.

/

~

u

~

Insulaled gale

u

bipolar Transistor

(IGBT)

Symmetrical switch

(IGBT)

...

l,~

.

H.

I. I) II

COII

trolleel

anel

controlJed electronic switdlC::;

III

I.he

pax(.

Lhe

fllllct.ion

of

controlled

d(~cl.J'()lIi

('

x

w;Ldws

li

as been realised

with

vm;"lIs

dl'VÚ'('

S Ill,tkiug

\l*~

o[

diJkr<H1t.

plt YHin

tl

dkds;

ali tlw:>e eurly

Il

rtllll,ioll:

;

nll'

1I1

l/l

11

kf,('

(.odny.

Siul't

:

l'I(\('.(.I'OIl

Llrl

1

oc

'lI

wít.1t

Itl.'it!.l:d·

cal.hodl~s

cOllld

1I"t.

Il

lIpply

"II"II

I\

IJ

l '

IIIT"IIt.

1,0

driv

o

IIIOt.OH

I

II

JltI

\\11

'

1<

'

pl

n

l~ll<'d

wil,lr lri((

1r

voll,-

'I',"

cll"I

" I " 1

11

ri

I,!

I,!'

,

III

1""11 d

ll

d

l'l

l"

Y,

1.111'

,

1'

W

"I

CI!

,

III""I

III

t'C("d

I,

,Y

J'

~

1U

1

lil

ll

..

,

c)I

.II

I'

('

I!

8.1 Electronic Switching Devices

with

heated

cathode, called

thyratronsj

however, power rating, life

time

and

reliability were also

rather

limited so

that

they

were only useful for small

drives. High power in

the

MW-range, such as needed for rolling mills or mine

hoists, could only be supplied by mercury-arc valves having a liquid

mercury

cathode.

These

electronic valves were developed

to

high perfection

by

the

1950's

but

the

liquid Hg-cathode limited

their

use in vehicles

and

remained

a disadvantage, as well as

the

large size

and

the

relatively high voltage

drop

across

the

electric arc connecting cathode

and

anode (20-25 V).

Outside

a

narrow

operating

temperature

range, controlled mercury-arc valves showed a

tendency

to

occasional spontaneous arc-backs, i.e. conduction in reverse di-

rection resulting in

short

circuits

and

high over- currents.

This

required close

temperature

control by heating or cooling

the

steel vessels. AIso, mercury-

arc

converters needed relatively long intervaIs for deionisation which

made

them

unsuitable for

operation

at

higher frequency. After a brief interlude

with

magnetic amplifiers where magnetic materiaIs with

an

approximately

"square" hysteresis loop were used as "switches", alI these difficulties were

completely overcome with semiconductor switching devi ces (Transistor 1948,

Thyristor

1957) which have become available

at

steadily increasing power

and

with improved performance since

about

1960.

They

have replaced mercury-

arc valves even

at

the

highest power ratings in high volt age DC transmissions

(HVDC).

The

physics

of

a

thyristor

are

extensively dealt

with

in

the

literature,

e.g. [17,24], a simple description may suffice here.

ln

Fig. 8.2 a a small slice

of highly purified monocrystalline silicon having four electrons in

the

valence

band

is shown, which is doped with materiaIs of valence

three

or five, for

instance Boron or Phosphorus, so

that

three

P - n

junctions

are

created,

separating

the

layers

of

P -

and

n - conducting material.

The

inner layers

are relatively weakly doped while

the

outer ones, forming

the

anode

and

cathode

terminaIs, contain more doping materialj

the

inner

P2

layer is also

connected

to

a

gating

electrode supplied by a low impedance control circuito

The

following

steady

state

operating

conditions exist:

•

With

negative voltage between anode

and

cathode, u <

O,

the

diode junc-

tion

PI -

nl

is

reversely biased; this corresponds

to

the

reverse blocking

state

seen in Fig. 8.1 b.

•

If

positive voltage is applied (u >

O)

but

without

gate

current (u

g

<

O),

the

diode

nl

-

P2

has reverse bias which

constitutes

the

state

of

forward

blocking. Again, only negligible current flows in

the

load circuito

•

With

positive voltage, u >

O,

and

with a sufficiently large

gate

current,

ig >

O,

the

junction

nl

-

P2

becomes conducting,

with

the

load

current

i s

atnrating

the

region

nl

from

both

sides with charge carriers.

Thus

the

lhyri

::;

tor assumes very low resistance in forward direction.

When

i has

l')(

('e

(

~(

kd

;t

cl'/'I.ailllatching threshold

iL,

the

conducting

state

persists even

afl.e/'

Lh

e' (',

;tl

,(' (

',

lIl'rmlt:

is removed.

Werner Leonhard Control of Electrical Drives

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