II
1
:\H
R.
Static
Converter as a Power
Actuator
for
DC
Drives
iI!
fi
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fifi
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Fil!:.
8.34.
Step
response
of
current
controlloop
with
adaptive
current
controller
CIlITCnt
(o:
= o:d
the
situation
is differentj
the
interval, where
UD(T)
>
O,
is
st.ill illcreasing when
o:
is reduced by
Llo:,
but
so
is
the
time, where
UD(T)
<
O.
I
ÍPJl
c:e
,
the
voltage gain dUD /
do:
is
much smaller
than
in
the
case of continuous
ClllT
ent.
The
block diagram of
the
current
control loop for discontinuous
current
colTesponds
to
that
shown in Fig. 8.31
but
the normalised voltage gain of
the
(·()Jlvcrt.
er is now considerably lowerj also,
the
lag element with time
constant
'/'i> is
ll
OW replaced by a lag having a time
constant
approximately equal
to
\.II<'
liliul-\
interval T
D
~
3.3 ms, which
is
about
l/lO
of
the
former value.
(
:I(
'arly
t.hi
s calls for a different design of
the
controllerj for example,
an
illl.(
'I',I:\1 colltroller with
the
transfer function
I
,'
,
(
...
)
1
with
Ti
=
G'o
(Te
+
T1)
,
(8.47)
.',' .
7."
i S
,ILII
:,('l.l'Iis
(,,1
hy a small
time
constant
TI,
i.e. high gain, could now be ex-
1>1'
11.""
1.,)
pr()duce fast
and
well
damped
closed loop transients.
III
md<T
t.o
adapt
the
controller
to
the new
situation,
the
condition of in-
1,
"I'IIIi1
.
I.(
'
IIt.
Cl.lrrent must
be
detected which requires a few additional electronic
, ,l'('lIil.s
IJIII.
presents no
major
difliculty.
'1'11('
, realisation of
an
adaptive
current
controller
is
particularly
attractive
ir
UI<'
('()ut.rol
is
performed by a microprocessor because
then
the
controller
II"
rallwt.ers
<.:an
be
adapted
by changes in
the
program,
i.e. by software.
ln
'ci
g,
1).
31 a measured oscillogram of a
current
transient
with a 6-pulse con-
vert.er
is
shown, where
the
entire control, including
the
firing, was performed
hy a llli(Toprocessor [M4, M5].
ln
both
regions,
with
continuous as well as
discolll.illllo
US
current, satisfactory transients are now obtained. Besides
the
n"il.pl.i
v
( ~
cmrcllt. control,
the
initial condition of
the
integral controller
chan
-
ud
\\'<1.
,';
,tI:
;
()
pr
\!
sd.
ou
th(~
lmsÍs of the current reference
and
the
reconstrud
ed
1';1d\ voll.al',"
Lo
IH'('Vl'lIl.
a Ial'ge [irs
\:
cl1rr(~I1t.
()v(~rsh()ot
which,
would
loe
oh-
i,','I,I
IIlI
l
d.l"
(III
1i
,'
I'
VII
dl'lv,':
: 'I'hi:; (I,'xil,ílil.,Y Ú, il.U
iIllporl
,1l.
nt.
kn.(,IIJ'(,
III'
dil
~
il,al
('
IHII,I,"
I
I)
\,
IIl1
j
' I
I'III'It(
" '
lh
,tll
"',
9.
Control
of
Converter-supplied
DC
Drives
Static
converters are ideal electronic
actuators
for
DC
drives because of
their
practically unlimited
output
power
and
excellent controllability.
The
speed
of response is usually
adequate
to
handle
the
electromechanical
transients
occurring in drives. Line-commutated, phase controlled converters or, as
they
are also called, converters with
natural
commutation,
are
the
most
frequent
choice for industrial applications, where a three-phase supply
is
availablej
this
is due
to
the
simplicity of
the
circuits requiring a
minimum
number
of
active
and
passive components.
For vehicle drives, where no
AC
catenary
or independent
AC
supply
is
available or where
the
reactive current
and
the
harmonics caused by a line-
commutated
converter would be unacceptable,
it
may
be necessary
to
employ
forced
commutated
converters having a more complex circuitry
and
involving
higher lossesj a special
situation
exists also with small DC servo drives, where
the
response of a line
commutated
converter
may
be insuflicient
to
cope with
the
stringent
dynamic
demands
and
where a chopper converter supplied by
a
DC
link
and
operated
with a higher switching frequency
is
necessary.
9.1
DC
Drive
with
Line-commutated
Converter
When
connecting a line-commutated converter, for example
the
three-phase
bridge circuit
in
Fig. 8.18,
to
the
armature
of a
DC
machine (Fig. 7.2),
it
should
be
kept
in
mind
that
the
converter
can
only
operate
in two
quadrants
of
the
uD/iD-plane
as seen in Fig. 8.23. Because
of
UD
= U
a
,
iD = ia
it
follows from Eqs. (5.6, 5.8)
that,
assuming
constant
sign of
the
flux
<Pe,
the
machine
can
run
in
both
directions,
but
the
torque
is unindirectional.
The
current/speed
cascade control scheme shown
in
Fig. 9.1
is
therefore capable
of
operating
in
two
quadrants
of
the
torque/speed
plane, hence
it
is
called a
two-quadrant
drive, Fig. 9.2.
Since
the
converter
cannot
produce negative
output
current,
there
is
no
point in
admitting
a
command
signal calling for negative
armature
currentj
with an illtegmting
cmrent
controller
this
would only cause unnecessary wait-
iHg
illL('l'vah
wll(
~
lI
L1w
current
rdcrcnce
becomcs positive again ("controllcr
wind
'
lIp")_
'1'1111
11
,
\.1)('
O\ltput
sig
llal
01'
\.III
: tip
l'
t'
d (:ollt.l'OlIer
shonld
be
limil.(~d
d,
\.III'
111'1"'1
'
j).
:1 w,'lI
l1
H
\.1)('
10\\"'1' ('
1111
,
()
. i"
11
'1
1 . i"
""
" .