fiK
5.
Separately
Excited
De
Machine
1
~
1+(W)2b
n1 = D
r:-:-;;;:
(5.25)
yl+V
,'-)O
IIIC
drives are subject to disturbances by periodic components in
the
supply
V(
,11.a.
ge or
the
load torque, causing continuous oscillations of
armature
current
alld speed
around
their
steady
state
valuesj
the
transfer function
permits
a
::
i
IIlpl
e
computation.
When
a
torque
consists of a
constant
and
a sinusoidally
varying component
'ln
r-
mLl
~
- - =
--
+
--
coswmt
'11/,0 mo mo
('ae h variable of
the
linear drive will in steady
state
contain a periodic com-
plllll'llt
of
the
sarne frequency
that
may
be computed by linear superposition.
Wil.h
the
complex expression
7nL
~
.
.
COSW
m
t =
---
[e
JWm
t +
e-JWm
t]
mo
2mo
wc
lilld for instance from Eq. (5.18)
the
peak
value of
the
alternating
current
COlllpOllent
(kL =
O)
..1mL
/h " I b I
(5.26)
'i"
,,
' - T
m
Ta (j w
m
)2
+
Tmj
W
m
+b
2
mo
;;;"Iil;(r! y
1./11
' plta
.se
of
the
oscillatory current
may
be determined.
If
the
fre-
'111(
'
''
(''y
..
r ;\. lwriodic disturbance lies near
an
eigenvalue of a poorly
damped
"I
I v(
·)
ii
rc
'
so
llallt
condition could exist causing large amplitudes of
the
oscil-
l:d
,II
I"
Y ('III'I"C ' II(. a
ll(l
speed deviations.
'1'11
1'
dY
II<l.lIli
c behaviour of separately excited DC motors will be further
"i
::r
11
!;;;
<:c1
in
laL
er sections in connection with control problems.
6.
De
Motor
with
Series
Field
Winding
This
motor
differs from
the
one previously discussed only by
the
design
and
the
connection of
the
field winding which, according
to
Fig. 6.1, carries now
part
or all of
the
armature
current.
Ral
is
the
armature
resistance, possibly
increased by
an
ex
ternal
resistor,
Rp
is
an
adjustable
shunt
resistor for field
weakening.
Because of
their
operating
characteristics, series-wound
motors
of larger
rating
are restricted
to
traction
drives.
ln
urban
transportation,
such as trol-
ley buses or subways,
motors
up
to
about
200
kW
are used
but
for
main
line locomotives motors of 1000
kW
are
common.
ln
principie, series wound
motors can be fed with either direct or
alternating
current
but
the
construc-
tional
design would be quite differentj for example, a
laminated
stator
and
special provisions with regard
to
commutation
are required for
the
AC
mo-
tor.
When
AC
traction
was developed
at
the
begin of
the
century,
the
current
induced by
transformer
action in
the
armature
coil
temporarily
shorted
by
the
brushes
during
commutation,
Fig. 5.2, caused problems, which were
then
solved by choosing a lower supply frequencyj this
is
the
reason for
the
16
2
la
Hz power supply used on central
European
railways until
today
. Since com-
pact
and
rugged power electronic components have become available,
main
line locomotives with AC supply were often equipped with rectifier- fed DC
motors.
ln
future, all these schemes are likely
to
be supplanted by AC motors
fed from variable frequency inverters.
ia-ie
ie
Re
cI>e
U
a
R
n
-R
,
11
mM,W,E
Fi)
·
:.
O.
I.
1)( '
""
.
<.
.... w
i<.h
,
,,
'ri
"H
fi..!d Willdillf(