
25-6 Power Electronics and Motor Drives
case is not suitable now, since no inertia element with electromagnetic time constant T
a
is available to
c
ompensate
t
he
n
umerator
t
erm
i
n
P
I
c
ontroller
t
ransfer
f
unction.
B
ecause
k
a
(i
a
, α) ≪ K
p
K
a
, a control
d
esigned
f
or
c
ontinuous
c
urrent
h
as
a m
uch
l
onger
r
ise
t
ime
w
ith
d
iscontinuous
c
urrent;
i
ndeed
a
t
s
mall
c
urrent
v
alue,
t
he
r
ise
t
ime
c
an
b
e
o
f
t
he
o
rder
o
f
s
econds.
I
f
t
he
c
urrent
c
ontrol
i
s
d
esigned
f
or
a s
hort
r
ise
t
ime
a
t
l
ow
d
iscontinuous
c
urrent,
t
hen
i
t
b
ecomes
u
nstable
o
n
t
ransition
t
o
c
ontinu-
ous
c
urrent.
T
o
a
void
t
his
i
nstability,
t
he
c
urrent
c
ontroller
s
hould
b
e
c
hanged,
t
o
p
resent
t
he
s