
Multiphase AC Machines 3-21
only (n−a) voltage equilibrium equations to consider, since zero-sequence current cannot ow in any
o
f
t
he
t
hree-phase
w
indings.
S
ince
n = 3a, t
hen
t
he
t
otal
n
umber
o
f
e
quations
i
s
2a. A
s
t
he
rst
p
air
i
s
a
lways
f
or
d − q c
omponents,
t
hen
t
he
r
esulting
n
umber
o
f
x
–y
v
oltage
e
quation
p
airs
i
s
o
nly
(a−
1).
is
c
omes
d
own
t
o
o
ne
d − q p
air
a
nd
t
wo
x
–y
p
airs
f
or
a
n
a
symmetrical
n
ine-phase
m
achine
w
ith
t
hree
i
solated
n
eutral
p
oints.
H
ad
t
he
n
eutral
p
oints
b
een
c
onnected,
t
here
w
ould
h
ave
b
een
t
hree
p
airs
o
f
x
–y
e
quations.
As
a
n
e
xample,
c
onsider
a
n
a
symmetrical
n
ine-phase
m
achine,
w
ith
d
isposition
o
f
s
tator
p
hase
magnet ...