Environmental Engineering Reference
In-Depth Information
The permanent magnets used in this design study will be sintered rare earth
type. Table 5.6 lists the RE-magnet properties of most interest in a hybrid propul-
sion application, and these are its remanence as a function of temperature, tem-
perature coefficient of remanence flux and bulk resistivity.
Table 5.6 Properties of permanent magnets
Reversible Temperature
Coefficient
T
op
,
max
(
C)
Magnet
type
BH
mx
(kJ/m
3
)
B
r
(T)
H
c
(kA/m)
B
r
(%/
C)
H
c
(%/
C)
NdFeB
200-290
1.2
870
180
0.13
0.60
0.045
0.25
SmCo
5
130-190
1.0
750
250
Sm
2
Co
17
180-240
1.05
660
250
Alnico
70-85
1.2
130
500
0.02
+0.01
0.20
Ceramic
27-35
0.4
240
250-300
+0.40
The best magnet for an electric motor would be SmCo, owing to its high
induction and simultaneous high coercive force and high operating temperature.
Moreover, its reversible temperature coefficient on induction is sufficiently low to
hold airgap flux density nearly constant over the normal operating temperature
range of most M/Gs in use. The issue is cost; SmCo permanent magnets cost from
two to three times as much per unit energy than rare earth, NdFeB. This has
resulted in SmCo magnets being applied in only the most performance sensitive
applications such as aerospace and spacecraft.
The discussion to follow is meant as a brief introduction to the overall process
of designing an M/G for a hybrid propulsion system, in this case, an integrated
starter generator (ISG). For ease of explanation, an SPM machine is selected. The
permanent magnet material will be NdFeB having a remanence of 1.16 T, a coer-
cive force of 854 kA/m and a recoil permeability,
m
r
¼
1.08. Equation (5.10)
summarizes the calculation of induction,
B
d
, versus applied field intensity,
H
d
, due
to current in the stator windings:
B
d
¼ m
r
m
0
ð
H
c
H
d
Þ
ð
5
:
10
Þ
Ni
l
e
H
d
¼
where
H
c
is the magnet coercive force.
With a suitable magnet mounted to the SPM rotor, the resultant flux induces a
voltage into the stator windings,
E
0
, when the rotor speed is at its corner point,
n
0
.
For a given rotor speed in per unit, pu, the
d
- and
q
-axis voltages are
i
s
¼
i
q
ji
d
p
P
60
n
0
l
dr
u
qs
¼
n
pu
ð
E
0
X
d
i
d
Þ
u
ds
¼
n
pu
X
q
i
q
E
0
¼
ð
5
:
11
Þ
