Environmental Engineering Reference
In-Depth Information
that hybrid M/Gs are generally liquid cooled (use of engine coolant and/or trans-
mission oil) completely supports this trend. Generally speaking, liquid cooling
boosts the torque production, so that the output of liquid-cooled machines is four
times that of air-cooled machines, all else being equal. The fundamental purpose of
any electric machine is to deliver torque. If the machine package volume is con-
strained, then a metric of torque per litre is valid, but this is not as universally
applicable as the more specific torque per unit mass, Nm/kg. High torque to mass
implies high power density and also high acceleration capability.
Some comments about the application choice of the electric machine types
listed in Figure 5.16 are in order before proceeding with a more detailed design of
the SPM. The SPM design may be sinusoidal or trapezoidal back-emf. Sinusoidal,
brushless ac designs tend to require higher inverter rating than the trapezoidal,
brushless dc designs. Both brushless dc SPM and the IPM designs tend to have high
torque ripple. The IM has the lowest torque ripple of any other type, but it requires
a supply of magnetization current from its inverter, thus increasing the inverter
kVA rating. IMs for hybrid propulsion require that a rather large fraction of input
VAs be dedicated to magnetizing the machine. IMs of several hundred to thousands
of horsepower have a much small fraction of input VAs dedicated to magnetizing
the machine. The doubly salient machines have many desirable features for hybrid
propulsion and are beginning to be looked at more seriously. The VRM has had
hybrid propulsion advocates for many years, but structural design to maintain the
tight airgaps necessary and controller algorithms capable of real-time current
waveshape control based on rotor position have been problematic. The VRM is
capable of the wide CPSR as is the IPM, and so should find application to power
split and other hybrid propulsion architectures.
To summarize the comparisons of the various electric machine technologies
for application to hybrid propulsion, it is necessary to comment on their torque-
producing mechanisms. We close with comparisons of electromagnetic torque
production in the machines evaluated so far (Table 5.4).
Table 5.4 Electric machine torque production
Definitions
Expression for torque
P
= number of poles
M
= number of electrical phases
I
p
= phase current
L
p
= phase inductance
l
= flux linkage
L
m
= magnetizing inductance
L
r
= rotor inductance =
L
mr
+
L
lr
q
= rotor angle (rad)
Synchronous machine (SPM,
IPM, SyncRel)
m
2
P
2
ðl
dr
I
qs
l
qr
I
ds
Þ
T
em
¼
m
2
P
2
L
m
L
r
l
dr
I
qs
Asynchronous machine (IM) under rotor
field oriented control (i.e.
l
qr
=0)
T
em
¼
2
I
p
dL
p
ð
q
Þ
1
Variable reluctance machine (VRM)
T
em
¼
d
q
