Environmental Engineering Reference
In-Depth Information
The acronyms defined in Figure 4.17 will be used throughout this topic. These
are IM for induction machine of cage rotor (cast aluminium or copper) or wound
rotor (i.e. slip rings) or all iron rotor, IPM for interior permanent magnet, SPM for
surface permanent magnet and VRM for variable reluctance magnet, doubly salient
designs. There are many variants of these electric machines such as drum versus
axial, normal versus inside out, and rotary versus linear, and various excitation
dependent nuances such as trapezoidal versus sinusoidal waveshape and many
other distinctions. The intent of Figure 4.17 is to capture the high level differences
in machine types and to showcase the origins of the four most popular types.
It is also important to re-emphasize the fact that the constant power speed
range of these four electric machine types ranges from 1.6:1 for the SPM without
use of a novel cascade inverter (the dual mode inverter concept (DMIC) discussed
later) to 3:1 for IM and VRM, to 5:1 for IPM. Wide CPSR in these machines comes
at a price: generally, IPMs with 5:1 CPSR are physically larger and heavier than
their counterparts having the same power rating. One difference is the DMIC power
electronic driver for an SPM in which 6:1 CPSR has been demonstrated, provided
the rotor structure can withstand the stress.
4.2.6 Machine sizing
We now turn our attention to M/G sizing for a hybrid propulsion system. As is well
known, the electric machine is physically sized by its torque specification. Since
electric machine torque is determined by the amount of flux the iron can carry and the
amount of current the conductors can carry plus the physical geometry of the machine,
the following can summarize the sizing process. Torque is proportional to scaling
constants times the product of electric and magnetic loading times the stator bore
volume. Electric loading is defined as the total ampere-conductors per circumferential
length (
A
, in units of A/m) - in effect, it is the description of a current sheet. The
electric loading is limited by thermal dissipation of the conductor bundles. Magnetic
loading is set by the material properties of the lamination sheets (
B
, in units of Wb/m
2
)
and of the physical dimensions of the airgap. The product of electric and magnetic
loading is a volumetric shear force,
AB
(Nm/m
3
). The stator bore volume,
D
2
L
, defines
the airgap surface area (
p
DL
) times the torque lever arm (
D
) of the rotor on which the
volumetric shear force acts. The scaling constants and coefficients are absorbed into
the proportionality constant for M/G torque in terms of its design loading and geo-
metry. For electric machines of interest to hybrid propulsion, the volumetric shear
force ranges from 25,000 to 80,000 Nm/m
3
. The relationship for machine torque is
T
¼
kABD
2
L
ð
4
:
7
Þ
where
k
is a constant that includes geometric variables, and excitation waveshape
variables for voltage and currents. The bore diameter,
D
, or more accurately the
rotor, OD, is the main sizing variable in electric machine design. Sizing is con-
strained by four fundamental limits. Two of the fundamental sizing constraints
have been discussed thus far: electric and magnetic loading. To further explain
these sizing constraints, it is important to understand the limitations on current
