Environmental Engineering Reference
In-Depth Information
machine and power electronics but of the mechanical system components as well,
including gear sets and differential.
The hybrid propulsion system considered in this section must develop 35 kW
of continuous power and 70 kW of peak power for 90 s, operate from a 350 V
nominal power source and deliver full torque in less than 125 ms. Furthermore,
torque ripple must not exceed 5%. In a hybrid propulsion system, the most
important system metrics other than meeting cost and package targets are the need
for fast transient response, wide constant power speed range and minimum torque
ripple [37].
The onboard power source will determine to a large extent the transient cap-
ability of the ac drive system in meeting torque, power and constant power speed
range metrics. The controller to a large extent determines the transient response
above and beyond those of mechanical system constraints set by rotor inertia and
gear ratios. Torque ripple characteristics are predominantly a machine constraint
with some mitigation effects attributable to the controller and driveline. In the
assessment to follow the gearbox is a two-stage design with ratio coverage ranging
from 6 to 12:1. Pertinent data for the comparisons are listed in Table 5.12. Notice
that the SPM machine is added to the comparison matrix. The four electric
machines are all 3-phase and the inverter is 3-phase, but in this comparison the
inverter package is sized to match machine drive current requirements. The VRM is
a 12/8 (two repetitions of 6/4 design).
Table 5.12 Hybrid propulsion system ac drive system attributes
Attribute
IM
IPM
SPM
VRM
M/G mass (kg)
44
43
46
57
Poles
4
8
12
12
Rated speed (rpm)
13,000
8,200
8,500
16,200
Specific power (kW/kg)
1.59
1.63
1.52
1.23
Inverter current (A
rms
)
350
350
280
500
Inverter mass (kg)
8
10
9
19
Specific power (kVA/kg)
14
11.2
12.4
5.9
Gear ratio coverage (
x
:1)
11
6
7
12
Mass of gear box (kg)
25
19
16
19
Total system mass (kg)
77
72
71
95
The inverter specific power, kVA/kg, is taken as machine power of 70 kW
scaled by a factor of 1.6 to account for power factor and efficiency losses. It is clear
from this comparison that, in terms of system total mass, inverter specific power
density and gear ratio cover required that the VRM system requires further devel-
opment in order to be a viable technology for hybrid propulsion.
The permanent magnet machines provide the highest specific power density
and have the lowest total mass for hybrid propulsion. However, the cost of the
