Environmental Engineering Reference
In-Depth Information
process information and that power electronics uses information to process power.
In this section, we describe how power electronics is sized to match the electric
machine to the vehicle ESS.
Figure 4.19 is a schematic of a hybrid ac drive system consisting of on-board
energy storage, power processing according to control algorithms and traction
actuation via the M/G and vehicle driveline.
R
d
Power electronics
R
i
V
f
,
I
f
V
b
T
w
Transmission
Driveline
Control electronics
C'mds
Controller, comm.
gate drives, pwr supply
Figure 4.19 Schematic of hybrid ac drive system
The essentials of ac drive system operation are that power from a dc source
such as a fuel cell, battery or ultra-capacitor is converted to variable voltage,
variable frequency ac power at the M/G terminals,
V
f
and
I
f
. The M/G then con-
verts this electrical power to mechanical power in the form of a torque and speed at
the transmission input shaft,
T
and
w
. The power electronics is an electrical
matching element in much the same manner that a gearbox processes mechanical
power to match the engine to the road load requirements. The power inverter
matches the dc source to the mechanical system regardless of torque or speed level,
provided these quantities are within its capability.
The power processing capability of power inverters is directly related to the dc
input voltage available. Higher voltage means more throughput power for the same
gauge wiring and semiconductor die area. Figure 4.20 captures the power
throughput versus voltage given the system constraint on current of 250 A due in
part to cable size, connector sizes and contactor requirements. The reader will
appreciate that practical contactors rated in excess of 250 A
dc
interruption cap-
ability are far too bulky and expensive for hybrid vehicle applications. In the case
of BEVs, contactors using high energy permanent magnet arc suppression are used
effectively to 500 A
dc
.
In Figure 4.20(a) notice that as automotive voltages move to 42 V PowerNet,
the sustainable power levels approach 10 kW. For hybrid propulsion the chart
illustrates the recommendation that voltages in excess of 150 V are advisable. With
recent advances in power semiconductors there is ample reason to move to voltages
beyond 300 V, provided ESS does not suffer and complexity is manageable.
