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In addition to two-wheelers, commercial vehicles with low speeds
(such as construction machines, industrial trucks, city buses), in
particular, could profit from a good cost/benefit ratio and package
advantages [Schäfer (2010)].
2.2.2.3 POWER ELECTRONICS
The power electronics module has the task of enabling the flow
of energy in the vehicle. In this regard, tasks include converting a
voltage to a different level (stepping a voltage up or down), inver-
ting a voltage (inverting AC voltage to DC voltage or vice versa)
or a combination of both. In addition to regulating the propulsion,
the power electronics is also used to provide the recuperation or
the charging process, as well as voltage conversion within the on-
board power supply. The power electronics thus are significant
components that influence the economy and efficiency of hybrid
and electric vehicles.
Although electric motors can be operated for a short time in the
overload range without overheating, the power electronics must
be configured to accommodate the maximum currents. This inevi-
tably results in increased cost and greater effort. In the design of
the power electronics, the costs, availability, performance, weight
and packaging of the system must be optimized as far as possib-
le, in order to achieve a result that is optimal for the respective
requirements.
In the hybrid or electric vehicle, three-phase propulsion systems
are used almost exclusively, as described in the preceding section.
For this, the electrical energy from the energy storage device must
be adapted to the requirements of the electric machine. Control
of the energy between battery and electric machine requires an
electronic adjustment device, the inverter. Here, the energy of the
battery (direct current) is converted for operation of the electric
motor (multi-phase AC voltage); activation of the motor requires
a variable AC voltage and this needs to have a variable frequency
for rotation speed adjustment.
The inverter consists of a power module (with high-power semi-
conductor circuits, e.g. IGBTs, SJ-MOSFET), capacitors, control
electronics (controller, software), power connections, cooling
and housing. The most frequently used circuit technology is the
B6 bridge circuit with 6 power switches. To smooth the input vol-
tage an intermediate circuit capacitor (usually a film capacitor) is
required. Due to the high voltage and required capacitance, the
capacitor can reach a considerable volume. Thus it has a signi-
ficant influence on the power density of the entire inverter. The
capacitor is also a critical component which, together with the
power module, crucially determines the reliability of the inverter.
Both components (power module and capacitor) are also respon-
sible for a major share of the cost of the inverter [Cebulski (2011)].
14 The authors’ own illustration based on Hoffmann (2010)
Fig. 13: Possible topology - power electronics with HV battery and LV on-board electrical system
14
Chapter 2