Environmental Engineering Reference
In-Depth Information
Table 5.1
Characteristics of power switching components
Transistor
type
Maximum
blocking voltage
(V)
Maximum
steady current
(A)
Maximum operating
frequency (kHz)
Easy of
paralleling
Cost
Thyristor
6000
4000
1
Not required
Low
GTO
6000
6000
1
Not required
High
Mosfet
1000
100
1000
Easy
High
IGBT
1200
400
100
Easy
High
Adapted from Ref.
[
12
]
torque by measuring stator currents and deducing the air gap flux from the motor
phase voltages and then controlling the whole electric drive in order to produce the
maximum load torque for the minimum value of current. Snubber circuits can be
connected across each power module to control the shape of the switching
waveform, they generally consist of a resistance-capacitor in parallel with the
power module and an inductance in series with the power module. A capacitor
bank is generally connected between the positive and negative input terminals of
the inverter to filter the DC input voltage and to reduce the ripple of the high
frequency currents generated during the PWM switching.
Before 1990s, thyristors and transistors were used as power modules for AC
drive systems. Since then other semiconductor switching power modules have
been used, such as GTO, MOSFETs, IGBTs, and combination of them. Nowadays
IGBT and MCT-based inverters are used by electric drives manufacturers for the
electric car market. The key characteristics of the different types of technology
used for power electronic component are reported in Table
5.1
. However, other
power switching modules are currently under development [
12
].
The power dissipated by the semiconductor components is considerable for an
electric road application and for that reason heat-sinks are essential. This means
that the voltage drops across the semiconductor component and this value of
voltage drop is an important characteristic to be considered in the selection of the
component for the specific application. Moreover, it should be taken into account
that in AC drives each current passes through two power components in series
rather than through a single component required in DC converters.
Since the same voltage drops across each electronic switch when it is con-
ducting, it means that the losses in AC inverters are significantly higher than those
in DC converters. Another consideration to be taken into account for the design of
an inverter for electric cars is the necessity of a cooling system. Natural air, forced
air or liquid cooling systems could be used, the dimensions of the heat-sink being
designed by the efficiency of each cooling method.
5.3 Storage Electrochemical Batteries
The term storages batteries or accumulators is used for devices able to
supply electric energy stored in active materials by means of electrochemical
