Environmental Engineering Reference
In-Depth Information
Variable speed drives are generally based on a set of an electric machine driven
by power electronic converters and controls, which are implemented in electronic
cards. The whole system makes possible the speed regulation of the electric
machine.
In Fig.
5.7
, the scheme of a generic DC electric drive supplied by a battery pack
is shown, and it also represents the traction drive for a BEV.
The DC-DC converter changes the electric characteristics of voltage and cur-
rent supplied by the electric source in order to feed the electric machine and satisfy
the load mechanic requirements.
AC electric drives require more sophisticated converters when they are supplied
with DC sources, because electric machines requires periodic voltage and current
waves with a variable frequency depending on the load requirements. In Fig.
5.8
,
the scheme of an example of three-phase induction motor driven by a pulse-width-
modulated inverter is reported. In this scheme a three-phase bridge connection
with six power modules is shown to form the so-called inverter. Each power
module can be composed by a number of power switches connected in parallel to
carry higher currents. Across each power switch (IGBT) a parallel diode is con-
nected to provide a return path for the phase current when the power module is
switched off.
For the design of an electric vehicle, it is important taking into account that the
use of high battery pack voltage affects the design of the power electronics. In fact,
high values of voltage mean low current and then reduced losses due to voltage
drops in the power electronics. However, the values of voltage are limited by
safety considerations.
The voltage waveform which feeds the motor is controlled by pulse-width
modulation (PWM) of each of the six power modules, by means of control signals
optimizing the motor phase currents. The PWM is controlled evaluating the motor
Fig. 5.7
DC electric drive of
Ib
Im
a BEV
DC-DC
CONVERTER
ELECTRIC
MOTOR
V
b
V
m
Fig. 5.8 Scheme of a three-
phase induction motor driven
by inverter
+
T
3
T
1
T
2
D
1
D
2
D
3
I
1
I
2
I
3
A
ELECTRIC
MOTOR
V
DC
C
B
C
T
5
T
6
T
4
-
D
5
D
6
D
4
