Environmental Engineering Reference
In-Depth Information
Equation (4.12) gives the fundamental ISA motor phase voltage for modulation
depth,
m
. Modulation depth will be in the range, 0
<
m
<
1:
m
p
V
d
sin
ðw
b
t
Þ
V
rms
Þ
V
ph1
¼
ð
4
:
12
Þ
where
V
d
is the dc link voltage. For a 42 V battery under load, (4.12) predicts a peak
phase voltage of 14.85 V. For the stated conditions, the ISA phase current will be
p
P
e
3
V
ph1
ð
A
rms
Þ
I
ph1
¼
ð
4
:
13
Þ
This calculates a 254 A
rms
phase current into the ISA for the case of 8 kW
power level in boosting at 2,400 rpm at the engine. For a 10-pole ISA, the funda-
mental frequency will be
f
= 200 Hz, as given by (4.14):
P
n
120
ð
Hz
Þ
f
¼
ð
4
:
14
Þ
For this example, ramp comparison (also, sine-triangle) modulation will be
used in the inverter controller to generate the inverter bridge switching waveforms.
Ramp comparison is a technique of encoding an analogue signal, in this case the
motor phase voltage at its base frequency of 200 Hz, into digital pulses that are
applied to the power semiconductor gates. Inverter current will then flow into or
out of the motor according to which switches in a 6-switch inverter are activated.
Figure 4.27 gives a schematic of the inverter switch arrangement, the controller and
load as well as the control signal generation.
The process of generating digital switch waveforms representing the magni-
tude of an analogue controlling signal is pulse width modulation. Figure 4.27
illustrates the case of modulation depth
m
= 0.80, showing how the switch con-
duction periods (state 1) versus its off periods (state 0) are defined.
In Figure 4.27, the corresponding phase A current is plotted over one cycle.
During the positive portion of
I
a
(
x
), the switch current is shown occurring for the
duration of the switch on time. The negative current in phase A represents diode
conduction. Capacitor ripple current is the summation of
I
a
(
x
)+
I
b
(
x
)+
I
c
(
x
) and
consists of pulses as shown in Figure 4.27(d).
4.4 Selecting the energy storage technology
The choice of ESS technology is interleaved with vehicle tractive effort for the
customer usage pattern anticipated. An example will help clarify the process. In this
example a 27-seat city bus is converted to a series hybrid by adding a generator to its
CNG fuelled ICE. The bus is assumed to have standing room for an additional 25 pas-
sengers. The bus has a total length of 12.5 m, height of 2.85 m and a width of 2.5 m, and
it weighs 17,500 kg with no passengers and a half tank of fuel. Loaded, and for a 34-66%
split front to rear, the resultant axle loads are 7,300 and 14,200 kg. Maximum speed is
