Environmental Engineering Reference
In-Depth Information
Table 6.4 Comparison of PWM techniques
PWM type
Carrier
frequency
Comments
Carrier based
Suboscillation
method
Constant
f
c
Sine-triangle modulator in each machine
phase
m
max1
¼
p
4
¼
0
:
785
m
max1
þ
third
¼
p
¼
0
:
907
Modulation index of fundamental is
increased with addition of triple harmonic
6
p
Space vector
Constant
T
=½
f
s
Only adjacent vectors to the reference vector
are selected. Subcycle period is half of the
inverter switching frequency and modula-
tion depth is 0.907
Synchronized
carrier
N
p
=f
c
/f
Subject to current transients during
N
p
'gear-
shift' as fundamental frequency changes.
Performance may be inferior to sine-
triangle when the pulse number,
N
p
, is low
Carrierless methods Frequency
modulated
Form of modified SVPWM in which inverter
state ON time is modulated
Overmodulation
All
Inscribed circle is limit of full modulation,
m
0.785. With added third harmonic, the
space vector will extend closer to hexagon
vertices,
m
<
0.952, and space vector
velocity along hexagon sides increases to
infinity at six step square wave mode
<
Feedforward PWM
(offline PWM
techniques)
N
p
=f
c
/f
Switching angles are pre-calculated to achieve
desired performance effect of harmonic
elimination or harmonic minimization
Feedback PWM
Hysteresis
f
c
is unconstrained Hysteresis current regulators are typically
implemented one per phase in the stationary
reference frame. There is no linkage
between modulators leading to a tendency
to high frequency limit cycle behaviour and
potential lock-up
Predictive control
Eliminates current error in steady state by
machine-model-based feedback of machine
back-emf. Requires high speed DSP
Trajectory tracking
A look-up table method in which the machine
current vector trajectory is predicted based
on a mathematical model of the machine
and used to implement trajectory tracking
HEVs, the converter output will also be at 42 and 14 V to support conventional
loads and higher powered loads such as EPS for SUV that needs higher voltage.
Schupbach and Balda [13] make a comprehensive comparison of dc/dc con-
verter types suitable for HEVs. In their study, the authors put special emphasis on
