Environmental Engineering Reference
In-Depth Information
R
s
I
qsm
Stator resistive
drop
U
dc
V
sm
-
m
i
V
qsm
+
Σ
V
sm
-
V
ds
2
2
Back-emf
limiting value
sL
s
I
qsm
E
qs-limit
V
dsm
-
E
q
*
w
e
+
Σ
X
dE
q
N
w
e
D
Flux increment
dl
s
-
*
l
s
+
*
1/
L
s
I
ds
I
qs
K
*
D
T
e
*
N
Figure 7.14 Controller refinement for efficiency optimization and peak torque/
ampere tracking
variables. A flux programme is required in any M/G controller that enters field
weakening. In the case of IM and IPM machines, in particular, the flux programme
must hold machine flux constant during the voltage ramp up phase of control
(i.e. during constant torque operation), then decrease flux command inversely with
speed during field weakening operation. The flux command during field weakening
should also be adjusted so that efficiency is optimized as high flux levels in an M/G
at high speed tend to exact higher core losses.
To ensure that the M/G controller holds its commands for flux in a range
appropriate for the system limitations and speed regime, two signals are monitored:
M/G stator resistance
R
s
and the energy storage system dc link voltage
U
dc
. It has
been shown elsewhere in this text that mobile energy storage systems from lead-
acid batteries to fuel cells have a characteristic that the available dc link voltage
droops from its unloaded, open circuit value, to approximately 70% of this
value when loaded to peak power delivery. During regeneration the system voltage
increases by 30% or more. The resulting nearly 2:1 variation in dc link voltage
results is a wide variation in the maximum voltage that can be synthesized by
the power inverter and applied to the M/G stator. To accommodate this wide
variation in energy storage system voltage, the M/G controller responds by
adjusting the maximum command values for
d
- and
q
-axis voltages as shown in
Figure 7.14. The block,
m
i
, computes the maximum stator voltage based on the
sensed dc link voltage
U
dc
. For current regulated PWM (CRPWM) the useable
modulation index is 0.577 and for space vector PWM (SVPWM) it is 0.637 or 10%
higher because of better bus voltage utilization. The maximum torque command
generating voltage,
V
qsm
, is then calculated by subtracting the maximum bus vol-
tage deliverable voltage,
V
sm
, from the speed dependent flux generating voltage,
V
dsm
. The voltage command that results in machine flux is calculated as the product
