Optimizing Performance in Switched Reluctance Drives
J. Reinert
R. Inderka
M. Menne
R.W. De Doncker
Institute for Power Electronics and Electrical Drives
Aachen University of Technology, Jagerstr. 17-19,52066 Aachen, Germany
www.rwth-aachen.de/isea
Abstract - It is demonstraited how the overall performance
of a switched reluctance drive can be improved by using
different control strategies for different regions of the
torque-speed diagram. For a 4-phase 30 kW machine these
strategies are fitted by connecting different optimizing
routines with a simulation program. The importance of a
correct tuning of the control parameters like turn-on angle,
turn-off angle, reference current and size of the hysteresis
band is demonstrated for a current controlled drive.
Furthermore, the test bench and the measurement
procedure for the experimental verification and comparison
to the simulation are presented.
I. INTRODUCTION
Due to their rugged brushless design switched reluctance (SR)
drives combine a high reliability and an outstanding performance
over a wide speed range. The SR-machine has been built for
drives ranging from a few watts up to 300 kW. Taking into
account some detailed design and control considerations, the SR
machine is ideally suited fix high performance applications at
low costs.
The performance of an elec.trica1drive can be characterized by
efficiency, torque, torque-ripple, noise, power-weight ratio and
power density. The importance of each of these criteria is
weighted according to the drive application. In addition, having
a specific application, the weight of each criterion can change
over the working area of the drive. An optimized performance
of a drive can only be reached with a correct balance of the
criteria, because the improvement of one criterion will most
surely worsen another one.
Analyzing the losses in SR machines is complicated, because of
the non-sinusoidal waveforms of the flux-linkage as well as
current and voltage. A low volt-ampere requirement can only
be reached if the iron is subjected to substantial saturation.
Therefore, the core is expo!jed to high average remagnetization
velocities and extreme non-linearities.
The optimization of the performance of the drive is based on
the calculation of the losses in the machine and converter. In
optimizing a certain performance criterion, i.e. torque ripple or
efficiency, it is absolutely necessary to consider the machine
design, the controller abilities and switching strategies together.
A low torque ripple for example can be obtained by either
0-7803-4340-9/98/$10.00
(3 1998 IEEE.
implementing current profiling [ 11 or by adapting the design of
the laminations [2], but even better, by a combination of both.
In addition, it is advantageous to have a flexible controller
structure as different control strategies can be implemented to
optimize further drive performance. With different values of
the control parameters turn-on angle, turn-off angle, reference
current and size of the hysteresis band
ireDAi), it is
possible to obtain the same operation point (torque-speed)
thereby influencing the performance criteria, such as efficiency,
torque ripple, noise, etc.
In this paper, a prototype drive (4 phase, 8/6 configuration) for
an electric vehicle (EV) is presented, where the efficiency is the
main performance criterion. However, the torque ripple,
particularly at low speeds, is also important. Attention is drawn
to the implementation of different controller strategies and their
effect on the drive performance, while having a fixed hardware
design of machine and controller. During the design stage,
however, the interaction of machine geometry and controller
structure was considered.
Section I1 of this paper treats the requirements to EV-traction
drives and the consequences to the control strategies of a SRdrive. The selection of these control strategies for different
operating regions in the torque-speed diagram is also discussed.
To test different control strategies a specialized simulation
program was constructed which is outlined in section 111. The
simulation results and the implementation of the optimizing
strategies are discussed in section IV. The measurement
methods and the test bench to verify the simulation results are
presented in section V of the paper.
11. DESCRIPTION
OF CONTROL STRATEGIES
In many applications, the drive is operated over the entire
operating area as, for example, in EV-traction drives. The
converter and the machine are subjected to a wide range of
requirements, with the associated efficiencies for each working
point. For this application, the energy consumption of a specific
driving cycle has to be minimized and it is therefore necessary
to implement control strategies with which the optimal drive
efficiency qd is reached for each working point.
Although the efficiency is a dominant criterion for an EV-drive
over the entire operating region, other drive performance
criteria, as for example the torque ripple factor,
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have a larger weight in some of the operating areas. Evaluating
the weight of different criteria leads to distinguished areas over
the torque speed range, as shown in Fig. 1 (here only the first
quadrant is shown). The boundaries between these areas are
obviously not as clear as in the figure, but can be identified
satisfactorily.
\
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80
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Rotor angle (deg)
40
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20
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Rotor anglc (dcg)
40
50
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Fig. 2:
Fig. 1:
Identified areas of different control strategies
Different switching strategies
As mentioned before, it is compulsory in the design of high
performance SR drives to consider the machine design
(lamination geometry and thickness, air gap, phase- and pole
number), the power converter (topology, semiconductor
devices) and the flexibility of the controller (microcontroller or
DSP) together. For the EV-application described here it was
found, that a good torque ripple factor at satisfactory efficiency
can best be achieved with a four phase machine. Once having
designed the machine and converter and having obtained the
drive criteria arrangement, the switching strategies of the
controller guarantee the highest degree of freedom to optimize
performance.
In area1 a low torque ripple is required, to prevent speed
oscillations during low-speed operation. A low torque ripple,
while using current controlled operation, can be achieved by
additional profiling of the phase current waveforms to obtain a
smooth total instantaneous torque as seen in Fig. 2. This current
profiling can reduce the efficiency considerably depending on
the number of poles and phases of the SRM. Therefore, it is
important to adapt the weights of the performance criteria so as
to optimize the efficiency, while keeping the ripple below a
predefined percentage.
Example of operation point with torque ripple factor
kpipp=28.5 %, N = 500 rpm; T = 68.3 Nm
The difference between I1 and I11 is that the drive operates with
a conventional current fed chopping mode in area I1 while it is
in single-pulse mode in area 111. Therefore, in region I1 the
control parameters, i.e. turn-on (O,,) and turn-off (e,,) angles,
reference current iref and size of hysteresis band Ai can be set
for each working point in the torque-speed diagram to obtain
the maximum efficiency. In area111 the performance can be
optimized only by changing e,, and e,,
At higher speeds and lower loads as in area IV a technique has
been found to minimize motor losses. It corresponds to a region
where the material starts to saturate. By switching only every
second phase of the machine these active phases are carrying
higher currents, thereby driving the material deeper into
saturation, as seen in Fig. 3.
0.2:
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In contrast to area I, the importance of a low torque ripple
factor in area11 is reduced. At higher speeds, a high torque
ripple does not lead to noticeable speed oscillations, due to the
kinetic energy in the system, which is proportional to the
square of the speed. In the regions 11, I11 and IV the efficiency
is the key criterion for the drive performance.
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Fig. 3:
40
50
Current (A)
4-phase versus 2-phase operation at N
and T,,, = 17 Nm
= 4300
rpm
After the saturation poinit the iron losses of one excitation do
not increase significantly. However, due to the fact that only
every second phase is activated, the overall iron losses are
reduced considerably. The overall copper losses increase
slightly due to their squared dependency on current. Simulation
and measurement have shown, that the total losses in the
machine for this region ciln be reduced by up to 4%. Since the
drive of an EV is regularly being driven in this region, an
increased drive efficiency lowers the energy demand notably.
The high torque ripple piroduced during this operation can be
tolerated because at these speeds the inertia of the mechanical
system cancels out unwainted speed oscillations. Fig. 3 shows
an operating point of 4700 rpm for the operation with four
phases (smaller loop) and with two phases active (bigger loop).
The torque produced with one phase is doubled by going from
the 4-phase to the 2-phase operation. No chopping occurs in the
2-phase operation, due to the fact that the induced voltage
equals roughly the supply voltage minus the resistive drop over
the phase windings in lhis operating point. This mode of
operation is not possible at high speeds, see area IV in Fig. 1,
because the induced voltage is to high.
Iv. IMPLEMENTATION OF OPTIMIZING STRATEGIES AND
SIMULATION RESULTS
To reach an optimal drive performance over the entire torquespeed range as discussed in section 11, the control parameters
can be precalculated with a simulation program or,
alternatively, adjusted on-line as done in [4]. In this section a
method is outlined which optimizes the control parameters in
advance by simulation. The optimized values can then be
stored as look-up-tables in the microcontroller unit. For the
generator operation the values of the control parameters have to
be optimized in a similar way.
In optimizing the parameters for the different areas two
strategies can be defined. One strategy for area1 of Fig. 1
where the torque ripple factor has to be minimized and the
other for the areas 11, 111, IV where the drive efficiency is
maximized.
111. SIMULATION
The use of an accurate sirnulation program in the design of SR
drives is indispensable. The simulation as well as the
optimization method is implemented in MatlabTM.With this
custom tailored program it is possible to fit the structure and
program features to specific needs. As basis of the simulation
program the magnetic properties of the machine are used.
These can be obtained by a finite element analysis or by
measurements on existing machines. The simulation does not
only calculate the machine behavior, it also considers the
different loss mechanisms in a SR drive. For the calculation of
the iron losses a novel method has been implemented [3] using
the average remagnetization velocity. The losses in the power
devices are calculated b s e d on measurements of 600Vl600A
IGBT devices used in the converter. Information from the data
sheets was not used as this tends to be somewhat optimistic
with respect to conduction losses. The calculated losses
occurring in the machine and the converter are used in the
optimization routine. With this procedure a fast calculation for
different operating situal ions is possible. These simulation
techniques offer a considerable time reduction compared to a
pure experimental method, because the measurements need to
be carried out in fewer predefined operating points. The drive
parameters for an optimiil machine behavior can readily be
determined. Changes in the controller hardware can be
evaluated and considered beforehand. The different switching
strategies are simulated and analyzed, which can be obtained
experimentally only with c;onsiderable effort.
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Rotor angle (de@
Fig. 4:
Torque production capability for three phases.
Contours of constant current are shown
In Fig. 4 the torque production capability of the drive is
depicted for 3 phases with 3 different constant current values
each. In a 816 configuration, 24 excitation-pulses per
mechanical revolution are needed (4 phases times 6 rotor poles)
and thus each pulse should contribute to torque over 15". The
rotor and stator poles have pole arcs of 20" each. However, due
to the leakage flux a positive torque can be developed over 30'
with one phase. Therefore two phases could always contribute
to the average torque. It can be seen in Fig. 4 that at 0" phase 1
is in the aligned position (not able to produce a positive torque)
while the phase 2 already has a high torque production
capability.
Minimizing the torque ripple the currents in the two phases
during a commutation process are calculated such that the best
overall efficiency is obtained. By consecutively adjusting a pair
of currents the sum of the torque produced with each phase is
held equal to the average torque. This simulation is repeated for
each rotor positions. To reduce calculation time the current in
the phase with the higher torque production capability is always
increased first while the other current is decreased. At the same
time, the rate of change of current which is possible at the
corresponding speed and rotor position is considered in the
calculation.
the energy ratio [5] is maximized. The higher copper losses,
caused by the very high current in comparison to the two other
waveforms, are more than compensated. The torque ripple is of
course much worse than that of the other curves, but this
performance criterion has no weight in these regions. The low
tum-on angle and the restricted current maximum of curve B
are the reasons for the poor efficiency, because a lot of reactive
power is needed between the angle of 4" and 9". The value of
the tum-on angle at waveform C is too high to build up the
current to its maximum level when the rotor reaches the
position with the highest torque-current ratio. Nevertheless, the
efficiency of control method C is still higher than with
control B because the reactive power is much smaller.
In Fig. 2 an example is given. It can be seen that the current
increases at the end of the conduction phase, thereby exploiting
the superior torque production capability of this phase
compared to the following phase for the corresponding rotor
positions. Here a specified torque ripple was tolerated in order
to keep an acceptable efficiency.
For the remaining areas of Fig. 1 the key performance criteria
is efficiency. Finding the best value for the control parameters,
provided speed and voltage are constant, is difficult because it
possible to attain the same working point with different control
parameters. For example to obtain the working point 13.5 Nm
4000 rpm three different simulated current waveforms are
shown in Fig. 5.
This example shows the procedure for optimizing one
performance criterion. Together there are four control
parameters to be adapted, all having an influence on the drive
behavior. A consideration of all the possibilities to change the
drive performance leads to high calculation times. To reduce
the calculation time it is advantageous to use a gradient-type
algorithm. All gradient-type algorithms require suitable starting
values for the control parameters. To obtain good starting
values a prestudy of the influence of the parameters on the
performance criteria must be made. The influence of the tumon and tum-off angles on drive efficiency and torque is
presented in Fig. 6 and 7.
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Fig. 5 :
Type
A
B
C
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Rotor angle (deg)
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In these two figures all control parameters (voltage, reference
current, hysteresis band) and speed are constant. By changing
the turn-on and turn-off angle a maximum of efficiency can be
found (eon= 6"; eOff
= 21") in Fig. 6 . In comparison to Fig. 7 it
is obvious that the torque is increasing by shifting the tum-on
angle to lower rotor angles than 6 degrees. Consequently, a
larger angle produces a higher average torque with worse ratio
of torque to current. At lower rotor angles a lot of reactive
power is needed leading to a lower efficiency.
60
Three currents and resulting torques producing the
same average torque of 13.5 Nm at N = 4000 rpm
eon
eoff
irefinax
3
4
18.5
23
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35
22
123
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irefmin
Taw
13.4
13.5
113.6
qd
93.4
88.8
190.1
~~
~~~~
~
Table 1: Control parameters and corresponding efficiency for
the current waveforms of Fig. 5
In Table 1, the control parameters e,,,
irefmax,
irehln and the
two drive criteria efficiency and average torque are presented.
Of all three waveforms shown the best efficiency is reached
with the parameters of curve A. Indeed, with these parameters
Fig. 6:
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Efficiency versus e,,, eOff
at 3000 rpm. Other control
parameters are constant
L"
10
Fig. 7:
"
Torque versus turn-on and turn-off angle at
3000 rpm. Other control parameters are constant
Fig. 9:
To obtain a higher average torque it is advantageous to increase
the reference current as long as possible and if the maximum
current (mostly restricted by the converter or by the maximum
winding temperature) i5i reached the turn-on angle must be
reduced.
However, as shown before, another reference current or
hysteresis band with other switching angles may lead to higher
efficiency at the same torque. Hence a consideration of all four
parameters is necessary for the current feed operation mode.
This typical trade-off between efficiency and torque is also
shown in Fig. 8 and 9 where the reference current and the turnon angle are the two changeable parameters while keeping the
other parameters constant. It can be seen that at low currents the
turn-on angle has no big influence on the torque or efficiency.
This changes considerably at higher currents, where a precise
turn-on angle is required.
By a gradient-type algorithm the performance criteria in each
defined area (Fig. 1) are optimized. The simulation results are
verified by experiments. These are carried out on a test bench
described in the next section.
V. TESTBENCH
To verify the simulation results and to optimize the control
strategy, the prototype 4-phase machine (8/6) with a nominal
power of 30 kW is installed on a test bench [6] as shown in
Fig. 10.
Setting the speed of the load machine and the torque of the
SRmachine via a master computer any point in the operating
region can be reached. By means of torque- and speed
measurements the mechanical output power is obtained. The
input power delivered to the entire drive as well as the motor
are measured via a high precision power meter. Therefore, the
efficiency of the machine, the converter and the overall
efficiency can be determined and compared to the simulation
results.
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Fig. 8:
Torque versus current reference and turn-on angle at
3000 rpm. Other control parameters are constant
Efficiency versus current reference and turn-on angle
at 3000 rpm. Other control parameters are constant
An oscilloscope with 2,5 GSampledsec connected to the
evaluation unit is also recording the torque of the drive versus
time. This measured value can be compared with the simulation
results. The torque-meter has a bandwidth of up to 1500 Hz.
Hence, the torque-ripple of frequencies up to about 500 Hz in
the low speed region can be measured.
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REFERENCES
N'
oscilloscope
4200rpm
550 Nm
evaluation
100 kW
L
I
1
M. Alakiila, L. Sjoberg, P. Johansson: A 40 kW Switched
Reluctance Engine StartedGenerator System f o r an Electric
Hybrid Vehicle, EPE' 97, Vol. 4, pp. 717 - 720.
300V DC
300A
battery current
battery voltage
master comwler
Fig. 10:
''
I
121
T. Schencke: Drehmomentenglattung von geschalteten
Reluktanzmotoren durch eine angepaJ3te Blechschnittgestaltung, Dissertation TU Ilmenau, February 1997.
[31
J. Reinert, R. Inderka, R. W. De Doncker: A Novel Method for
the Prediction of Losses in Switched Reluctance Machines,
EPE '97, Vol. 3, pp. 608 - 612.
[41
Kjaer P.C., Nielsen P., hdersen L., Blaabjerg F.: A new
Energy Optimizing Control Strategy for Switched Reluctance Motors, Proc. of APEC '94, pp. 48-55, 1994.
Test bench for measurements
The switching strategies influence the efficiency of both the
machine and the converter and also the torque-ripple. Each of
these influences can be detected by the measurements described
above. Different switching strategies can therefore be tested in
the whole operating region of the drive. The best switching
strategy can be chosen and the area of its implementation
defined.
[51
T. J. E. Miller: Converter Volt-Ampere Requirements of the
Switched Reluctance Motor Drive, IEEE Transactions on
Industry Applications, Vol. U-21, No. 5, pp. 1136-1144,
[61
P. Mauracher: System-Optimization of the Drive Train of
Electric Vehicles to Reduce the Energy Consumption, EVS 13, Vol. 1, pp. 70 - 77, 1996.
1985.
Furthermore, the control parameters of the SR drive can be
changed in each operating point. This tuning is implemented
on-line, while the test bench is running, by means of the user
interface of the SR drive's control unit. Thereby, the optimal
control parameters obtained by simulation can be verified.
VI. CONCLUSION
In this paper a technique to optimize performance of switched
reluctance drives is presented. The strategy is implemented on a
4-phase 30 kW drive for electric vehicles. The operating area of
the drive is divided into four regions, where different control
strategies have to be adapted in order to maximize a chosen
performance criterion. It is shown how these criteria can be
influenced by adapting the control parameters e,,, 8,ff, iref,Ai.
In helping to find the best possible combination of control
parameters for each working point a optimization program was
developed and successfully implemented.
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