INTER-NOISE 2016
Performance Invariant Noise Reduction of a Plug-In Hybrid Electric
Drive Using an Innovative Skewing Concept
Andreas WANKE1; Gustavo DIENSTMANN2; Peter SPRINGMANN3; Dirk LIESKE4;
Martin DOPPELBAUER5; Frank GAUTERIN6
1,2,3,4
DAIMLER AG Stuttgart, Germany; 5,6 Karlsruhe Institutes of Technology, Germany
ABSTRACT
In this paper, a review of skewing options for noise reduction in permanent magnet synchronous machines is
given. The electromagnetic force excitation in electric machines can be reduced with a rotor skew for
particular orders. The skew leads to a global reduction of the radial force modes acting on an e-machine stator
creating a phase shift between the skewed segments. Even with perfect global force compensation, the forces
still act locally. This changes the purely radial excitation into a radial-axial mixed mode. The skewing pattern
defines the axial component of the excitation mode. The Eigen modes, frequencies and structural resistance
against the excitation depend on the axial component. Different layouts are investigated to find an optimized
design. Next to electromagnetic and structural simulations, in-car and component test bench measurements
prove of the mentioned effect. This way a performance and application neutral design was found that
significantly reduces the level of the main noise order.
Keywords: Electric motors, Automobiles
1. INTRODUCTION
In the automotive industry, the increasingly popular use of electrical traction drives demand
innovative concepts to provide the essential balance between of a good acoustic comfort, high power
density and space limitations demands innovative concepts. In purely electrical driving mode, no
combustion noise covers the noise of auxiliary systems, gears or the magnetic noise of the e-machine.
This phenomenon is investigated in this paper. Due to frequencies in the kHz range, the resultant
noises from these components can be annoying even at low sound levels inside and outside the vehicle.
Even if the feeding current of the three-phase AC permanent magnet synchronous machine
(PMSM) is purely sinusoidal, higher harmonics in the field caused by stator slot effects, winding
distribution, rectangular magnets, magnetic saturation and other effects occur (1), (2), (3). These
higher field harmonics cause higher harmonic force excitation in the air gap between stator and rotor.
The forces can be calculated in 2D electromagnetic FEM with COBHAM Opera using Maxwell’s
Equations (Maxwell Stress). These forces act on the stator teeth radially and tangentially and cause
structural vibration at the steel lamination of the stator core. This vibration is transmitted to the
housing and power train and causes both air borne and structure borne noise in the vehicle. (see Figure
1).
2
,
(1)
The calculation of the Maxwell Stress and forces is assumed as given for this paper. Equation (1) is
a simplified expression of the Maxwell Stress as a result of the radial and tangential flux density in the
air gap. The full expression can be found in (1). The focus of this article is on the mechanical excitation
1
2
3
4
5
6
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[email protected]
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of the structure in simulation and on the experimental verification. Depending on the electromagnetic
design of the machine, different radial modes can occur. The naming conventions can be seen in Figure
2(a). The ‘breathing’ mode zero (
0) is the dominant excitation since it has the longest wave length
(2) and thus a high radiation efficiency can be assumed for this mode. As mentioned above,
radial-axial mixed modes are relevant. Examples can be seen in Figure 2 (b, c).
Fig. 1 – Excitation principle and fundamental configuration of a PMSM
(a)
2 2 0 1 (b)
(c)
Fig. 2 – (a) Pure radial modes (b) Mode (2,0) from structural simulation in ANSYS 16.2,
(c) Mode (2,1) from structural simulation in ANSYS 16.2
To define these mixed modes, Blum (4) proposed an adapted naming convention, describing the
radial
and axial wave number . Blum (4) theoretically investigated the effects of different
stepped-skew rotor segment configurations. The conclusions and technical effects are already
described in (4). In parallel to Blum’s theoretical work, the authors of this paper approached the topic
from an experimentalist perspective and proved the effect of a noise improvement from a changed
segment order in simulation and hardware.
2. Effect of Skew
To reduce the excitation of particular orders, the rotor segments of a PMSM can be shifted by a
certain mechanical angle. Theoretically, a rotor skew could minimize a particular force harmonic to
zero. The ideal skewing angle
can simply be calculated for the order that shall be reduced. As
a logical result, whole multiple orders of will be ideally minimized as well. The other excited orders
are generally reduced but not minimized reduced. Assuming a discrete skew with a constant shift
between the segments, the nominal skewing angle
for
depends on the number of
segments
. A simple linear skew can be seen in figure 3. Skew patterns and optimized angles with
a not constant increment can be found in (5).
.
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360°
(2)
1 (3)
Fig. 3 – Definition of rotor skewing angle
The forces of the segments and the phase shift between them can be plotted in a polar diagram or as
a bar diagram for each tooth and order at particular working points.
100
X = 40
Y = 71.6
Nomalized tooth force in dB
80
X = 60
Y = 60.7
60
40
20
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Mechanical Order
(c)
(b)
(a)
Fig. 4 – (a) Radial force main order at 25 Nm, 400 rpm, (b) Tangential force main order at 25 Nm, 400 rpm,
(c) Normalized tooth forces all relevant orders at 100 Nm, 4000 rpm
The forces of an electrical machine under load do not completely reduce to zero as can be seen in
figure 4. Each axial segment can be seen as a separate machine with the same stator current but a
different phase between the stator and the rotor field (electrical load angle). So electromagnetically
each segment has a slightly different electrical working point with a different field in the air gap,
especially for high loads and in the field weakening domain for higher speeds.
Due to space and cost limitations, e-machines in the automotive industry must have a very high
torque and power density compared to industrial standard drives and tend to saturate
electromagnetically for high loads. This leads to additional field deformation and differences between
the higher harmonic forces of the segments. However the skew still reduces the total force for all the
segments globally over the whole length of the e-machine.
3. Different segment orders
As mentioned above, the skew reduces the harmonic force excitation globally. Locally, at each
segment, the full radial and tangential forces act for the particular radial excitation modes (Figure 2(a)).
This leads to an axial excitation force which is investigated in detail in this section.
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For simplification only the radial force vectors are plotted. The structural simulations are done in
NASTRAN with all relevant forces taken into consideration.
3.1 Initial configuration – linear skew
As mentioned above, the linear skew leads to a global reduction of the radial and tangential forces.
The tangential forces form a torque ripple, which should be ideally reduced for the main order.
1 2 3 4 5 6 7 8 9 10 11
(a)
(b)
(c)
Fig. 5 – (a) Linear skew segment configuration, (b) 3D structural with linear skew – stator assembly at
resonance, (c) Cross section of one tooth, linear skew with radial force vectors for a particular time
at resonance
In figure 5 (b, c) a clear mode (0,1) can be seen. So although the total radial and tangential forces
compensate to a high extent, a tilting movement occurs. In order to show this behaviour, measurements
with a Polytec laser scanning vibrometer were made on a component test bench (see figure 6). The test
bench housing is based on series configuration gearbox housing for this hybrid application. About one
fifth of the circumference was scanned above the stator assembly. The laser scanning measurements
show a clear axial wave with relatively high surface velocities reaching up to 1 mm/s.
Fig. 6 – Laser scanning vibrometer measurements at mode (0,1) resonance
3.2 Simulation of different segment orders
To suppress the axial wave, different segment orders were derived from the polar diagram (figure
4 (a, b)). The aim was, not to redesign the e-machine and its subcomponents, but only to change the
rotor assembly. Each of rotor segments has indicators defined by the stamping tool. The magnetization
process might be affected by the new segment order. For a linear skew, the rotor can be assembled with
passive magnets one skewed pole of the rotor can be magnetized with one coil. For the prototypes,
active magnets were inserted into the segments.
Changes were avoided in the stamping process, however theoretically a new segment order should
not affect the fundamental total flux of the rotor and thus the certified torque, power and the flux tables
for the control application.
When looking at the polar diagram (Figure 4), the force vector of segment 1 is almost opposite to
segment 7, segment 2 opposite to 8 etc. So a configuration having these opposite force vectors as direct
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neighbours should create a maximal local compensation of the forces. Such a configuration can be
seen in figure 7 and was given the name variant B for these investigations. Compared to figure 5 (c), a
reduced tilting movement can be seen in figure 7 (b).
1 7 2 8 3 9 4 10 5 11 6
(a)
(b)
Fig. 7 – (a) Variant B segment configuration, (b) Cross section of one tooth, variant B skew with radial force
vectors for a particular time at resonance
A V-shaped skew is a more conventional approach to shift the axial excitation to a higher mode.
The number of segments is uneven. An optimized V-skew approach for an uneven count of segments
can be found in (5). As mentioned above, the segments were supposed to remain the same compare to
the initial linear configuration. So an unsymmetrical V-skew was implemented, that can be seen in
figure 8. This order also significantly reduces the tilting movement of the teeth.
1 3 5 7 9 11 10 8 6 4 2
(a)
(b)
Fig. 8 – (a) Unsymmetrical V-skew segment configuration, (b) Cross section of one tooth, unsymmetrical
V-skew with radial force vectors for a particular time at resonance
The structural simulation was performed for a whole powertrain model, not only for the e-machine
parts or the hybrid module. The geometries of the gearbox housing and combustion engine were
simplified, rotating and oscillating parts were removed from the model.
(a)
(b)
Fig. 9 – (a) Structural geometry, (b) Integral surface velocity for different configurations
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The whole structure is only excited by the forces acting on the stator teeth. The solid lines in figure
9 (b) represent the whole structure, the dashed lines only the surface of the hybrid module. This hybrid
module is highlighted in purple on figure 9 (a). To verify the accuracy of the simulation, a calculation
for a previous hybrid project is plotted in black and can be neglected. Compared to the linear skew
machine, both variant B and the unsymmetrical V-skew show an improvement of 11-12 dB at the main
resonance around 3000 rpm. The V-skew has a slight advantage in simulation.
4. Measurements of the new variants
Both new variants were built up and tested on at an electromagnetic test bench. Variant B showed
a reduced induced voltage and magnetic total flux. The two dimensional approach in electromagnetic
simulation was shown not to be valid for big shift angles between neighbouring segments. Detailed
investigations with flux measurements and in electromagnetic 3D FEM have shown parasitic
transversal stray flux paths between the rotor segments. However these flux investigations are no
subject to this paper. Nevertheless variant B did not achieve the performance and objected application
invariant optimization and therefore was not continued.
The unsymmetrical V-shaped skew was electromagnetically almost identical to the initial linear
skew and was installed into a car and compared to linearly skewed sample.
Fig. 10 – Vib. measurements in the same car (new E Class W213) for comparable load points, quadratic
average of vector amount surface velocities of two reference triaxial accelerometers on the hybrid
module
In figure 10, a significantly reduced surface vibration on the hybrid module at the resonance point
can be seen. These in-car measurements performed on a test track verify the effect of a changed mode
shape.
5. CONCLUSIONS
These investigations in simulation and measurement have shown that the axial mode shape of a
mode 0 excitation is very relevant for the sound level at the main resonance of an e-machine.
Significant improvements can be achieved using a re-designed skew without reduction in
performance.
Although the rotor magnetization and assembly get more challenging, this improvement can be
achieved within an existing production line without the need to change stamping and machining tools.
No additional space or damping material is required. For future projects, a design rule should be to
take a V-skew whenever this is feasible.
For further validation, more statistically reliable measurements must be executed in different
vehicles and on different test benches. Based on these data, the simulation models can be refined, so
the prediction quality can be improved.
The electromagnetic effects of 3D flux paths of step skewed rotors must be investigated in future
work. A more detailed knowledge could allow more advanced skewing options, improvement of
performance and reduction of electromagnetic loss.
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ACKNOWLEDGEMENTS
A particular thank to Dr. E.P. Bowyer for her encouragement to publish this topic at Internoise 2016.
The authors also thank Mrs. Christina Schöll for her thoughts on skew and force wave compensation.
Special thanks to Mr. J. Fischer for his support with the laser scanning vibrometer measurements
and the colleagues at EM-motive GmbH (A Joint Company of Daimler and Bosch), Hildesheim,
Germany for providing the prototype hardware.
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machines on noise and vibration. Proc ELECTRIC DRIVES PRODUCTION CONFERENCE (EDPC)
4; Sept. 30 - Oct. 1 2014; Nuremberg
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DE102014017305 A1, 2015
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