International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 7 (2017) pp. 1371-1376
© Research India Publications. http://www.ripublication.com
Cogging Torque Reduction of IPM Motor using Skewing, Notching, Pole
Pairing and Rotor Pole Axial Pairing.
Fatihah Shafiqah Bahrim1,*, E. Sulaiman1, Laili Iwani Jusoh1, M. Fairoz Omar1 and Rajesh Kumar1
Dept. of Electrical Power Engineering, Faculty of Electrical and Electronic Engineering ,
Universiti Tun Hussein Onn Malaysia (UTHM), P.O Box.86400 Parit Raja, Batu Pahat, Malaysia.
1
Abstract
Interior Permanent Magnet (IPM) motor has been widely used
in a range of industrial application due to high power density
and good flux-weakening capability. In many of the
applications, the high strength of permanent magnets causes the
undesirable effects of high cogging torque that can aggravate
performance of the motor. Large cogging torque is significantly
produced by IPM motor due to the similar length and the
effectiveness of the magnetic air-gap. Therefore, the reduction
of cogging torque becomes an increasingly vital subject in IPM
motor. In this paper, to minimize the effect of cogging torque in
IPM motor, four common techniques for cogging torque
reduction such as skewing, notching, pole pairing and rotor
axial pole pairing has been analyzed in order to compare the
cogging torque effect. 3D finite element analysis (FEA) by
JMAG software is carried out for each technique. The results
showed the reduction of cogging torque up to 99%, followed by
81.90%, 43.2% and 39.01% compared to the initial model
design.
Keywords: Interior Permanent Magnet motor (IPM);
Permanent magnet (PM); cogging torque reduction; cogging
torque
INTRODUCTION
Permanent Magnet (PM) motors have been an invaluable part
of the industry today as these motors can save nearly any
industry a great deal of time and money under the right
circumstance. In various PM motors, for an interior-type PM
(IPM) motor used for variable-speed drives, a separation of PMs
caused by the centrifugal force at high speed can be avoided
since the PM is inserted into the rotor core [1]. Magnets for IPM
motors are inserted inside the rotor rather than bounding the
surface that leads to the advantages of robust design and high
speed operation. The advantages of IPM motor over Surface
mounted Permanent Magnet motors (SPM) is some of their
inherent characteristics which IPMs combines the reluctance
torque that is generated by rotor saliency and the magnetic
torque due to the PM. Therefore, they have the benefits of high
torque density, high efficiency, and their compactness, etc [2].
However, fluctuations of torque, which are the essential source
of variation, noise and the fluctuations of speed in IPM motors
are large. These speed variations can be divided into two factors
which are torque ripple and cogging torque. The main reasons
for torque ripple is the fluctuations of the field distribution and
the armature MMF and the major sources are cogging, PWM
current harmonics, non- ideal back-EMF waveforms, and
converted related issues. Torque ripple is usually filtered out by
the system inertia at high-speed levels, but at low speeds, it may
result in undesirable speed variations, vibrations and acoustic
noise that way effect the significant performance of the machine
[3]. The minimization of cogging torque become an
increasingly critical issue as it may cause vibration and acoustic
noises. Cogging torque of the IPM motor arises from
interactions between PM on the rotor with the stator slot
opening even without powering the motor. Cogging torque or
also called detent torque and ‘no-current’ torque is one of the
inherent characteristics of PM motors. These cogging torque
acts as a disturbance, superimposing over electromagnetic
torque produced during machine operation even produces zero
networks. Number of cycles for cogging torque can be derived
from:
Np = Nr/HCF [Nr, Ns]
Ne = Np Ns/Nr
Ne is referring to the number of cogging torque cycles, Np is a
constant, Nr is the number of rotor poles and HCF is the highest
common factor. The resulting cogging torque is the sum of the
elementary torque produced by the interaction between each
magnet and the edge of the slot opening. Therefore, the low
value of Np leads to high cogging torque while higher values of
Np lead to low cogging torque. For cogging torque reduction in
PM machine, numerous methods are proposed and investigated,
such as skewing the slots/poles [5]–[7] or the magnets [8],
shifting the magnet [9], optimizing the pole arc coefficient [10],
[11], slot openings [12] and the combination of the number of
slots and poles], teeth paring , and teeth notching [13], [14], and
so on. Variety of design factors and methods also influence the
effect of cogging torque and among the factors, the length of the
air gap, magnetic pole pitch and slot opening plays an important
rule. Technique for reducing cogging torque plays an important
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 7 (2017) pp. 1371-1376
© Research India Publications. http://www.ripublication.com
Table 1: Design Parameter for basic motor design of 6S-4P
IPM motor
role in the motor design and numerous studies have been carried
out on the prediction and reduction of cogging torque. Hansel
man D. C. have presented the effect of slot count, pole count
and skew on Brushless DC (BLDC) motor for cogging torque,
radial force and back EMF in year 1997 while in 2004, the
techniques for designing the magnetic poles design, dummy
slots, skewing for reducing cogging torque has been verified by
Mohammad Islam et al. Then, the next 3 years, Fei. W. and Luk.
P.C.K have designed the axial magnet pole pairing method
aiming the cogging torque reduction. Based on these previous
study, the cogging torque of IPM motor can be reduce since the
IPM motors produces high cogging torque value. In this paper,
several cogging torque technique in IPM motors are carried out
using 3d FEA.
6S-4P IPM
Motor
Parameters
REDUCTION METHOD FOR COGGING TORQUE
Several techniques used in this paper to minimize the cogging
torque effect in IPM are rotor skewing, rotor pole notching,
rotor pole pairing and rotor pole axial pairing. The performance
of all IPM motors is compared by a conventional spoke-type
6S-4P IPM motor which is the basic model as shown in Fig.1.
A six slot stator with concentrated windings is shown in Fig.1
(a) and circumferentially magnetized NdFeB magnets in a
spoke-type array is inserted in the rotor as in Fig. 1(b).
Stator pole/ slot numbers
6
Rotor pole numbers
4
Outer radius of stator (mm)
44
Stator pole shoes width (mm)
2.1
Air gap length (mm)
0.5
Rotor pole inner arc length (mm)
15.9
Rotor pole outer arc length (mm)
32.9
Outer Radius of rotor (mm)
24.6
Shaft radius (mm)
12.6
Inner radius of stator (mm)
25.1
Permanent magnet width (mm)
4.2
Stator tooth width (mm)
9.43
Speed (rpm)
4800
Table 2: Design Parameter for skewing, Notching, Pole
Pairing and rotor pole axial pairing of 6S-4P IPM motor.
Parts
(a)
(b)
Figure 1: Basic model a) Stator with coil winding
b) rotor with magnet
The motor specification for the initial motor design model are
listed in Table 1 and Table 2 shows the specification of each
motor design techniques for cogging torque reduction. To
examine and quantify the 3-D effects exhibited in the machine
under consideration, all the 3-D design models were created
using a commercial FEA package. The JMAG Designer version
14.0, of commercial FEA package released by the Japan
Research Institute (JRI) is used as 3D-FEA solver for this
design. There are two steps in designing which is geometry
editor and JMAG designer. The circumferential magnetized
NdFeb magnet in a spoke-type array is inserted in the Nippon
Steel 35H250 type rotor.
6S-4P IPM Motor
Materials
Conditions
Rotor
Nippon Steel
35H250
Motion: rotation
Torque: nodal
force
Stator
Nippon Steel
35H250
—
Armature
Coil
Conductor Copper
FEM Coil
Permanent
Magnet
Neomax-35AH
(irreversible)
Motion: rotation
Torque: nodal
force
Materials and condition for all motor models are as shown in
Table 3. Rotation motion, torque and arrangement of FEM coil
is set under condition setting. Arrangement of the FEM coil are
linked with a three phase circuit (Fig 5) and a rated speed of
4800rpm is used for magnet and rotor rotation in this paper.
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 7 (2017) pp. 1371-1376
© Research India Publications. http://www.ripublication.com
Table 3: Materials and conditions
Where n is the number of skewing steps, Q is the number of
stator slots, and Nc is the period of cogging torque as mentioned
in equation 2. In this paper, skewed model with an angle skew
of 2.8° as shown in Fig. 4(a) and Fig. 4(b) has been designed
and analyzed.
Rotor pole
inner arc
15.9
length (mm)
Rotor pole
outer arc
32.9
length (mm)
Permanent
magnet width 4.2
(mm)
Skewing
2.8
angle (Ɵ)
15.9
32.9
4.2
3.9, 25.8
10.2, 49.0
Rotor pole axial
pairing
Pole Pairing
Notching
Parameters
Skewing
6S-4P IPM Motor Design
10.0, 17.7
28.0, 33.0
(a)
4.2
2.0, 10.0
-
-
(b)
Figure 2: Skewing rotor model a) Isometric view
b) Rotor top view
B. Notching
COGGING TORQUE REDUCTION ROTOR DESIGN
IPM motor focuses on the variations of rotor, air gap length, and
stator parts in order to study its cogging torque reduction. The
cogging torque reduction techniques are normally chosen with
the conditions of easy implementation, minimum cost and
machine performances. This paper deals with the optimal shape
design of IPM motor and the design parameters are derived by
an analytical method and the rotor shape is optimized by using
FEA. There are four methods related to the permanent magnet
that can reduce the cogging torque, which is rotor skewing, rotor
notching, rotor pole pairing, and rotor pole axial pairing. These
optimal design approach will dramatically reduce the cogging
torque of IPM motor.
Notches are dummy slots in either stator or rotor of the machine.
This technique can be effectively applied to minimize the
cogging torque effect in IPM motor by decreasing the air-gap
flux. Air-gap flux is directly reduced as the magnet flux density
is lowered by changing the magnet grades. The number of the
notches in the rotor pole is chosen as 11, as shown in Fig.3 (a)
and 3(b). Rotor pole-notching will decrease the variation
amplitude and increase the variation periods, resulting in a
reduced peak value of the cogging torque. The notch depth and
the width could be carefully chosen as it influences the cogging
torque effect. The optimum width and depth values are chosen
for IPM motor are 0.5mm x 1mm and 2mm x 1mm.
A. Skewing
Skewing of either the rotor or the stator stack is a widely applied
method for reducing cogging torque and torque ripple in
permanent magnet motors. The basic idea behind skewing is to
influence the interaction between the stator slots and the rotor
magnets. This method is basically making the dR/dθ zero over
of each magnet face. In theory, the cogging torque can be
completely eliminated. In reality, it may not perfectly reach
zero, but be significantly reduced. Skewing can be implemented
on either the magnets or the slots. Both have disadvantages.
Skewing the magnets increases the magnet cost. Skewing the
slots increases the copper losses due to increased slot length,
resulting in the longer wire. The skewing angle for the skewed
model is between two adjacent steps which is equal to
Ɵskewing = 2π/nNcQ
[3]
(a)
(b)
Figure 3: Notching rotor model a) Isometric view b) Rotor
top view
C. Pole Pairing
Rotor pole pairing is a technieque of pairing the two different
size or width in the rotor design. The variable magnetic
resistance of air gap and rotor not only change the waveform
of cogging torque, but also reduce the amplitude of cogging
torque. To obtain smaller amplitude of the cogging torque, it is
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 7 (2017) pp. 1371-1376
© Research India Publications. http://www.ripublication.com
(a)
NO LOAD ANALYSIS FOR COGGING TORQUE
REDUCTION
Cogging torque characteristic effecting the torque pulsation
which corresponds to unacceptable vibration, acoustic noise,
poor position, performance degradation and running failure can
be determined at no load condition which there is no supply
from the stator. The cogging torque for 5 model design with an
optimized stack length of 54mm are shown in Fig. 6 below.
Cogging Torque (Nm)
also necessary to choose the proper width ratio of armature teeth
to magnet pole. The rotor with different pole widths assembled,
as illustrated in Fig. 4, can be used to lessen the cogging torque
effect. Width of these 2 poles could be optimized to minimize
the cogging torque by this scheme.
(b)
Figure 4: Pole Pairing rotor model a) Isometric view
b) Rotor top view
D. Rotor Pole Axial Pairing
Skewing
Axial Pairing
Notching
30 60 90 120 150 180 210 240 270 300 330 360
Mechanical angle(degree)
Figure 6: Cogging torque of Basic model, Skewed model,
Notching model, pole pairing model and axial pairing model.
Fig. 6 demonstrates the peak to peak cogging torque value
against mechanical angle. For the analysis result, cogging
torque has been reduced as a result of the approach rotor-PM
technique. Obviously, the cogging torque for the initial or basic
model is the highest compared to other model as the peak to
peak cogging torque value is 0.88Nm. Skewed model reduced
the cogging torque about 99(%), followed by rotor teeth axial
pairing 81.90(%), rotor pole notching 43.2(%), and rotor pole
pairing 39.06(%).
Flux Linkage (Wb)
Rotor pole axial pole pairing is another technique of polepairing by altering the stack length of design, proportional to
the magnitude of cogging. The phase of cogging torque can be
reversed as magnet pole arc changes, thus, total cogging torque
can be reduced by conjoining magnets having the same stack
length but different pole arc. Cogging torque is examined by
altering the value of permanent magnet width and pole arc.
Firstly stacks length is fixed and only pole arc is varied so that
cogging torque using axial pole arc pairing is examined as
results. An optimal stacks length pair of IPM obtained should
be carefully chosen so the cogging torque produced by axially
pole arc paired magnets can be reduced. The rotor pole axial
pairing model is as illustrated in Fig. 5(a) and Fig. 5(b) below.
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1 0
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
Basic
Pole Pairing
0.012
0.01
0.008
0.006
0.004
0.002
0
-0.002 0
-0.004
-0.006
-0.008
-0.01
-0.012
Basic
Skewing
Pole Pairing
Axial Pairing
Notching
30 60 90 120 150 180 210 240 270 300 330 360
Mechanical angle(degree)
(a)
Figure 7: Coil arrangement
(b)
Figure 5: Rotor Teeth Axial Pairing model
a) Isometric view b) Rotor top view
The coil arrangement test is used to set the armature coil
position and normally performed to validate the operation
principle of the 3-phase IPM motor. Therefore, each armature
coil is tested separately with the counter-clockwise direction of
winding. Every different coil is compared to define the armature
coil phases according to the conventional 3-phase system, the
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 7 (2017) pp. 1371-1376
© Research India Publications. http://www.ripublication.com
U, V and W phases. Fig. 7 illustrates the 5 model U-phase
sinusoidal waveform of torque versus mechanical degree.
Model basic and pole pairing are expected with the highest flux
amplitude of approximately 0.011Wb followed by rotor pole
axial pairing (0.0085 Wb), pole notching (0.004), and PM
skewing (0.003 Wb). Skewed rotor-PM technique almost
producing zero cogging torque or an optimal cogging torque
and also improved the back EMF waveform as illustrated in Fig
8 but the flux linkage produces by skewing technique is low and
thus increasing the leakage inductance and stray losses.
Basic
Pole Pairing
Skewing
Axial pairing
Notching
80
60
Back EMF (V)
40
20
0
-20 0
30 60 90 120 150 180 210 240 270 300 330 360
-40
low losses and the motor will offer a significant performance
and efficiency by eliminating a high vibration or noise.
CONCLUSION
Cogging torque has been a subject of recent investigations, in
the pursuit of methods of minimizing torque ripple in various
configurations of electrical machines. In this paper, the cogging
torque effect of spoke-type 6S-4P IPM motor design has been
investigated and discussed based on 3D FEA JMAG. Different
rotor-PM pole design techniques, which are rotor skewing, rotor
pole nothching, rotor pole pairing and rotor teeth axial pairing
techniques, has been analyzed to compare the cogging torque
effect and the best technique for cogging torque reduction
techniques for IPM motor is rotor pole axial pairing techniques.
With these excellent design approach has dramatically reduced
the cogging torque of IPM motor. Hence, further improvement
of cogging torque reduction technique in IPM motor for better
power and torque performance should be carry out in future
study.
-60
-80
Mechanical angle(degree)
ACKNOWLEDGMENT
This research was supported by FRGS (Vot 1508) by Universiti
Tun Hussein Onn Malaysia (UTHM) and Ministry of Higher
Education Malaysia (MOHE).
Figure 8: Back EMF phase comparison
Rotor pole pairing shows the best result among 4 techniques of
cogging torque reduction based on the optimal value of cogging
torque, flux linkage, back EMF and average torque as shown in
figure 9.The Back EMF for 5 models are not following the
requirement of sinusoidal shape, but can be improved by the
optimization method for the future study.
Basic
Pole Pairing
Skewing
Axial pairing
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