Comparison of PM Machines with Concentrated
Windings for Automotive Application
Sachar Spas, Gurakuq Dajaku, Dieter Gerling
Φ
Abstract – This paper presents a comparison of two inlet
permanent magnet synchronous machines with concentrated
windings. A novel 24-teeth/28-poles machine design with flux
barriers in the stator teeth is compared to a common 30teeth/20-poles design, which is widely used for hybrid electric
vehicle (HEV) applications. Detailed comparison of important
machine characteristics such as torque ripple, iron losses,
eddy current losses in the magnets, axial length and machine
weight is presented.
According to the study, the novel 24-teeth/28-poles machine
design offers a high saving potential concerning the space
requirement and production costs. At the same time this
machine topology generates a smoother torque compared to
the 30-teeth/20-poles topology.
poles topology using flux barriers in the stator teeth to
reduce undesired sub-harmonics (Fig. 1(b)). Both machines
are designed under the same geometrical and electrical
conditions and are analyzed with finite element method
(FEM). According to the study, the novel 24-teeth/28-poles
machine design offers a high saving potential regarding
machine length, weight and consequently space
requirement and costs.
Index Terms— efficiency, finite element method, hybrid
electric vehicle, magnetic flux barrier, MMF harmonics,
permanent magnet synchronous machines, tooth concentrated
winding
I.
INTRODUCTION
In the last years, the requirements for better
performances, safety, reliability and lower costs in
automobiles have driven the development of electrical
machines using new design techniques. Depending on the
application in automotive different requirements, such as
cost, limited space, efficiency, torque density, wide
operating speed range, have to be considered. Regarding
these requirements permanent magnet (PM) synchronous
machines turned out to be the best solution to satisfy the
demands. Moreover, using PM machines with tooth
concentrated windings instead of conventional distributed
windings implies advantages, such as short and less
complex end-winding, high filling factor, low cogging
torque, greater fault tolerance and low manufacturing costs.
However, the magnetic field produced by concentrated
windings has more space harmonics, including subharmonics. These unwanted harmonics cause negative
effects, such as localized core saturation, eddy current
losses in the magnets [1],[2], noise and vibration [3]-[5],
which are the main drawbacks of these windings. To
overcome these disadvantages several methods have been
developed and investigated in the last period of time [6][8].
This paper compares a widely used 30-teeth/20-poles
PM machine topology (Fig. 1(a)) with a novel 24-teeth/28-
Fig. 1 a): Conventional 30-teeth/20-poles PM machine
topology (A+, B+, C+)
Sachar Spas is with FEAAM GmbH, Werner-Heisenberg-Weg 39, D85579 Neubiberg, Germany (e-mail: Sachar.Spas@feaam.de)
Gurakuq Dajaku is with FEAAM GmbH, Werner-Heisenberg-Weg 39,
D-85579 Neubiberg, Germany (e-mail: Gurakuq.Dajaku@feaam.de)
978-1-4799-4775-1/14/$31.00
©2014 IEEE
1990
Dieter Gerling is Head of the Institute of Electrical Drives at the
University of Federal Defense Munich, Germany (e-mail:
dieter.gerling@unibw.de).
Fig 1 b): Novel 24-teeth/28-poles PM machine topology
(A+, C-, B+, A-, C+, B-)
II.
ANALYSIS OF MAGNETOMOTIVE FORCE DISTRIBUTIONS
As well-known, the main information about an electrical
machine can be deduced by analyzing the stator winding
magneto-motive force (MMF) and its space harmonics.
Important machine characteristics, such as air-gap flux
density, electromagnetic torque and torque ripple, magnetic
radial forces and so on are directly related to the stator
MMF characteristics. For the 30-teeth/20-poles (10 times
3-teeth/2-poles) winding topology the MMF distribution
and the corresponding MMF spectrum are shown in Fig. 2.
1.5
depending on the harmonic order ν, I is the phase current
amplitude, δ is the load angle, ω is the angular frequency
and w is the number of turns per phase.
Analyzing the MMF of the 24-teeth/28-poles (2 times
12-teeth/14-poles) winding topology without flux barriers
in the same manner, leads to following MMF distribution
(2):
m 2wξ
π
I cos ωt − ν x + δ 2
2 πν
τ
Θ x, t =
with
ξ = sin "ν
MMF [p.u.]
1
π
$ .
12
Corresponding MMF distribution and MMF spectrum
are shown in Fig. 3.
0.5
0
1.5
1
-0.5
0
1
2
3
4
Angle [rad]
5
6
Fig. 2 a): MMF distribution of the 30-teeth/20-poles winding
topology over the stator circumference
MMF [p.u.]
0.5
-1
0
-0.5
-1
1
-1.5
0
1
2
MMF [ p.u. ]
0.8
3
4
Angle [rad]
5
6
Fig. 3 a): MMF distribution of the 24-teeth/28-poles winding
topology over the stator circumference
0.6
0.4
1
0.2
0
0
2
4
6
8
10
12
Harmonic order
14
16
18
20
Fig. 2 b): MMF spectrum of the 30-teeth/20-poles winding
topology
Figure 2(b) illustrates that the space harmonics decrease
with increasing harmonic order. However, for a 20-poles
machine only the 1st space harmonic of the stator field
interacts with the rotor field, caused by permanent magnets,
to produce a continuous torque. In other words, with a
20-poles rotor the 1st stator field harmonic is chosen as the
working wave. The other space harmonics (2nd, 4th, 5th, 7th,
8th, etc.), that have relatively large amplitudes, are
undesirable and lead to above mentioned negative effects.
Using Fourier series the MMF distribution of 30-teeth/20poles winding is given by following equation:
MMF [ p.u. ]
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
12
Harmonic order
14
16
18
20
Fig. 3 b): MMF spectrum of the 24-teeth/28-poles winding
topology
Using this winding with a 28-poles rotor means that the
7th stator field harmonic is chosen as the working wave. The
other space harmonics affect the machine performance
negatively. According to Fig. 3 the 24-teeth stator without
flux barriers is not well suited for a 28-poles rotor, mainly
because of the high magnitudes of the 1st and the 5th MMF
harmonics. These sub-harmonics (compared to 7th) would
m 2wξ
π
Θ x, t =
I cos ωt − ν x + δ 1
produce high iron losses and eddy current losses in the
2 πν
τ
magnets, which would decrease the torque density and the
with
efficiency of the machine. But according to [9] the situation
π
ξ = sin "ν $ ,1991changes significantly applying flux barriers to the stator
3
teeth, as it is shown in Fig 1(b). Reference [9] describes and
investigates the influence of flux barriers on the resulting
where m is the number of phases, ξ is the winding factor
air-gap flux density. It is shown that the 1st and the 5th sub-
harmonics of the air-gap flux density are reduced by 73%
and by 19%, respectively. Furthermore, the 7th harmonic of
the air-gap flux density, which is the working harmonic, is
increased by about 16%, compare Fig. 4.
saves about 14,3% of active length compared with a
common used 30-teeth/20-poles design.
Table III
30/20-topology
300 A
270 Nm
1.69 kW
0.025 kW
0.33 kW
63 mm
@2500rpm
I&'(
M*
P,P'./
P01
Active length
Fig. 4: Comparison of the air-gap flux density spectrum of a
conventional stator (red) and a new stator with flux
barriers (black) [9]
III. COMPARISON OF SIMULATION RESULTS
Furthermore, as Fig. 5 illustrates, the electromagnetic
torque created by this innovative machine design is far
smoother than the one produced by the conventional
machine. The torque ripple of the novel design is about
2,33%, compared to 11,8% of the conventional one.
On the other hand, the losses of the 24-teeth/28-poles
machine tend to be higher than those of 30-teeth/20-poles
machine. Especially this holds for the iron core losses Pfe in
the stator and rotor. These increased losses arise from the
increased number of poles and the associated higher
frequency. However, if necessary, these losses can be
reduced by a different optimization target.
To compare the above mentioned machine designs both
PM machines have been designed and analyzed using FEM.
To ensure a fair comparison both machines are designed
under the same geometrical und electromechanical
boundary conditions, refer to Table I and Table II.
Electromagnetic Torque @ 2500rpm
Torque [Nm]
300
Table I: Geometrical Boundary Conditions
Inner rotor diameter
Outer rotor diameter
Outer stator diameter
Gap length
Maximum active length
200 mm
240 mm
300 mm
1 mm
63 mm
Irms = 300 A
nn = 2500 rpm
Mn = 270 Nm
Pn ≈ 71 kW
nmax = 6000 rpm
M(nmax) = 112.5 Nm
The comparison is done for the same output power
(mechanical power) at different rotational speeds. In either
case the effective value of the sinusoidal supply current is
fixed to Irms = 300A. In order to reduce the eddy current
losses in the magnets, like it is presented in [10], the
permanent magnets of both machines are considered to be
segmented eight times in axial direction.
250
200
M_24/28 @2500rpm
M_30/20 @2500rpm
150
0
1
2
3
4
5
6
Time [ms]
Fig. 5: Electromagnetic torque of investigated machines at
Irms =300 A and n = 2500 rpm
Table II: Electromechanical Boundary Conditions
Supply current
Nominal rotational speed
Nominal torque
Nominal Power
Maximum rotational speed
Torque @ nmax
24/28-topology
300 A
270 Nm
1.72 kW
0.075 kW
0.75 kW
54 mm
B.
Simulation results for operation at maximum speed
The same way the results for operation at maximum
speed are presented in Table IV and Fig. 6. As estimated
the iron core losses and the eddy current losses increase
with the speed. As Fig. 6 illustrates, the torque ripples at
this operating point are almost identical for both designs.
Nevertheless, it is important to keep in mind that torque
ripple is far less critical at high speed due to the damping
effect of the rotor mass inertia.
@6000rpm
I&'(
M
A. Simulation results for nominal operating point
P,Simulation results for the nominal operating speed (2500
P
rpm) under the same load condition (270 Nm) are presented1992
'./
in Table III. It can be clearly seen, that the 24-teeth/28P01
poles machine design satisfies the load requirement with a
Active length
significantly shorter active length. Using this topology
Table IV
30/20-topology
300 A
112.5 Nm
1.69 kW
0.2 kW
0.55 kW
63 mm
24/28-topology
300 A
112.5 Nm
1.72 kW
0.25 kW
1.8 kW
54 mm
V.
Electromagnetic Torque @ 6000rpm
Torque [Nm]
150
100
50
M_24/28 @6000rpm
M_30/20 @6000rpm
0
0
1
2
3
4
Time [ms]
5
6
Fig. 6: Electromagnetic torque of investigated machines at
Irms =300 A and n = 6000 rpm
C.
Comparison of material weights
The novel machine design provides a great advantage
concerning the material weights. This is caused by the
reduced axial length on the one hand side and the usage of
flux barriers in the stator on the other side. The research
shows that the total machine weight is reduced from 18.67
kg to 15.95 kg, refer to Table V. This means, that the
required amount of material for the motor production is
reduced, e.g. 14% less magnet material, which leads to a
significant decrease of production cost. Simultaneously, the
space requirement is reduced enormously due to the shorter
machine length and using flux barriers there are other novel
cooling methods applicable [11].
Table V: Material Weights
30/20-topology
Stator weight
(stack of sheets)
Rotor weight
(stack of sheets)
Copper weight
Magnet weight
Total weight
REFERENCES
[1]
M. Nakano, H.Kometani, "A study on eddy-current losses in rotors
of surface permanent magnet synchronous machines", IEEE
Transactions on Industry Application, vol. 42, No. 2, March/April
2006.
[2] N. Bianchi, E. Fornasiero, "Index of rotor losses in three-phase
fractional slot permanent magnet machines", Electric Power
Applications, IET, vol. 3, No. 5, September 2009.
[3] J. Wang, Zh.P. Xia, D. Howe, S. A. Long, “Vibration Characteristics
of Modular Permanent Magnet Brushless AC Machines”, IEEE IAS
Annual Meeting, 2006, Tampa, Florida, USA.
[4] M. Boesing, K. A. Kasper, R. W. Doncker, “Vibration Excitation in
an Electric Traction Motor for a Hybrid Electric Vehicle”, 37th
International Congress and Exposition on Noise Control
Engineering, Inter-Noise 2008, 26-29 October 2008, Shanghai,
China.
[5] Z. Q. Zhu, Z. P. Xia, L. J. Wu and G. W. Jewell, "Analytical
modelling and finite element computation of radial vibration force in
fractional slot permanent magnet brushless machines", IEEE
International Electric Machines and Drives Conference, IEMDC
2009, Florida, USA, May 3-6, 2009, pp. 157-164.
[6] G. Dajaku, D. Gerling, “A Novel 24-Slots/10-Poles Winding
Topology for Electric Machines”, International Electric Machines
and Drives Conference, IEMDC 2011, Niagara Falls (Ontario),
Canada, May 15-18, 2011, pp. 65-70.
[7] G. Dajaku, D. Gerling, “A Novel Tooth Concentrated Winding with
Low Space Harmonic Contents”, International Electric Machines
and Drives Conference, IEMDC 2013, Chicago, Illinois, USA, May
12-15, 2013
[8] G. Dajaku, D. Gerling, “A Novel 12-Teeth/10-Poles PM Machine
with Flux Barriers in Stator Yoke”, 20th International Conference on
Electrical Machines, ICEM 2012, Marseille, France, September 0205, 2013, pp. 36-40.
[9] G. Dajaku, D. Gerling, “Low Costs and High-Efficiency Electric
Machines”, 2nd International Electric Drives Production Conference,
EDPC 2012, Erlangen-Nuremberg, Germany, October 16-17, 2012.
[10] H. Toda, Z.P. Xia, J.B. Wang, K. Atallah and D. Howe, “Rotor eddycurrent loss in permanent magnet brushless machines”, IEEE
Transactions on Magnetics, vol. 40, no.4 , July 2004. pp. 2104-2106.
[11] A. Nollau, D. Gerling, “Novel Cooling Methods Using Flux
Barriers”, IEEE International Conference on Electrical Machines
ICEM 2014, Berlin, Germany, submitted paper.
24/28-topology
VI. BIOGRAPHIES
8.43 kg
6.76 kg
5.33 kg
4.56 kg
3.63 kg
1.28 kg
18.67 kg
3.53 kg
1.1 kg
15.95 kg
IV. CONCLUSIONS
Sachar Spas; M.Sc. Sachar Spas is with FEAAM GmbH, WernerHeisenberg-Weg 39, D-85579 Neubiberg, Germany (e-mail:
Sachar.Spas@feaam.de)
Sachar Spas was born in Lviv, Ukraine, in 1988. He got his M.Sc. degree
from the University of Federal Defense Munich, Germany in 2012. Since
2012 he is a Research Scientist with FEAAM GmbH.
Gurakuq Dajaku;Dr.-Ing. Gurakuq Dajaku is with FEAAM GmbH,
Werner-Heisenberg-Weg 39, D-85579 Neubiberg, Germany (e-mail:
Gurakuq.Dajaku@feaam.de)
Born in 1974, Dr. Dajaku got his diploma degree in Electrical Engineering
from the University of Pristima, Kosova, in 1997 and his Ph.D. degree
from the University of Federal Defense Munich, Germany, in 2006. Since
2007 he is Senior Scientist with FEAAM GmbH, an engineering company
in the field of electric drives. His research interest is in the field of
electrical machines and drives.
Dr. Dajaku received the Rheinmetall Foundation Award 2006 and the ITIS
(Institute for Technical Intelligent Systems) Research Award 2006.
The present study compares two inlet permanent magnet
synchronous machine designs for hybrid electric vehicle or
battery electric vehicle application. Under the same
geometrical and electrical boundary conditions the
conventional design and the novel design with flux barriers
Dieter Gerling; Prof. Dr.-Ing. Dieter Gerling is Head of the Institute of
in the stator teeth are considered and the obtained results
Electrical Drives at the University of Federal Defense Munich, Wernerare compared. In conclusion, the proposed machine design
Heisenberg-Weg 39, D-85579 Neubiberg, Germany (e-mail:
generates smoother torque and leads to a shorter and far
Dieter.Gerling@unibw.de).
Born in 1961, Prof. Gerling got his diploma and Ph.D. degrees in
less heavy machine. This results in lower production costs,
Electrical Engineering from the Technical University of Aachen, Germany
because one needs less material. The disadvantage of higher
in 1986 and 1992, respectively. From 1986 to 1999 he was with Philips
iron core losses compared to the 30-teeth/20-poles topology
Research Laboratories in Aachen, Germany as Research Scientist and later
as Senior Scientist. In 1999 Dr. Gerling joined Robert Bosch GmbH in
must be kept in mind, but nevertheless these losses can be
Bühl, Germany as Director, being responsible for New Electrical Drives
reduced through a corresponding optimization target.
1993
and New Systems. Since 2001 he is Full Professor and Head of the
Institute of Electrical Drives at the University of Federal Defense Munich,
Germany.
1994
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