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DESIGN STUDY OF LOW-SPEED DIRECT-DRIVEN
PERMANENT-MAGNET MOTORS WITH
CONCENTRATED WINDINGS
F. Libert, J. Soulard
Department of Electrical Machines and Power Electronics,
Royal Institute of Technology
100 44 Stockholm, Sweden
e-mail: florence@ets.kth.se
Abstract— In this study, different designs of radial-flux direct-driven permanent magnet (PM)
motors are compared. They should replace an induction motor and its gearbox for an industrial
application requiring 5 kW and 50 rpm. The goal is to show if the use of concentrated windings
brings some benefits for this application, the PM motors having a high number of poles.
Different combinations of the number of slots and number of poles are investigated for the
concentrated windings designs. Neodymium Iron Boron PM and ferrite PM are tested and the
cost of the different machines is estimated.
1. INTRODUCTION
Replacing induction machines with permanent magnet
(PM) synchronous machines has recently gained great
interest, as the environmental concern increases
worldwide and the price of PM materials decreases. For
low speed applications, using PM machines may further
eliminate the need of a gearbox, which is traditionally
coupled to a standard induction machine. Since the
gearbox is costly, decreases the efficiency of the drive,
and needs maintenance, a direct-driven PM machine is an
attractive solution that can compete with the induction
motor and the gearbox. Direct-driven PM machines are
nowadays mostly used for boat propulsion, wind turbines
and elevators [1].
In the present paper, the industrial application is a mixer
used in wastewater treatment plants, which requires 5 kW
and a rated speed of 50 rpm. Since the speed is very low,
the machines are lighter when they have a high number
of poles. Different radial-flux PM machines with high
pole number, distributed windings, and different PM
positions in the rotor were investigated in [2]. The
present study shows how concentrated windings can
improve the designs of high pole number direct-driven
PM machines. The advantages and drawbacks of the
concentrated windings are well known when the number
of poles is under 30 [3], [4]. With a higher number of
poles, the possibilities in the design of permanent magnet
machines with concentrated windings are wider.
Especially there are many more combinations of number
of slots and number of poles to choose between. The
choice of the number of poles and number of slots of the
investigated designs is therefore justified in detailed.
Designs with surface mounted and buried PM magnets
with different concentrated windings are compared using
results from finite element methods (FEM) simulations.
The cost of the active material of different designs is also
given and different PM materials are investigated. The
benefits that bring the use of concentrated windings in
the direct-driven PM machines are emphasized.
2. THE INVESTIGATED PM MOTOR CONFIGURATIONS
A. Description of the motor configurations
Surface mounted permanent-magnet (SMPM) motors are
the most common configuration of radial-flux PM motors
for direct-drive application. Both inner rotor and outer
rotor motor designs are investigated.
The other considered configuration has buried permanent
magnets. It is referred to as tangentially-magnetized PM
(TMPM) motor (figure 1). The salient rotor consists of
different pieces of iron and permanent magnets that are
fixed together on a non-ferromagnetic shaft. With a
ferromagnetic shaft, a large portion of the flux generated
by the permanent magnets would leak through the shaft.
With this configuration, the flux generated by the PM is
concentrated in the rotor iron. High airgap flux densities
can thus be achieved with a low PM weight.
Fig. 1. Cross-section of a TMPM motor (one pole pair)
2
The compared designs have Neodymium Iron Boron
(NdFeB) permanent magnets. Designs with ferrite
magnets are also investigated for the tangentiallymagnetized PM motors.
B. Design procedure
The designs are conducted by solving an optimization
problem. The objective function to be minimized is the
active weight of the motor. The optimized design should
fulfill different requirements and constraints. Thus the
exterior diameter is limited to 500 mm, a minimum
efficiency is guaranteed by limiting the copper losses to
15% of the nominal power of the machine and a limit is
set to 5 kg of PM material (for the NdFeB magnets)
because of the high cost of the PM. All the designs are
calculated for the same nominal torque and nominal
speed of 840 Nm and 50 rpm and the same amount of
copper losses (700 W). The different steps of the design
procedure are depicted in figure 2. More about this
procedure can also be found in [2].
The analytical design procedure is common for the
different configurations and adapted for each case. For
example, the reluctance torque is taken into account for
the tangentially-magnetized PM motor, even though the
reluctance torque is less than 5% of the rated torque. The
saliency ratio is indeed less than 3. NdFeB permanent
magnets or ferrite magnets can be used. The analytical
calculations of the fundamental airgap flux density and
flux density in the teeth are corrected using FEM
simulation results. The difference between the analytical
calculations and FEM simulations results of the
fundamental airgap flux density is less than 5% for
designs with distributed windings and less than 8% for
designs with concentrated windings, the number of poles
being between 20 and 80.
3. THE INVESTIGATED WINDINGS
A. Distributed windings
The higher the number of pole, the lower the active
weight of the direct-driven motor. Since the outerdiameter of the motor is limited and the number of poles
is high (over 20), the number of slots per pole per phase q
is set equal to 1. Otherwise, with a higher number of slots
per pole per phase, the teeth are too numerous and too
thin to guarantee a rigid structure.
With q = 1 and without any skewing, the torque ripple is
high, due to the interaction between the airgap flux and
the space harmonics in the magneto-motive force. As
table I shows, the torque ripple is the highest for the
tangentially-magnetized PM motors and the lowest for
the inner-rotor surface-mounted PM motors, but it is still
very high. The torque ripple is simulated with FEM.
Fig. 2. Followed procedure to optimized the design of low-speed PM
motors
Table I
Ratio between the torque ripple and the mean torque in %
Pole
number
p = 30
p = 50
p = 70
Inner-rotor
SMPM
19
16
10
Outer-rotor
SMPM
28
27
26
Tangentiallymagnetized PM
70
46
47
B. Concentrated windings
Using concentrated windings has several well-known
advantages: the end-windings are much shorter, the
windings are easy to mount and a low torque ripple can
be achieved [3], [4]. Furthermore, the concentrated
windings give the possibility of having a low number of
teeth, around three times lower than for windings with q
= 1. This is an important advantage when considering
high pole number machines with limited dimensions,
since the teeth are less saturated and more rigid.
Several investigations have been made on the choice of
the pole number (p) and slot number (Qs) of PM motors
with concentrated windings, [4], [5], [6]. The highest
winding factors are obtained for concentrated windings
with the number of poles close to the number of teeth.
3
Table II
Winding factors for different pole numbers p and slot numbers Qs
Table II shows the winding factors for windings with
different number of poles and number of slots. Windings
with the same layout pattern and the same number of
slots per pole per phase have the same winding factor.
Machines with concentrated windings, whose layouts are
not symmetrical, have problems with vibration and noise
because of an unbalanced magnetic pull. The
combinations of slot and pole number of such windings
are for example when Qs = 9 + 6k (k=0,1,2...) and
p = Q s ± 1 , [5]. The winding layout of these motors is not
symmetrical since the coils of one phase are all located
on one side of the stator. In this paper, the investigated
slot and pole number combinations give high winding
factors and windings layouts with many symmetries.
They are the combinations with the number of slots per
pole per phase q equal to 2/5 and 2/7 giving a
fundamental winding factor kw1 equal to 0.933, q = 3/8
and 3/10 with k w1 = 0.945 and q = 5/14 and 5/16 with kw1
= 0.951.
saturating the teeth. This allows the motors with
concentrated windings to be shorter and therefore lighter.
Fig. 3. Active motor weight as a function of the pole number for
SMPM motor designs with concentrated and distributed windings.
4. RESULTS FOR SURFACE-MOUNTED PM
MOTORS
A. SMPM motors with inner rotor
Figure 3 shows the active weight as a function of the pole
number for different values of the number of slots per
pole per phase q. The tendency for SMPM motors with
concentrated windings is that the active weight decreases
with an increasing pole number. Some points do not
follow this tendency due to the fact that the active weight
also varies with the number of slots. Thus, the designs
with q = 2/5 or 2/7 are slightly heavier than the others,
because of their lower winding factor. Figure 3 reveals
also that the motors with concentrated windings are about
15 kg lighter than those with distributed windings (q = 1).
Since there are fewer teeth in designs with concentrated
windings, both the slots and teeth can be wider, as figure
4 shows. The permanent magnets are allowed to be
thicker and the airgap flux density to be higher without
Fig. 4. Geometries of two 70-pole SMPM motors, one with concentrated
windings (q=3/10) and one with distributed windings.
All these designs have equal copper losses (700 W).
Moreover, the torque ripple is reduced from 9.3 % to less
than 3 % for motors with concentrated windings.
B. SMPM motors with outer rotor
With a larger airgap diameter, the SMPM motors with
outer-rotor are lighter than the inner-rotor SMPM motors
(compare figure 3 to figure 5). With concentrated
windings, the outer rotor SMPM motors are
approximately 20 kg lighter than with distributed
windings. The drawback of the outer-rotor SMPM
motors with q = 1 is their high torque ripple (table I).
4
With concentrated windings, it is successfully reduced
from 26 % to less than 5 %, as figure 6 shows.
Fig. 5. Active motor weight as a function of the pole number for
outer-rotor SMPM motor designs with concentrated and distributed
windings (q=1).
and 4.5 kg. Due to the flux concentration, the airgap flux
density reaches easily 1.1 T with less than
5 kg
permanent magnet.
Fig. 7. Active motor weight as a function of the pole number for
TMPM motors with concentrated and distributed windings (q=1).
Table III
Comparison of 70-pole tangentially-magnetized PM motor designs
with NdFeB magnets (FEM results)
q
3/10
2/5
5/14
1
Fig. 6. Torque of outer-rotor SMPM motors with concentrated and
distributed windings at load conditions.
4. RESULTS FOR TANGENTIALLYMAGNETIZED PM MOTORS
A. Tangentially-magnetized PM motors with NdFeB
magnets
Figure 7 shows the active weight of different
tangentially-magnetized PM motor designs with
concentrated and distributed windings. The difference in
weight between the designs with different concentrated
windings and same pole number is very small, although
designs with q = 2/5 and q = 7/5 are slightly heavier. As
for the surface-mounted PM motors, designs with
concentrated windings are much lighter than with
distributed windings. However, the tangentiallymagnetized PM motor designs with concentrated
windings have an important advantage compared to the
surface-mounted PM motors. In addition to a lower
active weight, the found optimized designs have lower
permanent magnet weights than the maximum allowed 5
kg. Their permanent magnet weight is only between 3.5
Active
weight
[kg]
55.9
57.9
56.2
76.4
PM
weight
[kg]
3.4
3.3
4.1
5.5
Torque
ripple
[%]
4.2
4.8
4.2
41.7
Active
length
[mm]
101
100
119
163
Outer
diameter
[mm]
500
498
500
500
Furthermore, the lower bound of the constraint on the
machine length, which is 100 mm, is reached for some
designs. The external diameter is then reduced and
becomes less than 500 mm. This can be observed in table
III that gives different features of 70-poles tangentiallymagnetized PM motor designs with different winding
types. The torque ripple is also considerably reduced with
concentrated windings, as shown in table III.
Tangentially-Magnetized PM motor designs with outerrotor are also possible and have been simulated. As for
the surface-mounted PM motor, the active weight is
lower for the outer-rotor designs due to a larger airgap
diameter. A 70-pole PM motor with outer rotor is thus 5
kg lighter than an inner-rotor design with 3 kg PM
weight. However the torque ripple is slightly higher with
6 %.
B. Tangentially-magnetized PM motors with ferrite
permanent magnets
For the tangentially-magnetized PM motor designs, the
NdFeB PM can be replaced with ferrite magnets. Indeed,
the flux concentration in the rotor allows reaching high
airgap flux density with a low amount of PM.
Furthermore, the designs with NdFeB magnets are light
5
thanks to the use of concentrated windings. With ferrite
magnets, the designs, though heavier, might still be
lighter than the induction motor and its gearbox that are
weighting 150 kg together. The ferrite magnets have
poorer magnetic properties than the NdFeB magnets but
they are much cheaper. The solution with ferrite magnets
can then be a better compromise between the machine’s
weight and the machine’s cost.
Ferrite magnets have other advantages. They are
electrically non-conducting, which means that there are
no eddy currents in these magnets. Their temperature of
demagnetization is high (over 200° C). The ferrite
magnets are also easier to handle when assembling the
rotor since they are weaker than the NdFeB magnets.
Finally their density is lower (5000 kg/m3 against 7500
kg/m3).
With ferrite magnets, the airgap flux density is chosen to
be lower than 1 T in order to use the PM efficiently.
Figure 8 shows the magnetization curve of the ferrite
magnet: for a 56-pole design with a PM height of 30 mm,
the operating point of the PM is close to the maximum
energy product. The PM thickness is also adjusted so that
the iron between two PM does not saturate. The obtained
airgap flux density for the 56-pole design with ferrite
magnet is then 0.96 T. In order to achieve the nominal
torque, the current loading is higher compared to a design
with NdFeB. This can be seen in figure 9. The slots of
the motor with ferrite magnets are bigger than the other
motor (they both have the same amount of copper losses).
Features from two designs with 56 poles, 63 slots and
different magnets are compared in table IV. Airgap flux
density, torque ripple and iron losses are calculated with
FEM. The design with ferrite magnets is 35 kg heavier
than the design with NdFeB. The ferrite magnet weight is
high with 16.4 kg but these magnets are cheap. The iron
losses are slightly higher for the ferrite magnet design
due to the bigger quantity of iron, but the eddy current
losses in the NdFeB magnets were not taken into account
whereas the ferrite magnets have no eddy current losses.
Fig. 8. Magnetization curve of the ferrite magnet material with operating
points of designs having different magnet widths wm and same magnet
thickness. Bδ is the fundamental of the airgap flux density
Fig. 9. Geometries of two 56-pole tangentially-magnetized PM motors
with concentrated windings, one with NDFEB magnets, one with ferrite
magnets.
Table IV
Comparison of 56-pole, 63-slot tangentially-magnetized designs with
NdFeB and ferrite PM.
Results
Airgap flux
density
Magnet width
Magnet height
Torque ripple
PM weight
Rotor active
weight
Total active
weight
Iron losses (stator
+ rotor)
Units
NdFeB
Ferrite
T
1.16
0.96
mm
mm
%
kg
10.2
9.4
3.7
4.8
30
14
3.7
16.4
kg
12.1
32.2
kg
62.2
97.2
W
73.9+
6.3
76.7+
12.8
5. COST
The different designs have been calculated to give the
lower active weight with a limited PM weight for the
NdFeB magnets. The total cost of the motor is also very
important. An estimation of the cost of the machine is
therefore made by calculating the cost of the active
materials, which are the stator and rotor iron, the copper
and the permanent magnets. The prices of these materials
per kilogram are given in table V.
6
The costs of the active material of the different machines
with 56 poles are given in table VI. These machines have
the same external diameter and amount of copper losses.
As can be seen, the active materials of the motors with
concentrated windings are cheaper than the motor with
distributed windings. Tangentially-magnetized PM
motors are lighter and cheaper than the surface-mouted
PM motors, due partly to the lower PM weight needed to
reach the demanded torque and speed. The machine with
ferrite magnets is about 100 € cheaper than the cheapest
motor with NdFeB PM. It is also about 40 kg heavier. If
the cost of the machine is an issue and not the weight, the
ferrite magnets are a solution to decrease the price of
active material.
Table V
Material cost
NdFeB
Price
[€/kg
]
55
Ferrite
Lamination
plates
Copper
wire
5.5
1.65
3.1
Rotor
SMPM
Inner
SMPM
Inner
SMPM
Outer
SMPM
Outer
TMPM
Inner
TMPM
Inner
TMPM
Outer
TMPM
Inner
NdFeB
Active
weight
[kg]
98
Cost of
material
[€]
483
NdFeB
79
425
NdFeB
87
461
NdFeB
66
385
NdFeB
78
440
NdFeB
62
377
NdFeB
59
356
Ferrite
97
251
Winding
Type
PM
Material
DW q=1
CW
q=3/8
DW q=1
CW
q=3/8
DW q=1
CW
q=3/8
CW
q=3/8
CW
q=3/8
The results show that high pole number PM machines
with well-designed concentrated windings are very
attractive for low-speed direct-driven applications. High
pole number motors with concentrated windings are
lighter than the conventional motors with distributed
windings. They are also cheaper because the PM weight
required to reach the torque is lower at same value of
copper losses. The construction of the stator is also made
easier because of the bigger and less numerous slots.
Buried PM machines with tangentially-magnetized PM
and concentrated windings are appropriate for the
application. Because of the flux concentration in the
rotor, cheaper PM can be used. Combined with
concentrated windings, the cost and weight of the highpole number tangentially-magnetized PM motor is very
attractive.
REFERENCES
Table VI
Material cost for various designs with 56 poles
Motor
6. CONCLUSION
[1] T. Haring, K. Forsman, T. Huhtanen, M. Zawadzki,
“Direct Drive – Opening a New Era in Many
Applications”, Pulp and Paper Industry Technical
Conference, pp. 171 –179, 16- 20 June 2003.
[2] F. Libert, J. Soulard, “Design Study of Different DirectDriven Permanent-Magnet Motors for a Low Speed
Application”, Nordic Workshop on Power and Industrial
Electronics Trondheim, Norway, 2004.
[3] D. Ishak, Z.Q. Zhu, D. Howe, "Comparative Study of
Permanent Magnet Brushless Motors with All Teeth and
Alternative Teeth Windings”, IEE International
conference on Power Electronics and Electrical Machines
(PEMD), Edinburgh, United Kingdom, 2004.
[4] J. Cros, P. Viarouge, “Synthesis of High Performance PM
Motors With Concentrated Windings”, IEEE Transactions
on Energy Conversion, Vol. 17, Issue 2, pp. 248-253,
2002.
[5] F. Libert, J. Soulard, "Investigation on Pole-Slot
Combinations for Permament Magnet Machines with
Concentrated Windings", International Conference on
Electrical Machines, Cracow, Poland, 2004.
[6] D. Ishak, Z.Q. Zhu, D. Howe, "Permanent Magnet
Brushless Machines with Unequal Tooth Widths and
Similar Slot and Pole Numbers", Industry Applications
Conference, 39th IAS Annual Meeting. Conference Record
of the 2004 IEEE, Vol. 2, pp. 1055-1061, 3-7 Oct. 2004.
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