Comparative performances analysis of different rotor types for pmsg used in wind turbine application

International Journal of Advances in Power Systems (IJAPS) Vol. 1, No. 1, June 2013
ISSN : 2335-1772
1

Abstract—PMSG provides a high performance, compact size,
light weight, and low noise, without forgetting its simple
structure, high thrust, and ease of maintenance, allow replacing
steam catapults in the future. Most turbine generators at low
wind speed are presented PMSGs, These it has advantages of
high efficiency and reliability, since there is no need of external
excitation and loss of drivers are removed from the rotor. In this
paper, a comparative PMSG performance study's with several
rotor topology is presented, each topology rotor has its own
permanent magnet structure that is width, thickness and angle.
These results are obtained by finite element method (FEM); this
approach is a powerful and useful tool to study and design
PMSGs, as represented in this paper.
Index Terms—Electrical machine, Design and simulation,
Finite element, PMSG, Permanent Magnet, Rotor type, Wind
Energy.
I. INTRODUCTION
he development of modern wind power conversion
technology has been going on since 1970s, and the rapid
development has been seen from 1990s. Various wind turbine
concepts have been developed and different wind generators
have been built [1].
In specialized literature several types of the machine was
developed; we can find a PMSM with internal rotor [2-3] or
with external rotor [4], even the comparative studies between
the two preceding topology was made [5], Also in [6] it
presents a study of interior rotor IG by FE, DC machine [7],
external rotor SRM [8], in the references [9-10] we find a
study of DFIG internal rotor by FE and external rotor in [11].
Even of the special structure machine like a doubly stator or
doubly rotor, this special machine is intended for special
applications [12].
Recent studies show a great demand for small to medium
rating (up to 20 kW) wind generators for stand-alone
generation-battery systems in remote areas. The type of
generator for this application is required to be compact and
light so that the generators can be conveniently installed at the
top of the towers and directly coupled to the WTs [13]. In
addition there are several reasons for using variable-speed
operation of WTs; the advantages are reduced mechanical
stress and optimized power capture. Because of the variable
speed operation, the direct-drive PMSG system can produce
5–10% more energy than the fixed two-speed concept, or 10–
15% more than the fixed single-speed concept [14].
II. PMSG IN WIND TURBINE APPLICATION
Compared to a conventional, gearbox coupled WT
generator, directly coupled generators has a series of
advantages, such as a much reduced size of the overall system,
a rather low installation and maintenance cost, flexible control
method, quick response to the wind fluctuation and load
variations, etc. However, a directly coupled generator needs to
have a very low-speed operation to match the WT speed and,
at the same time, to produce electricity in a normal frequency
range (10-60 Hz) [13].
Compared with electrically excited machines, PMSG have a
number of economical and technical advantages, so that they
are becoming more attractive for direct-drive WTs, these
advantages can be summarised as follows according to
literatures [3]:
 Higher efficiency and energy yield,
 No additional power supply for the magnet field
excitation,
 Improvement in the thermal characteristics of the PMM
due to the absence of the field losses,
 Higher reliability due to the absence of mechanical
components such as slip rings,
 Lighter and therefore higher power to weight ratio.
However, PMMs have some disadvantages, which can be
summarised as follows:
 Relatively new and unknown technology for applications
in larger MW-range
 High cost of PM material,
 Difficulties to handle in manufacture,
 Low material reliability in harsh atmospheric conditions
(offshore)
 Demagnetisation of PM at high temperature.
On the other hand, in recent years, the use of PMs is more
attractive than before, because the performance of PMs is
improving and the cost of PM is decreasing [14].
Currently, Zephyros (currently Harakosan) and Mitsubishi
are using this concept in 2 MW WTs in the market.
PMM are not standard off-the-shelf machines and they
Comparative Performances Analysis of Different
Rotor Types for PMSG Used in Wind Turbine
Application
H. Mellah and K.E .Hemsas.
Department of Electrical engineering Ferhat Abbas University, Sétif, Algeria
Laboratoire d’Automatique de Sétif, LAS Ferhat Abbas University, Sétif, Algeria.
has.mel@gmail.com, hemsas_ke_dz@univ-setif.dz
T

ISSN : 2335-1772
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allow a great deal of flexibility in their geometry, so that
various topologies may be used [14].
We can notice two problems of PMSG used in wind power.
First is the inherent cogging torque due to magnet materials
naturally attractive force. This kind of torque is bad for
operation, especially stopping WT starting and making noise
and vibration in regular operation. The other one is the risk of
demagnetization because of fault happening and overheating
of magnets. This risk is very dangerous and the cost for
replacing bad magnets is much higher than the generator itself
[3].
III. PMSG DESIGN METHODOLOGY
Traditionally, the study and design of PMSGs is based on
the equivalent magnetic circuit method (EMCM). The EMCM
is of advantages of simplicity and fast computation, but its
disadvantage is also marked: it relies too much on empirical
design experience, such as flux leakage coefficient, armature
reaction factor, etc. Meanwhile, under certain circumstances,
EMCM is not competent for the analysis and design of
PMSGs. For example, EMCM cannot be employed to study
the cogging torque of PMSGs with fractional stator slots [15].
Numerical methods, such as finite-element analysis (FEA),
have been extensively used in study and design PMSGs [11-
15], Furthermore, owing to its precision and simplicity, the
two-dimensional (2-D) FEM has approximately dominated the
FEM study of PMSGs. By using FEM, many design curves
and data, such as the PMSGs’ output voltage, no-load leakage
flux coefficient, and cogging torque etc., can be obtained and
used to design PMSGs[11], In addition, many commercially
available computer-aided design (CAD) packages for PM
motor designs, such as SPEED, Rmxprt, and flux2D, require
the designer to choose the sizes of magnets. The performance
of the PM motor can be made satisfactory by constantly
adjusting the sizes of magnets and/or repeated FEA analyses
[10].
IV. PMSG GEOMETRIC DIMENSION AND DESIGN
PARAMETERS
Our goal is to see and analysis the effects of the rotors
topology on the designed generator performances, and
accurately the permanent magnets such as PM positioning,
thickness, width and bending angle, to properly isolate the
interaction PMSG phenomena, all machines are designed with
the same stator in terms of size, dimensions and materials, this
approach which makes it possible to compare machines
together.
A. Stator Topology
The operation principle of electric machines is based on the
interaction between the magnetic fields and the currents
flowing in the windings of the machine.
Fig. 1. Stator and coil structure of all designed generators.
The following table shows some common rated values,
geometric parameters of all prototype PMSGs.
TABLE I
SOME COMMON RATED VALUES, GEOMETRIC PARAMETERS OF ALL PMSG
Rated Voltage (V) 120
Outer Diameter of Stator (mm) 120
Inner Diameter of Stator (mm) 75
Number of Stator Slots 24
Number of Stator Slots 24
Length of Stator Core (Rotor) (mm) 65
Stacking Factor of Stator Core 0.95
Stacking Factor of Iron Core 0.95
Frictional Loss (W) 12
Operating Temperature (ÛC) 75
The following figure shows the b-h non linear characteristic
of the steel core both for stator and rotor.
Fig. 2. stator and rotor core B-H characteristic
B. Rotor Topologies
In this paper our study is carried out for various rotor
topologies. They are listed below:
The rotor types under study, depicted in Fig. 3, are:
Type 1: PMSG with a rotor with surface mounted thinnest
magnets with a vertical edge (SMTV-PMSG).
Type 2: PMSG with a rotor surface mounted thinnest
magnets with bending angle edge (SMTB-PMSG).
Type 3: PMSG with a rotor surface mounted thickness
magnets with bending angle edge (SMB-PMSG).
type4: PMSG with a rotor with radial arranged internal
magnets (RI-PMSG).
Type 5: PMSDG with a rotor with internal magnets (I-
PMSG).
0 1 2 3 4 5 6 7 8 9 10
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
B[tesla]
stator and rotor core B-H characteristic
H [10 K AM]

ISSN : 2335-1772
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Fig. 3. Several rotor used in designed PMSG
Rotational Machine Expert (RMxprt) is an interactive
software package used for designing and analyzing electrical
machines, is a module of Ansoft Maxwell 12.1 [10].
The following figure shows ¼ 3D of PMSG prototype with
rotor type1.
Fig. 4. 3D view of the PMSG designed with rotor type 2.
V. SIMULATION RESULTS
The FEA model of electromagnetic field is built by
Maxwe1l2D, This simulation is obtained by Terra pc
(QuadroFX380, i7 CPU, 3.07 GHZ, 8 CPU, 4 G RAM), and
the simulation time is take some hours. Our model of PMSG
used in Maxwell environment has 19699 triangles.
Fig. 5. Power vs angle degree for several PMSG rotor type.
Fig. 5 illustrate the PMSG power for several rotor type, the
maximum PMSG power produced by the two rotors 4 and 5
do not exceed 4kw, however, the use of the rotor type 1 or 2
improve the power almost than 150%, the magnificent of all
rotor cases is the operation of PMSG with rotor type 3; where
the power produced by PMSG using the rotor type 3 is almost
three times the power in case 2.
We can say that the type rotor 3 is suitable designed to give
maximum power compared to all rotor.
-20 0 20 40 60 80 100 120 140 160 180
0
2
4
6
8
10
12
Power
ang[deg]
Power[kW]
Type 1
Type 2
Type 3
Type 4
Type 5
Type1 Type2
SMTV-PMSG SMTB-PMSG
Type3 Type 4 Type 5
SMB-PMSG RI-PMSG I-PMSG

ISSN : 2335-1772
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Fig. 6. Induced phase voltage vs electrical degree for several PMSG rotor
type.
Fig. 6 illustrates the induced phase voltage with respect to
the electrical degree for several PMSG rotor topologies, by
comparison the rotor type 1 and type 5 give the same induced
voltage of value 182V, a small decrease of 7V (≈4%) can be
noticed for both cases 2 and 3 rotor topologies, the choice to
use the rotor type 2 or type 3 make the PMSG produce exactly
175V.
In the fourth case the PMSG produce only 167V, so a
decrease of more than 8% compared to the case 1 and 5.
Fig. 7. Efficiency with respect to angle degree for several PMSG rotor type.
Fig. 7 illustrates the efficiency with respect vs angle degree
for several PMSG rotor topologies, the curvature can be
divided in two categories; the first one is slim curves with a
significant peaks the first categories include the rotor type 1,
type and type 3, the efficiency are respectively 90%, 89.31%
and 88.06%.
The second one is wide curves with a small peak this
characteristic is really corresponding to the both rotor type
cases 4 and 5, the maximum of efficiency are 86.65% and
78.23%.
By comparison the first category has an important
efficiency greater than 70% but not for all rotor angles just for
the first 40 degree, however the second hasn’t an important
peak but guard the efficiency up 70% for more than 120
degree.
Fig. 8. Cogging torque vs electrical degree for several PMSG rotor type.
In Fig. 8, the computed cogging torque is compared for
several rotor types, we can see that the 1, 2 and 4 rotor type
produce a great peak 0,8 Nm, however a small cogging
torque is remarked in the both rotor type case 3 and 5 the
minimum value is 0,2 Nm, in all rotor cases the curvature are
similar.
We can say that the SMB-PMSG is suitable designed to
give minimum cogging torque compared to all rotor type.
Fig. 9. One coil voltage vs electrical degree for several PMSG rotor type.
Fig. 9. Show the one coil voltage vs electrical degree for
several PMSG rotor types, the rotor type 1, 2 and 3 have
almost the same curvature, but the rotor type 4 and 5 gives a
reverse voltage with substantially small amplitude.
Fig. 10. Show the flux density vs electrical degree for
several PMSG rotor topologies, the rotor type 1, 2 and 3 have
almost the same curvature, but the rotor type 4 and 5 gives a
reverse curve with substantially small amplitude.
0 50 100 150 200 250 300 350 400
-200
-150
-100
-50
0
50
100
150
200
Induced voltage vs Electrical Degree
Electrical Degree [deg]
Inducedvoltage[V]
Type 1
Type 2
Type 3
Type 4
Type 5
-20 0 20 40 60 80 100 120 140 160 180
0
10
20
30
40
50
60
70
80
90
Efficiency
ang [deg]
Efficiency[%]
Type 1
Type 2
Type 3
Type 4
Type 5
0 50 100 150 200 250 300 350 400
-1000
-800
-600
-400
-200
0
200
400
600
800
1000
Cogging torque vs Electrical Degree
Coggingtorque[mNm]
Type 1
Type 2
Type 3
Type 4
Type 5
0 50 100 150 200 250 300 350 400
-300
-200
-100
0
100
200
300
Coil Voltage vs Electrical Degree
CoilVoltage[v]
Type 1
Type 2
Type 3
Type 4
Type 5

ISSN : 2335-1772
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Fig. 10. Flux density distribution with respect to electrical degree for several
PMSG rotor type.
VI. CONCLUSION
Finite element analysis (FEA) is a frequently used method
for analysis of electromechanical converters. As a numerical
analysis method, FEA allows for including any practical
material, external excitation, inclusion of motion, and
nonlinear effects such as magnetic saturation and eddy current
effects.
A several PMSAG rotor types are presented such as SMTV-
PMSG, SMTB-PMSG, SMB-PMSG, RI-PMSG and I-PMSG,
designed, modeled, solved and some simulation results is
given and commented.
This work is the necessary preparations for design and
development high reliability and high security of PMSG
applications.
It can be seen that each structure has these advantages and
these disadvantages, so the need of optimization is a
mandatory step for optimal results and maximized the most
PMSG performances, so in the future work we think to use
fuzzy logic or genetic algorithms.
REFERENCE
[1] H. Li, Z. Chen, "Overview of different wind generator
systems and their comparisons", IET, Renewable Power Generation,
vol. 2, pp. 123–138, 2008.
[2] J. Krotsch, B. Piepenbreier, "Radial Forces in External Rotor
Permanent Magnet Synchronous Motors With Non-Overlapping
Windings", IEEE Transactions on Industrial Electronics, vol. 59, pp.
2267-2276, May 2012.
[3] H. Mellah, K. E. Hemsas, “Simulations Analysis with
Comparative Study of a PMSG Performances for Small WT
Application by FEM,” International Journal of Energy Engineering,
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[4] P. sergeant, F. de belie, J. melkebeek, "Rotor geometry
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[6] S. Seman, "Transient Performance Analysis of Wind-Power
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[11] H. Mellah and K. E. Hemsas, “Design and Analysis of an
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[12] K.T.Chau, Y.B. Li, J.Z.Jiang, C. Liu, "Design and Analysis
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[13] İ. Tarımer, C. Ocak, Performance Comparison of Internal and
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[14] H. Li, Z. Chen, Overview of different wind generator
systems and their comparisons, Renewable Power Generation, IET
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[15] Z. guo, L. chang, FEM Study on Permanent Magnet
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0 50 100 150 200 250 300 350 400
-800
-600
-400
-200
0
200
400
600
800
Flux Density vs Electrical Degree
FluxDensity[mT]
Type 1
Type 2
Type 3
Type 4
Type 5

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