A Low Speed, High Torque, Direct Coupled Permanent Magnet
Generator for Wind Turbine Application
J. Y. Chen and C. V. Nayar
Centre for Renewable Energy Systems Technology Australia
Curtin University of Technology
GPO Box U 1987
Perth 6001
Western Australia
Telephone:
+61 (0)8 9266 3369
Facsimile:
+61 (0)8 9266 3107
E-mail:
pchenj@alpha2.curtin.edu.au
Abstract
There is a growing market for battery charging wind generators for remote area power systems in
Australia and overseas. The technology based on high speed, gear driven induction generators has not
been cost effective and reliable in the low power range. Mechanical simplicity and high efficiency can
be achieved by employing direct coupled, high torque, low speed generators using high strength
permanent magnets. This paper presents the design details of a 20 kW permanent magnet generator
using Neodymium-Iron-Boron magnets for direct coupled wind turbine applications. A prototype has
been built and tested. The agreement between experiment and computation shows the feasibility of the
design method.
1
INTRODUCTION
Wind energy is regarded as an important resource of the renewable energy used for electrical generation. The amount of
electricity produced by wind energy can even be comparable with that produced by diesel generating power station in
many parts of the world. Most of the wind turbine generators in the world are connected with the power grid, rather than
to serve for a local load. The grid readily absorbs the electricity produced by wind energy. Thus, such machines have
not been cost effective and reliable in the low power range. There is a large potential market for isolated small wind
generator systems. These wind generators are in small size, usually less than 5 kW, and are located in remote areas
which are not connected to any electrical grid. There is a demand for higher rating
wind generators used for battery charging in small hybrid power system in
developing areas.
Mechanical simplicity and high efficiency can be achieved by employing direct
coupled, high torque, low speed generators using high strength permanent
magnets. There have been a variety of high-performance permanent magnet
generators [1-4] developed for significant applications. The paper describes the
design of a 20 kW low-speed, permanent magnet generator for direct-coupled
wind turbine application. This type of generator has an outer-rotor rotating around
its centre stator, instead of the conventional machines with their rotors inside of
the stator. The Fig.1 shows the outer-rotor, and Fig.2 the prototype 20 kW
permanent magnet generator.
Fig. 1
Outer-rotor
The generator uses sintered Neodymium-Iron-Boron for the magnets so that it can stand a high magnetic loading. All of
the radially magnetised square magnets are evenly arranged along the inside periphery of the drum. While the machine
is running, the centrifugal force of the magnets applies a pressure to the bonding media, thus, increasing the reliability
of their glued joint. The blades of wind turbine can be simply bolted to the front face of the outer-rotor. Due to the
absence of gearbox, it was estimated that about 4% of input wind energy could be saved[2] for electrical generation.
A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications
Chen et al
This type of generator may be used to charge battery-bank in an isolated WindSolar-Diesel-Battery hybrid power system that incorporates wind turbines,
photovoltaic panels, battery storage and diesel engines. Reliable and simple
mechanical configuration makes this sort of machine compact and lightweight,
and easy to be installed on a tower.
2
WIND ENERGY ASSESSMENT
Fig. 2
It is well known that the wind power Pw in an air
stream with a density ρ and a velocity ν through a
cross-area is:
0.5
0.45
1
ρAν3
2
(1)
A wind turbine, with this swept area in the air stream,
will develop a power Pt, and Pt/Pw is the conversion
efficiency, sometimes called the power coefficient Cp.
Mathematically
1
Cp = Pt/Pw = Pt/ ρAν3
2
(2)
0.35
0.3
0.25
0.2
0.15
0.1
Assuming a circular cross sectional area πR2 with a
radius R of blade, then
1
Pt = CpρπR2ν3
2
0.4
Coefficient Cp
Pw =
20 kW PM generator
(3)
0.05
0
0
1
2
3
4
5
6
7
8
9
10
Blade-Tip-Speed Ratio B
More detailed analyses of wind turbine performance[5]
Fig. 3 Cp-β curve for a 30 kW horizontal axis wind turbine
indicated that maximum power from the wind is
obtained when the final down wind velocity is one third of the up wind velocity. The value of power coefficient is then
16/27 or 0.593. This is known as the Betz limit, after the German aerodynamicist who defined it in 1927. Practical wind
turbines designed for wind generation have Cp value generally below 0.45. The power coefficient is a function of a
turbine‘s wind blade tip speed to speed ratio, given as below
β = νt/ν= ωR/ν
(4)
where, νt is the turbine blade tip speed and ω the turbine shaft angular speed. It can be seen, from the Fig. 3, that a
single maximum Cpmax occurs when the tip-speed ratio takes a particular value βmax. Obviously, if the turbine is to
extract maximum power from the wind, the shaft speed will vary as
ω = βmax ν/R
(5)
When the turbine is running and keeping the tip speed ratio at βmax, the output power from the turbine is,
Pt = (
1
ρπ Cpmax R2) ν3
2
(6)
The expression in the bracket is constant, and hence the output power varies as the cube of the wind speed. It is
assumed that wind speed follows a Weibull distribution[6]. In designing a generator, rotating speed is one of key input
data for electromagnetic calculation. Substituting Equation (5) for ν, Pt can be alternatively expressed in terms of ω as
Pt = (
1
ρπ Cpmax R5/βmax3) ω3
2
(7)
A predicted annual energy output versus mean wind speed for a 30 kW wind turbine was used for wind energy
assessment. The mean annual wind speed of 7.1 m/s is derived form the previous statistic distribution of wind speed in a
detailed area (Western Australia), and the rotational speed of 170 rpm is initially determined as normal speed of the
wind turbine. So, the 68 Hz is determined in the design of the 20 kW permanent magnet generator with 48 poles.
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A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications
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DESIGN CONSTRAINTS
3.1
Temperature Rise
Chen et al
Since the system is to be work in remote location, it was specified that the generator should be completely enclosed as
protection against possible environmental damage, such as storm, sun radiation, dust, insects and chemical pollution,
etc. With this mechanical construction, there is almost no direct ventilation between inside and outside of the machine.
The generator can only emit the extra heat energy into environment through the appearance of the drum.
A maximum ambient temperature of 50°C is supposed in user’s area and the glue material fixing the magnets is able to
afford 120°C at its up-limitation. Thus, a maximum temperature rise is no more than 70°C for the operation. All of the
thermal sources must be catered for the assumed condition and restricted at the bearable level.
3.2
Demagnetisation
The magnets can be partially demagnetised by over current or excessive temperature, or a combination of both.
Generator is specified to suffer no more than a certain percentage demagnetisation at a possible maximum current and
temperature. The procedures should ensure that the demagnetisation current is applied at all orientations so that the
worst case is definitely covered. The partial magnetisation effects may be roughly calculated by manual calculation
methods and more accurate computation involves finite-element or boundary-element methods.
4
ELECTROMAGNETIC DESIGN
Due to so many important aspects of design, it is not easy to derive an optimum design in some simple forms. But, it is
possible to obtain an optimum choice of machine construction just keeping in mind the requirement to achieve
sufficient output power from a given dimensions, the availability to be able to predict performance at variable load and
a prediction of voltage regulation due to winding resistance and inductance. Considering the possible non-linearity, an
iterative numerical method is adopted to determine the operating point in magnets. The approaches involve
discretisation of magnetic circuit and integration of loop flux path. Repeated calculation and optimised device aim at to
obtain possible large output, good performances and low cost of product. The leakage flux resulting from the
polarisation of the magnets affects the density in critical parts of the magnetic circuit and main flux. It is necessary to
set up a simultaneous equation related to the balance between m.m.f. and reluctance drops throughout the entire path.
∫ H dl − ∫ H dl − ∫ H dl − ∫ H dl − ∫ J dΩ = 0
m
s
r
g
(8)
Ω
Where, subscript m, s, r, and g express the integral paths across magnet, stator, rotor, and air gap respectively. Ω is the
integral area surrounded by the entire integral path. The first and last items in the equation (8) are refereed to m.m.f. of
magnet and armature reaction.
4.1
Open-circuit operating point
Referred to the case of open-circuit of this permanent magnet
generator, the flux in the air gap, linking the stator winding, is solely
produced by the magnets and induce an open-circuit voltage Eo in the
armature, which corresponds to the open-circuit operating point of the
magnets. To plot this point on the B/H characteristic, it is assumed that
the demagnetising curve is a stabilised one with a single valued
relationship. Namely, the demagnetisation within the range of
operation is reversible. Normally, Nd-Fe-B material has a linear curve
in the second quadrant on B/H characteristic. Although field strength H
is negative in the second and third quadrants, it is preferred to regard H
as a positive number for more convenient calculation. The B/H line can
be defined as
No-load operating
Λ ex point with nil
current
Φr
Λ∑
Λσ
Φm
O’
Φ∑
Φσ
α’ex
Fc
Fm
Fig. 4 Static operating point
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A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications
B = -µ‘H + Br
Chen et al
(9)
So far as a magnet with the given geometry, its Φ/F characteristics can be found from the relations: Φ = BAm and F =
Hhm where Am and hm are the magnetic area and height of the magnet. This Φ/F characteristic is linear with intercepts
equal Φr = BrAm and Fc = Hchm, as shown in Fig. 4. Integral paths are selected both for the main flux and for leakage
fluxes, and a balance is applied between the reluctance drop and the m.m.f. The total flux Φ, supplied by permanent
magnets, equals the sum of main flux Φ∑ and leakage flux Φσ distributed along the flux path. It is expressed as:
Φ = Φ∑ + Φ σ
= Fm/(Rg+Rs+Rr) + FmΛσ
= FmΛΣ + FmΛσ
(10)
The main flux path within a pair of poles consists of rotor yoke, two magnets, two sections of air gap, stator teeth and
yoke. Because of that the flux density in the teeth is unsaturated and the permeability µr in steel is far greater than unit,
the reluctance drops in the steel of stator and rotor are only a small part comparing with the magnetic potential
difference across the air gap. The resultant permeance of the entire magnetic circuit can be expressed as:
Λex = Φ/Fm = ΛΣ + Λσ
(11)
The Λex is the slope of no-load line and represents the external magnetic affection to the magnet. The operating point
corresponding to the open-circuit load is plotted at the intersection of no-load line and demagnetising curve, as shown in
Fig. 4. The corresponding coordinate on Φ-axis represents the total flux coming out from the magnet. The angle of the
no-load line is αex = arctan pex.
4.2
Dynamic operating point
With the current flowing through the winding, the magnets of the
generator is significantly affected by an external magnetomotive
force which is caused by armature reaction, and its operating point
vary with the exerteded external field strength. The corresponding
position of the dynamic operating point depends on both the
direction and amplitude of the armature reaction. The air gap field,
set up by the armature current, possesses a direct-axis component
and a quadrature-axis component, which bear a close relation to the
direct-axis and quadrature-axis inductances.
Open-circuit operating
point with nil current
Br
Pex
load operating point with
armature reaction
O
Bm
L
Load line
αex
Hc
αex
Had
Fig. 5 Dynamic operating point
The direct-axis flux passes across a magnet face and either add to or
subtract from the magnet flux, depending on the direction of the
direct-axis current which is defined by the load-angle θ between Eo and load current I. A positive sign for Id means a
magnetising action and a negative sign a demagnetising action with respect to the magnets. In the case of this generator,
more attention is paid to latter. The demagnetising field strength caused by the direct-axis armature current can be
deduced from a set of equations. It is known that the amplitude of m.m.f. produced by the three-phase D-axis armature
current under one pole one phase is
Fad =
W k
3 2 2WΦ k w
I cos θ = 2.7 Φ w I cos θ
π
p
p
(12)
Before being used for plotting the load operating point directly on the B/H characteristic of the magnets, equation (12)
is needed to be divided by hm:
Had = Fad / hm = 2.7
WΦ k w
I cos θ
phm
(13)
To determine the magnetic operating point on the B/H characteristic, the field strength Had is caused by armature
reaction and can be directly iterated to the open-circuit characteristic. The no-load line pex is shifted along the H-axis for
a Had and intersects the magnetic demagnetisation curve at point L shown in Fig.5. The L is the wanted load operating
point of the magnet.
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Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
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A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications
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Chen et al
INVESTIGATION OF MACHINE PERFORMANCE
5.1
Open-circuit characteristics
The open-circuit test was made under various rotational
speed or frequencies. Fig.6 shows output RMS line-line
voltages from computation and measurement. It is seen
that the computed results from the method described in
section 4.1 has a very good agreement with measured data.
The test results shows that the output terminal EMF was
linearly proportional to the rotational speed, and so the
machine operated at unsaturated region which is identical
with the flux density in the design.
Output line-line voltage, V
To investigate the performance of this type of generator, a prototype machine was built with an intention to deliver an
output power of 20 kW at a supposed rotational speed of 170 rpm. A comparison of the performance is made between
the results of the computation and measurement.
1000
900
800
700
600
500
400
300
measured
200
predicted
100
0
0
100
200
Fig. 6
5.2
300
400
500
600
Rotational speed, rpm
Open circuit characteristics
Performance of load test
The prototype generator was tested with a symmetrical 3-phase resistive load. Varying with the different load resistance
in the test, the armature reaction has significant influence on the output characteristics of the machine. At this resistive
load test, it was found that although the no-load output voltage waveform was not a satisfied sinusoid the generator
could produce a better sine wave output when the load current was increased beyond 13 A. The stronger the load
current was, the more standard sine wave the output waveform tended to be. It is a very interesting behaviour and the
expected performance that the generator is capable of supplying power system with a good sine wave over wide loading
range. This trend was promoted when load current was raised as a result of that either load resistance dropped at certain
working frequency, or frequency increased at the constant load resistance. Under either of the cases the inductive
component of load current was enhanced. This varying tendency could be easy to be detected through the tests. The
Fig.7 shows the obvious difference of waveforms between the cases at no-load test and at the resistive load test of 11Ω
per phase. Both pictures of the Fig.7 (a) and (b) were taken at the operating frequency of 68 Hz. The Fig. 7(b) obviously
has a more standard sinusoidal shape.
phase voltage
Vp-p=452 V
(a)
V p-p =379 V
Phase voltage at no-load
phase current
Ip-p=147A
(b) Voltage and current at resistive load
Fig. 7
Output waveform
A comparison of the load performances is made between the results of the computation and measurement. The designed
parameters, methods and a proper phasor diagram have been used to predict the output characteristics. The prediction
and measurement of output phase voltage varying with load current at 6 different speeds are shown in Fig. 8.
Comparing with the Xd and Xq at low operating frequency, stator armature resistance ra has a significant influence to the
performances of the permanent magnet machine. It is seen in Fig. 8 that the prediction of regulation at relative lower
rotational rang reaches a satisfied agreement with the measurement by taking account of ra. A positive voltage
regulation can been seen through the measurement. The computed results include stator resistance ra and armature
reaction with influence of Xq. A good agreement between computations and measurements is obtained within normal
Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
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A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications
Chen et al
load current range. With higher value of current, difference between the two voltages is gradually increased. The
possible reasons are:
200
Output voltage per phase, V
(1)
Variation
of
stator armature resistance
due to temperature rises.
In the computation, a
constant resistance ra at
constant temperature was
only used. But, increased
current yielded higher
temperature and caused
the ra changed a lot in the
test.
180
160
140
120
100
80
60
40
20
0
0
10
20
30
40
50
60
70
(2)
When frequency
Load current, A
increased, load angle and
power factor angle varied
Fig. 8 Out put voltage at resistive load
in a more complicated
function, which is related to
the features of magnetic material and symmetry of the machine.
75 rpm computed
75 rpm measured
100 rpm computed
100 rpm measured
125 rpm computed
125 rpm measured
150 rpm computed
150 rpm measured
170 rpm computed
170 rpm measured
200 rpm computed
200 rpm measured
The synchronous performance of this prototype machine was tested under varying load condition at 6 different
rotational speeds. The experimental results show that when the output power of the generator reached 20 kW at the
rotational speed of 170 rpm, the efficiency was 86%, load current 52 A and output phase voltage 134 V. After one
hour’s full load test, the measured temperature rise of the stator armature was 57°C at the ambient temperature of 31°C.
This exhibits that the machine is able to work at the supposed environmental condition with a reliable performance.
6
CONCLUSION
The paper has described the design of a direct coupled, outer-rotor, permanent magnet generator for wind turbine
application. A prototype machine was built and tested. The discussed essential parameters of this machine model was
determined by using computational and experimental methods. There is a reasonable agreement between computed and
measured results, which exhibits the feasibility of those approaches developed in the design.
7
ACKNOWLEDGMENT
The study was founded by MERIWA as Project No. E233. The authors appreciate the cooperation of Wester Wind and
the supply of experimental equipment from the Curtin University of Technology. Also, the authors thank the advice
from Dr W. W. Keerithpala.
8
REFERENCES
1
Chalmers, B.J. (1994), “Performance of interior-type permanent-magnet alternator”, IEE Proc.-Electr. Power Appl, Vol.
141, (4) 1994, pp.186-190.
2
Spooner, E. and Williamson, A.C. (1996), “Direct coupled, permanent magnet generators for wind turbine application”,
IEE Proc.-Electr. Power Appl, Vol. 143, (1), and pp.1-8.
3
Nayar, C.V. (1991), “Investigation of capacitor-excited induction generators permanent magnet alternators for small scale
wind power generation”, Renewable energy, Vol. 1, No.3/4, pp381-388.
4
Binns, K.J. and Low, T.S. (1979), “Performance and application of multistacked imbricated permanent-magnet generator”,
IEE Proc.-Electr. Power Appl, Vol.130, (6), pp. 407-414.
5
Golding, E.W., (1976), “The Generation of Electricity from Wind Power”, E&F Spon Ltd., London.
6
Johnson, G.L. (1985), “Wind Energy Systems”, Prentice-Hall, U.S.A.
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