1MW MULTI-STAGE AIR-CORED PERMANENT MAGNET
GENERATOR FOR WIND TURBINES
A.S. McDonald1, N. Al-Khayat1, D. Belshaw1, M. Ravilious1, A. Kumaraperumal1 A. M. Benatmane1,
M. Galbraith2, D. Staton3, K. Benoit4 & M. Mueller5
1. NGenTec Ltd. , 7/9 North David Street, Edinburgh, 2. Fountain Design Ltd., UK, 3 Motor Design Ltd, UK, 4. Motor
Engineer, France, 5 University of Edinburgh, UK
CORRESPONDING AUTHOR: Markus.Mueller@ed.ac.uk
NGenTec Ltd is developing a novel PM generator for the
offshore wind market. The concept has been proven at lab
scale, and now it is being scaled up to the MW level, where
the
technology
has
greatest
potential.
Detailed
electromagnetic, structural and thermo-fluid modelling is
described along with experimental investigation on full scale
modules. The experimental work provides confidence in the
design and modelling work completed allowing the design to
be finalised.
& McDonald [..] have demonstrated a PM generator
technology, which has the potential to be 30-50% lighter than
conventional generators. It exhibits a high degree of
modularity and through the use of air-cored windings, there
are potential benefits in component manufacture, assembly
and O&M compared to conventional PM direct drive
generator technology. The generator technology is being
commercialised by NGenTec Ltd, a spin out from the
University of Edinburgh. The generator concept has been
demonstrated at lab scale, but the benefits are mainly
applicable at multi-MW level. This paper describes some of
the development of a 1MW demonstrator being developed by
NGenTec.
1 Introduction
2 Generator Evolution
For offshore wind direct drive generators or low-medium
speed hybrid systems are being seriously considered by some
of the large wind turbine manufacturers, such as Siemens,
Nordex, Alstom, Areva, Gamesa and Vestas. The permanent
magnet synchronous generator (PMSG), rather than the field
wound synchronous generator (FWSG) as developed by
Enercon,dominates in these developments. The PMSG
topology used by the manufacturers represents an incremental
change from the FWSG, in that the electromagnetic poles in
the FWSG are replaced by permanent magnet poles. The
stator configuration of the PMSG is exactly the same as for a
FWSG, namely pre-formed copper coils are inserted into a
ferromagnetic laminated structure to form a 3-phase
distributed winding. There are two main benefits of the
PMSG over the FWSG: an increase in power density and a
reduction in rotor losses due to no rotor field current.
However, there are significant challenges with the PMSG
particularly during component manufacture and generator
assembly. The design of both PMSG and FWSG generators
has been well documented [1, 2 & 3]. It is well known that
such conventional direct drive generators are very heavy, with
the mass of the structural support dominating. McDonald [4]
showed that the structural mass could be as much as 80% of
the total mass. A generator in excess of 6MW will weigh
more than 100 tonnes, and even more for a FWSG.
Installation of such generators in the offshore environment
becomes a challenge. Should a fault occur on the generator,
the whole generator will have to be removed, leading to long
downtimes and the use of expensive O&M vessels. Mueller
The evolution of the generator topology concept is described
in reference .. In this section the authors will outline the
learning achieved thus far through the demonstration of small
scale lab prototypes. Based on this learning a 1MW
demonstrator has been designed and is currently being built
for testing in March 2012.
Keywords: PM generator, wind energy
Abstract
2.1 20kW, 100rpm, Radial Flux Prototype
In order to prove the concept a 20kW radial flux machine was
built. Figure 1 shows how the rotor and stator were assembled
for this machine. Mild steel blocks were cut and machined to
allow mounting of the permanent magnets. These blocks were
then assembled into a c-core, which were mounted to form a
rotor leaving an opening between the c-core limbs for the aircored stator. Coil blocks consist of a single concentrated coil
wound on a former and potted in epoxy. The coil blocks are
mounted to form a ring of coils, which can be inserted
manually into the rotor opening using in this case an engine
hoist. As there is no iron in the stator, no forces exist during
assembly.
2.2 Multi-Stage Axial Flux Machine
The generator topology can be both radial and axial. Due to
the high degree of modularity with this topology there a
number of design options, which are shown in Figure 2. In
each of the examples shown in Figure 2, there are 4 separate
generators each consisting of a number of PM c-core modules
and single coil modules. In Figure 2(A), four generators are
mounted at the same radius supported on two discs external to
Figure 1: Assembly process of the rotor and stator for a 20kW
radial flux machine
Figure 3: Rotor modules assembled onto the rotor structure
the air-cored stator. Such an arrangement leads to a long axial
length and it is also difficult to access the stator coils should a
fault arise. The axial length is reduced in Figure 2(B), with
pairs of machine mounted at the same radius. However, each
pair of machines will have a different design. In addition the
problem of access to the internally mounted stator coils is still
an issue. Figure 2(C) shows a multi-stage axial machine
consisting of 4 generators of the same design, but the stator is
mounted internally making access difficult. An outer stator
multi-stage axial flux machine is shown in Figure 2(D), in
which full access is now available. The topology in Figure
2(D) is the preferred option for the development of multi-MW
generators.
A 25kW 3-stage machine was designed, built and tested [ref
IET RPG] in order to investigate the multi-stage axial flux
electromagnetic and thermal characteristics. Rather than build
up the machine of individual PM c-core and coil modules, the
4 c-core modules shown in cross section in Figure 2(D) could
be merged in the axial and circumferential directions to form
a multi c-core module, reducing the number of parts. Such
modules could be cast, making them very cheap to
manufacture in volume. Figure 3 shows the outline of one
such cast module built for the 25kW 3-stage machine.
Figure 2: Multi-stage topology options
A number of coils could be potted into one mould to form a
multi-coil module as shown in Figure 4.
Figure 4: Stator coil module
In essence the generator topology developed provides a high
degree of flexibility to the machine designer, not available in
the conventional PMSG. Figure 5 shows the complete
machine installed on a test rig at the University of Edinburgh,
but with only one stator stage installed. The other two stator
stages were installed in-situ on the test rig to demonstrate the
ease of assembly due to the lack of magnetic attraction forces
between the stator and rotor. Subsequently it was found that
two stator modules in one of the stages had to be replaced,
which was done by hand. More details of the design and
testing of this machine can be found in refs [5,6 & 7]
Figure 5: 25kW prototype with only one stator stage installed
2.3 1MW Demonstrator.
NGenTec’s generator technology was specifically developed
for large diameter low speed multi-MW machines. The
concept and specific aspects of the design and manufacture
have been demonstrated at laboratory scale. The next stage in
the technology roadmap is to scale up to MW levels.
Ultimately NGenTec’s generator is being aimed at the
offshore wind market, at ratings of 6MW and greater. In order
to demonstrate the potential at this rating, a slice of a 6MW
generator is being built. The rating was chosen to be 1MW,
but at the same rotational speed of a 6MW, 12rpm, and at the
same outer diameter as would be expected at 6MW, 6.4m in
this case. Figure 6 shows a CAD model of the 1MW, 12rpm
prototype with one stator and rotor module highlighted. From
this it can be seen that there are 4 stages in the machine, so
that each stage is rated at 250kW.
Copper losses and PM loss due to eddy currents were
calculated from the FEA model. Table 1 outlines the losses in
the machine at the full load point, which are then used to feed
to thermo-fluid analysis. It is assumed that the iron loss is
negligible, as there is no iron in the stator winding. The total
machine torque with 3 stages operating at rated load and 1
stage with a full 3-phase short circuit was calculated, Figure
8. The steady state value, approximately 1500kNm, was then
used in the structural analysis.
.
Figure 7(a) 3D FEA model
.
Figure 6: CAD model of the 1MW, 12rpm prototype
.
3. Modelling the 1MW Prototype
In previous prototypes the focus was very much on proof of
concept and verification of the electromagnetic design tool.
As a result of scaling up both in terms of power rating and
physical diameter, the design challenge is structural and
thermal. Although no magnetic attraction forces exist the
stator and rotor structure have to withstand the rated torques
and extreme torques produced during fault conditions, such as
a short circuit fault. In terms of thermal performance the
machine has to remove more heat dissipated in the windings,
whilst the airflow due to the rotation of the machine is much
less. Detailed structural and CFD modelling has been
undertaken, and utilising results of loss distribution and fault
torques from detailed electromagnetic modelling. Such
detailed modelling is important to build up confidence in the
complete electromagnetic, structural and thermal design
before committing to manufacture.
3.1 Electromagnetic Modelling
Using the design tools and experience from the small scale
prototypes, a 1MW demonstrator was designed. In order to
verify the design tool further 3D finite element modelling
(FEA) was undertaken using the design tool to provide the
main dimensions. NGenTec collaborated with Motor
Engineer from France on the em modelling [refs], using Flux
3D [ref – CHECK]. Figure 7 shows the 3D FEA model for a
5 stage machine, the magnetic field distribution due to PM
excitation only, and the resulting no-load emfs. The latter is
for the 1MW 4-stage axial flux machine.
Figure 7(b) Magnetic Field Distribution in one rotor module
Figure 7(c) No load induced voltage
PIN (kW)
N (rpm)
T (kNm)
I rms (A)
Joule Loss (kW)
Magnet Loss (kW)
1002.8
12
798.2
84.3
51.2
2.71
Table 1: Loss calculation at full load
Figure 8(a) Rotor Module Concept 1
Figure 8: Rated torque in 3 stages and a full 3-phase short
circuit torque in stage 1
3.2 Structural Modelling
The structural modelling was performed in-house by
NGenTec engineers using ANSYS [ref]. Both the stator and
rotor modules were modelled in order to finalise their
structural design, and the overall structure required to support
the stator and rotor modules was also modelled.
Figure 8(b) Deflection in rotor module due to rated torque and
magnetic attraction forces
Within a single rotor module the steel plates making up the
module experience magnetic attraction forces as well as
torque. Figure 8(a) shows the first rotor module concept
mounted on the rotor support structure and Figure 8(b) shows
the deflections calculated in ANSYS. A maximum deflection
of 0.8mm into the airgap is calculated. Manufacturing
tolerances will lead to more deflection, so a modified concept
was developed to reduce the deflection due to magnetic forces
and torque. Figure 9 shows a ribbed structure on the external
place. The total module mass has increased slightly (2.5%
increase), but the deflection is now only 0.26mm.
The stator modules are cantilevered from the stator support
structure as shown in Figure 10. There are no normal
magnetic attraction forces between the stator modules and the
permanent magnet rotor plates, but the modules and structure
have to be designed to react fault torques, such as the
condition shown in Figure 8. Stress analysis performed using
ANSYS indicates acceptable stress levels and deflections.
Harmonic analysis showed no modal vibration issues. Figure
11 shows the deflection of the stator modules held fixed at the
mounting points when under rated torque conditions,
800kNm.
Figure 9: Rotor Module Final Concept
The resulting deflection with both torque and gravity
combined is shown in Figure 12(b). It should be noted that the
dummy stator blocks were modelled as separate blocks with a
small gap between each one so as to closely model the real
situation. The peak deflection is of the order of 0.5mm, and
further modelling showed that this was mainly due to the
torque. Gravity had negligible impact.
3.3 Thermo-Fluid Modelling
The stator support structure model is shown in Figure 12(a),
with the load conditions A-D shown in the figure. The
individual stator coil modules were not modelled in this case.
Dummy stator blocks exhibiting the same mass and centre of
gravity were modelled instead. A moment of 1500kNm was
applied to the faces of the dummy stator blocks, which
represents the steady state condition shown in Figure 8.
The loss in the machine is dominated by copper loss,
including eddy current losses in the winding. The coils are
potted in epoxy, which does not exhibit good thermal
conducting
properties.
Understanding
the
thermal
performance is therefore very important for this machine.
Figure 10: Stator modules cantlivered from stator support
structure
NGenTec collaborated with Motor Design Ltd [ref] to
develop a thermal model of the machine using Portunus
[ref].The thermal model was based on the T-network cube
element [ref Mellor & Wrobel]. Computational fluid
dynamics was used to determine heat transfer coefficients for
use in the thermal model. The permanent magnet rotor acts as
a very badly designed fan to stir the air in the machine and
provide some natural cooling effect. At rated load the Joule
losses are expected to be 51kW. This translates to 2.32kW per
module consisting of 4 stator blades. Using ANSYS Fluent
the heat flux was applied to the stator modules. The resulting
temperature on the blade surface and within the coils is shown
in Figure 13 indicating a maximum temperature of 64°C. An
airflow of 0.3m3/s is assumed to flow across the coils within
the machine.
Figure 13: Thermo-fluid modelling of a full scale 1MW stator
blade.
Figure 11: Stator module deflection under rated torque
conditions.
The heat transfer coefficient was calculated from the CFD
and used in the Portunus model, resulting in a maximum
winding temperature at the bottom of the stator coils of 64°C.
4. Experimental Investigation
Figure 12(a) Stator support structure load conditions
4.1 Structural Performance
Once the 1MW generator is complete deflections will be
monitored during testing. The structural design of the rotor
module was finalised based upon the ANSYS modelling. The
materials used in the rotor module are well understood in
terms of deflection under the expected forces. However, the
stator module consists of copper coils in epoxy in which the
temperature increases with loading. The stator module has to
be able to react the torque producing force. There is no
magnetic attraction force acting on the stator modules because
it is supported in a non-magnetic material. Current flowing in
the winding produces heating, which will affect the structural
properties of the support material. Figure 14 shows an
experimental arrangement of a stator module undergoing
deflection tests under a fault condition. The measured
deflection is well within the safe deflection specified, and the
analysis provides a good estimate of the expected deflection.
4.2 Thermal Performance
A test rig of a full scale module was built as shown in Figure
15. The coils were instrumented with thermocouples, and a
fan was installed to investigate forced cooling.
Figure 12(b) Stator support structure deflection under fault
conditions.
investigations on full scale modules provide confidence in the
final design, which is now being built, with testing expected
in February and March 2012.
Figure 14: Deflection investigation of stator modules under
fault conditions.
With a current flowing in the coils corresponding to full load
and a module dissipation of 2.2kW the maximum temperature
without any forced cooling was measured to be 104°C. Table
2 shows the maximum temperature for different flow rates
provided by the fan. At the time of the test the ambient
temperature was 20°C. The maximum temperature was
observed at the bottom of the coils in the same area as
observed in the CFD modelling.
Flow rate (m3/s) Maximum Temp (°C)
0.3
62.3
0.55
55.0
0.65
51.8
Table 2: Maximum temp at different flow rates for
2.2kW/module
Figure 15: Full scale module test rig.
At rated load the winding temperature for the various flow
rates shown in Table 2 are all well below the peak operating
temperature for electrical machines. However, the winding
has to withstand fault condition also. Figure 16 shows the
temperature results for an emulated fault condition lasting
100s, with and without forced cooling. For forced cooling a
flow rate of 0.3m3/s was chosen. I both cases the temperature
rise does not exceed 100°C. In the final 1MW prototype
demonstrator the performance of the complete machine will
be investigated with and without forced cooling.
5. Conclusion
The authors have described the learning that has taken place
to develop a 1MW prototype generator. Experimetnal
Figure 16: Short circuit test; coil temperature with and
without cooling.
Acknowledgements
The authors would like to thank NGenTec Ltd for permission
to publish, DECC for funding some of the work, SMART
Scotland for funding the 25kW prototype, Scottish Enterprise
for funding the original concept, and the University of
Edinburgh for providing facilities..
References
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