28-06-2012, 04:51 PM
Permanent magnet machine topologies for wind power generation
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Introduction
Wind energy is one of the world’s fastest growing renewable energy sources. It is
important to investigate various topologies and design concepts of generator both used in
real wind power market and laboratory experiments. In real wind power market, three types
of wind power system for large wind turbines exit. The first type is fixed-speed wind power
with a multi-stage gearbox and a traditional squirrel-cage induction generator (SCIG),
directly connected to the grid. The second one is a variable speed wind system using a
multi-stage gearbox and a doubly fed induction generator (DFIG), with a power less rating
of 30% of the generator capacity passing the power electronic converter from the rotor
winding. The same as SCIG, the stator winding of the DFIG is directly connected to the grid.
The third type is also a variable speed wind turbine, which is famous as a gearless or
direct-drive system. Usually it contains a low-speed high-torque synchronous permanent
magnet (PM) synchronous generator and a full-scale power electronic converter, which is
called direct-drive PMSG system.
Normally, a machine with a permanent magnet for exiting source can be called the
PM machines. But as for a wind turbine generator, the output voltage of the machine
usually must be sinusoidal. The types of PM machines studied in this paper focus on
traditional PM generators that can be divided into radial-flux and axial-flux machines,
according the flux direction in the air gap.
Radial-flux PM wind generators
Basically, there are two types of radial-flux PM wind generators in real application as
Rotor core
Stator core Magnets
Copper
Air gap
Magnets Rotor core
Copper Stator core
Inner rotor Outer rotor
Figure 1 Inner and outer rotor radial-flux PM generators
In a fact, only the two topologies are applied in MW wind system. There are a lot of
experiences and studies in the inner rotor PMSGs. They also show a good electrical
operation performance and better cooling effect for stator. As for the outer rotor radial flux
PMSG, they are easily directly coupled between wind turbine and generators
mechanically .This topology has a better cooling effect for rotor and magnets, but difficult
cooling for stator, while most of wind generator loss is on this side.
For dual stator and IPMSG, illustrated in Fig. 2, they are just used in small power wind
system, from 1kw-50kw. The dual stator structure has higher torque capability and the
IPM topology is good for resisting demagnetization. The IPM topology is widely used in
industrial application. But for a large power system, its large volume and the performance is
easily influenced by load change.
Stator
Magnets Rotor
Figure 2 Dual stator and IPM radial-flux PM generators
Double stator and single side axial flux PMSG are widely used in small wind power
system, and the structure can be seen in Fig. 3. Double stator axial flux PMSG has a higher
torque capability and the performance is superior to the single side one. Besides, the
topology is too complicated to useful for a large wind generator system. The sing side
axial-flux PM generator has simple construction and is easy for mass production. But when
it comes to large power and large blade radius, mechanical dynamic balance is a serious
problem.
Double stator axial flux PMSG Single side axial flux PMSG
Figure 3 Double stator and single side axial flux wind PMSG
Lastly, two problems of PMSG used in wind power are introduced. First is the inherent
cogging torque due to magnet materials naturally attractive force. This kind of torque is bad
for operation, especially stopping wind turbine starting and making noise and vibration in
regular operation. But It can be reduced as small as possible by reasonable design, for
instance fractional slot design. 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. Currently, this kind of risk
can be avoided by online re-magnetization or fault-torrent design. They are all my future
research tasks.
Conclusions
This paper simple introduces permanent magnet machine topologies used in wind turbine
in recent 15 years, including laboratory and application level. Radial and axial flux PM
machines are investigated as two main types. Advantages, disadvantages and
comparisons are given for understanding and reference for designers or researchers in
wind power generators.