AN INNOVATIVE TYPE OF SYNCHRONOUS GENERATOR
WITHOUT RARE EARTH MATERIALS
AND WITH HIGH EFFICIENCY
Table 1: comparison table
PO. ID
xxxx
Diego Artioli, Engineering R&D Manager
Sicme Motori SRL
Abstract
Since 2002 Sicme Motori started the project SICMEWIND for the development
and construction of synchronous PM generators for wind applications at low and
medium speed, up to 3 MW rated power.
The cost of rare earth materials dramatically increased since last spring, Sicme
Motori faced a challenge to find a cost-effective solution in order to continue
proposing synchronous machines for wind turbines.
Synchronous reluctance machines can be used as generator, even though they
are mainly known for motoring applications. This is a fact which doesn’t require to
be proven. The originality of Sicme Motori’s offer is in a new type of assisted
synchronous reluctance machine (ASR) for generator applications and without
rare earth materials. Sicme Motori, after acquiring a patent license for the design
and production of SR machines, continued to develop these concepts, specifically
investigating and prototyping the possibility to have Ferrite assisted machines, so
getting rid of rare earth materials, whose cost suddenly increased last year and is
always affected by finance speculating operations and, furthermore, facing the
difficult task to design SR machines for multi-polar solutions, such having a
product suitable for medium speed operation (hybrid machines and mini-wind
applications) and currently investigating the further development to low speed
applications (direct driven multi-MW generators).
Working Principle of the Synchronous Reluctance machine
The synchronous reluctance machine (SR) is made by a conventional three-phase
AC stator (the same as an asynchronous motor, for instance) and by an
anisotropic rotor. As stated in [1], the principle of the “reluctance” is based on the
unwillingness of the magnetic flux to overcome the large air-gap in the spaces
between the salient poles. This is a very well known principle, which is obviously
considered and quantified even hen discussing salient rotor poles, such as in [4],
where a “reluctance torque” is added to the “cylindrical torque”, which is a peculiar
term to refer to the torque given by the excited rotor field.
A good design for the SR machine requires a peculiar rotor shape, which
maximizes the ratio between the reluctances of the magnetic circuit 90 electric
degrees away from each other, such maximizing the “anisotropic” component of
the torque. The rotor is designed to produce the smallest possible reluctance in
one direction and the highest reluctance in the “perpendicular” direction (referring
to the electric axis), with shapes as shown in figure 1, where a visual comparison
between a (copper) squirrel cage and an SR rotor is shown.
Advantages of the Synchronous Reluctance solution
SR is combining the advantages of permanent magnets and induction machines,
having both a “cold” rotor (thus meaning the rotor has no fundamental losses,
joule or iron components) and the field regulation, which allows a certain range of
regulation of the voltage, which is no more “fixed” by the permanent magnets’
flux. It has the robustness of an induction motor and the size, efficiency, and
synchronous speed operation benefits of permanent magnet motor technology.
From the user’s point view, SR can be seen as a synchronous machine with
magnetizing current. This means that its operative potential and flexibility can be
compared to the performance of an induction motor, but without rotor cage,
therefore with a rotor which is without fundamental losses (which typically are
about 30-40 % of the total losses).
FASR Ferrite Assisted Synchronous Reluctance generators
Assisted SR can be realized starting from a properly designed SR machine and
installing permanent magnets in order to improve the power factor and the
performance; in more technical
words, the PM flux compensates the
magnetizing current effect and moreover gives an additional torque contribution.
This is the ASR machine (Assisted, with permanent magnets, Synchronous
Reluctance), or simply ASR, which has the following features:
- No rotor windings, thus no rotor Joule losses, resulting in HIGH EFFICIENCY
- Very high torque density (almost the same as brushless motors)
- Good range of constant power speed regulation
- Low torque ripple, if properly designed (patented design)
- Very low inertia (very light rotor)
- Smaller dimensions compared with an induction machine
Permanent magnets could obviously be rare earth magnets, but Sicme Motori’s
train of thought was ‘can we use this solution with different materials?’ ‘Can we
find a way to improve the design of SR machines so that we have a
performance which is really close to the one of the PM generators, in terms of
efficiency and power factor, but eliminating rare earth materials?’
Sicme Motori’s Research and Development Department, together with the
Politecnico of Turin, demonstrated that a proper quantity of low cost permanent
magnets made of iron ferrites, without rare earth materials, can be installed in
the rotor “slots”, such giving a great opportunity to produce machines with
performance really close to that of standard synchronous PM generators, but
without using rare earth materials. The following table shows the comparison for
an application already designed by Sicme Motori.
PM-ASR
Torque
density
Efficiency
Figure 1: visual comparison between induction motor’s rotor and SR rotor
.
References
[1] Dr. Ing. Herbert Rentzsch, Elektromotoren – Electric Motors, ABB
[2] Vagati, Synchronous reluctance electrical motor having a low torque-ripple design, 1996
[3] Nicola Bianchi, Design, Analysis and Control of Interior PM Synchronous machines, IEEE 2004
[4] Stephen J. Chapman, Electric Machinery Fundamental, Mc Graw and Hill
95%
Wound rotor Brushless Induction
synchronous machine
machine
80-85%
100%
55-60%
92%
For
instance
95%
90%
94%
Conclusions
Sicme Motori has currently designed and engineered a few ASR solutions for
small wind turbines and bigger hybrid generators (at approximately 200-500
rpm), and has produced a first prototype at 300 rpm which is going to be tested
and compared in back to back electric and mechanical operation with a
catalogue product of the series SWCn which is just the same size. These
products can be now offered to clients as competitive solutions, in order to face
the important issue of the cost of rare earth materials. Very important is to
underline that this new solution is very cost-effective and would even be
competitive against traditional PM machines in case the rare earth’s cost came
back to the level of three years ago.
EWEA 2012, Copenhagen, Denmark: Europe’s Premier Wind Energy Event