machine building & automation - Industrial Technology Magazine

POWER TRANSMISSION
Gears & Gearboxes
Magnetic gear at the heart
of innovative CVT design
Applications as diverse as hybrid vehicles and wind turbines,
as well as industrial gear applications, could all benefit from
the latest developments in magnetic gear technology
Magnomatics has been working hard on its
magnetic gear since we last reported on it in
Industrial Technology and there have been a
number of new innovations that extend the
applications potential of this innovative technology. The
range has been extended to include fixed-ratio magnetic
gears, magnetically-geared motor generators (PDD) and
most recently a magnetic continuously variable
transmission (MAGSPLIT).
In operation, as with any gear technology, one shaft
rotates at a different speed to the other. However, instead
of using meshing teeth to connect the shafts and transmit
mechanical power, this gear uses the magnetic field of
powerful rare earth magnets. It has no contacting parts,
and so does not require lubrication. This means that the
gear offers inherently high reliability, with little
or no requirement for maintenance. The
gear is also more efficient because there
is no friction.
A magnetic gear is directly
analogous to a planetary, or
epicyclic, gear and consists of three
rings. The outer and inner rings have
powerful magnets, arranged in an
alternating north-south pattern,
whilst the middle ring consists of steel
segments which alter the magnetic
field between the inner and outer
magnet rings.
The inner ring consists of a low number
of magnets and is connected to the high speed
shaft. The middle ring consists of a number of steel
segments held within a mechanical structure and is
typically connected to the low speed shaft. The outer ring,
consisting of a high number of magnets, is usually held
stationary in conventional gear applications. There is no
physical contact between any of the rings as the motion
ICE
MAGSPLIT mCVT
Motor/Generator
(Right) Illustration of Magnomatics magnetic gear design.
(Middle) Magnomatics Pseudo Direct Drive (PDD)
(Bottom) Block diagram of MAGSPLIT for hybrid vehicles
is transferred across an air gap using the force of
the magnetic field.
The steel pole segments act as flux paths from
each of the rings of magnets. This has the effect of
creating harmonics in the fields produced by each ring of
magnets. By careful selection of pole numbers the magnet
arrays can couple via the harmonic fields creating a gear
ratio. The behaviour of this gear is exactly the same as a
mechanical epicyclic gearbox, and can provide a high
performance replacement for conventional
epicyclic gears in
demanding applications.
In high-torque applications,
torque densities comparable with
mechanical gears can be achieved with an efficiency of
greater than 99% at full load and with much higher part
load efficiencies than a mechanical gear. Magnetic gears
inherently protect against overloads by harmlessly
slipping if an overload torque is applied, and
automatically and safely re-engaging when the fault
torque is removed.
An evolution of the fixed-ratio magnetic gear was
realised by introducing a stator with electrical windings
around the gear. This creates a very
compact motor or generator that is
exceptionally powerful at low speed
operation compared to other technologies. This is
Magnomatics’ pseudo direct drive (PDD) and is said to be
ideal where a high speed motor would traditionally be
used with a mechanical gearbox.
This new type of electrical machine is able to drive a
large load without the use of a mechanical gearbox. It
offers a significant size reduction over conventional direct
drive machines, whilst the ultra high efficiency reduces
the need for ancillary cooling. Power factor is typically
To differential
greater than 0.9, and the PDD provides the same inherent
torque overload protection and reduced maintenance
requirements as the magnetic gear. Importantly, the PDD
is controlled using standard power electronics.
Most recently Magnomatics has extended the
magnetic gear concept to create a variable magnetic gear
which can be used as a power-split device within a
hybrid vehicle.
In a MAGSPLIT unit a wound stator is introduced,
the outer ring of magnets has a low number of pole
pairs and so becomes the high speed rotor, something
that is not possible with an epicyclic gear. This outer
high speed rotor is now free to rotate. By controlling
the rotational speed of this outer ring of
magnets one can control the gear ratio of the
inner magnetic gear. The ratio can be
continuously adjusted and even set such that the output
shaft is stationary or reverses. This is not a passive device
since power flow is always required through the stator,
either as a source or sink. Typically the maximum power
required in the control stator is 25% of the power being
transferred through the powertrain.
Applications for MAGSPLIT include hybrid vehicle
powertrains for cars, buses, trucks and off-highway
vehicles. There are already many vehicles with powersplit
drivetrains, e.g. the Toyota Prius, but these are
achieved using mechanical epicyclic gears in conjunction
with a motor generator. In recent simulations
Magnomatics has predicted that the MAGSPLIT will
provide somewhere between a 3% and 5% improvement
in fuel economy over a mechanical power split device.
In July this year, Magnomatics received a £100,000
Smart Award to develop a second generation of the
MAGSPLIT technology. The solution is attracting a lot of
interest from vehicle manufacturers and Magnomatics is
currently working with a leading global car manufacturer
on a project funded by the Technology Strategy Board
(TSB) to evaluate the application of the technology in
hybrid cars.
In a separate project, also funded by the TSB,
Magnomatics is working with a large, internationally
renowned truck and bus manufacturer to evaluate the
application of MAGSPLIT for larger commercial
vehicles.
www.magnomatics .com
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INDUSTRIAL TECHNOLOGY • July 2013