Until now, manufacturers of wind turbines and electric machines always had to find a balance between using generators with gearbox (meaning lower weight, but also lower reliability) and direct-drive systems (higher reliability and efficiency, but higher weight).
A new generation of low-weight and highly efficient direct drive motors and generators are on the horizon and they combine the best of both worlds. This new generation of electric machines is based on recent innovations in Axial Flux Technology.
Electric motors and generators are used in many different applications but it is not always clear for product developers which type of machine is the best fit for their specific application.
In this blog post we'll describe the difference between two different machine topologies: Axial Flux and Radial Flux technology, commonly used in direct-drive electric generators and motors.
In this particular case, we’ll be focusing on the larger diameter machines (high-torque – low RPM), which find application e.g. in wind turbines.
Getting rid of the gearbox
The requirement for high power at low rpm yields a high torque requirement (remember; power = speed*torque).
To achieve this, most applications have historically typically been using an induction motor with a gearbox.
These motors can only provide their rated power at high speed (e.g. 1500rpm) and low torque, so the gearbox is necessary to divide the rotational speed by the reduction ratio, thus acting as a torque multiplier.
Common best practices however, following the trend of maintenance reduction and reliability optimization, are moving away from motors or generators with gearboxes. Gearboxes have a lot of rolling and sliding parts, resulting in inevitable wear and tear, and an inherent decrease in efficiency. This is why the technology is evolving to "direct drive" machines.
Direct drive (DD) machines are capable of generating the required torque directly at their output shaft, without the need for a gearbox.
Here you can clearly see the difference in size between a traditional induction motor/generator with gearbox and an axial flux motor/generator as developed by Magnax. The difference with traditional direct-drive systems (radial flux) is even more significant. Radial flux direct-drive machines weigh at least 4.000kg (see further).
Radial Flux vs Axial Flux
Since the year 2000, we see an evolution in the direction of motors and generators fitted with permanent magnets in the rotor. (usually referred to as Permanent Magnet Synchronous Machines PMSM)
In any motor, interaction of the magnetic fields of the rotor and the stator create the rotational force that drive the machine.
Permanent magnets generate a constant magnetic field, and thus generate a flux by design. As a result, the power density and dynamic performance of PMSM machines is higher compared to induction motors where this magnetic field must be "induced" via electrical current.
Permanent Magnet machines can be divided in two different construction topologies which are shown in the picture below.
Radial Flux Motor
Most Permanent Magnet Motors and Generators, used today, are radial flux machines. We talk about "radial" flux because in this type of machine, the magnetic field, or flux, runs radially with respect to the direction of the rotor shaft.
For radial-flux machines, the process of manufacturing is well established since it has been gradually optimized through the enormous amount of induction machines manufactured during the 20th century.
The radial-flux permanent-magnet machine can be produced with basically the same manufacturing methodology as the induction machine, since the layout for both machine types are very similar. This is one reason why we have seen a natural evolution from induction machines to radial flux permanent magnet machines, but this doesn't mean that they are the optimal solution for all cases.
More Torque, Lower Weight: Axial Flux Motors
A number of companies have been working on axial flux machines in the past, but very few succeeded in developing a commercially viable product. At least until now.
There are four main technical challenges to overcome, regarding the construction of axial flux machines:
Effective positioning of the stator and windings, which is particularly difficult to achieve in an axial concept. This is mainly due to the magnetic forces of attraction over longer distances, while the air gap must be maintained at around one millimeter.
Effective cooling of the windings, which is a challenge since they are located between the rotor disks.
A difficult process to manufacture the axial flux machine stator, which is also very challenging to automate.
Flux calculations are particularly difficult to do. Solving these issues requires a huge amount of FEM simulation and engineering. Parameters such as terminal voltage, iron losses, eddy current losses in the permanent magnets, copper losses, torque ripple and cogging torque must be calculated before you can even start building a working prototype machine.
But when these challenges can be overcome, the benefits are particularly huge. Axial flux machines have some vastly superior characteristics when compared to the traditional radial flux machines.
The main reasons why axial flux topologies are better by design are:
- Higher torque due to a larger distance between the magnetic field interaction and the machine shaft/axis, with an equal outside diameter of the machine. Compare it to a lever: One can generate more torque with a longer moment arm.
- The flux paths are much shorter. As a result, axial flux machines have fewer magnetic losses and thus higher torque and efficiency.
To design the perfect axial flux machine, calculation- and simulation tools to define the design specifications and parameters have recently been developed.
In the meantime, a new concept for effective positioning of the cores and cooling of the windings has been invented by Magnax.
Magnax has, for the past years, been working intensively on a patented technology, which not only fully addresses the technical challenges, but also allows for a highly automated manufacturing process of their axial flux machines.
Best of both worlds
We already mentioned that axial flux machines have a much higher torque density than radial flux machines. But how does this translate into benefits for companies using these technologies, like wind turbine manufacturers? The answer is obvious, if we look at this from another perspective: axial flux machines are much lighter and more compact, for the same power and torque.
Let’s take a real-life example, for instance a generator in a 100kW wind turbine. We see the following numbers:
Weight and size for 100kW direct-drive wind turbine
Radial Flux PM
Axial Flux PM
Axial Length (mm)
Comparing both 100kW PM generators, the overall length is reduced with a significant factor of 5 or more, while the weight can be reduced with a factor of 4.
For wind turbines, reducing turbine mass is an ever-present aspiration for engineers, as it facilitates the ability to build taller wind turbines which are able to harness more powerful winds at greater heights. So, by lowering generator mass, towers and foundations supporting nacelles will also be engineered using less steel and concrete, thereby saving on manufacturing costs.
One can expect that this will open huge opportunities for future wind turbine designs.
Obviously, the birth of this new technology brings new possibilities for many other cleantech industries, too. The advantages of axial flux motors do also apply to other use cases such as elevators (no more need for a machine room, the motor can be placed inside the elevator shaft), electric vehicles and helicopters (lower mass means more battery life), direct drive production machinery, and so on.
100kW Axial Flux PM Electric Machine as developed by www.magnax.com
So to summarize, Axial Flux Permanent Magnet Machines are the better choice for applications where:
The axial length must be small
Very high efficiency is necessary
Highest torque density is needed
Weight must be kept as low as possible
More information can be found on the website www.magnax.com or contact us directly (+32 471 78 38 04)