Radial flux permanent magnet motor
Companies in today’s market are looking for ways to make their systems more efficient in terms of performance and cost. As such they are looking for ways to improve their system efficiency and for the optimal electric motor to integrate into their application. A common question that arises, in the search for best fitting electric motor, is the choice between a so-called radial flux or axial flux electric motor. At Magnetic Innovations we believe strongly in the radial flux permanent magnet motor topology. In this article we will explain why.
What is flux in a motor?
Let’s start by clarifying what the word ‘flux’ means. It refers to the magnetic flux which is present in electric motors and is caused by the magnetic field of the permanent magnets. The magnetic flux is measured in unit tesla (T). The magnetic flux is normally imposed by multiple permanent magnets. These magnets are often made of rare earth material, therefore they are called rare-earth permanent magnets. With sustainability in mind, it is best to reduce the volume of the magnetic material used in electric motors as much as possible.
What is the difference between an axial and radial flux motor?
The name ‘radial’ or ‘axial’ originates from the direction in which the permanent magnets impose the magnetic field. When the flux is imposed in the radial direction the motor is called a radial flux motor. When the flux is imposed in the axial direction, it is called an axial flux motor. A critical difference between the radial and axial flux motor, is the amount of permanent magnet material that is used for a given performance. An axial flux motor generally has a higher torque density than a radial flux motor. However the axial flux motor generally uses significantly more permanent magnet material to obtain this higher torque density.
Another important difference between the axial and radial flux motor is manufacturability and ease of construction. The construction of an axial flux motor is complex and involves challenges that are difficult to solve using verified production and construction methods. The radial flux permanent magnet motor benefits from a far more straightforward construction. This translates into an easier installation and a benefit in cost when it comes to the mechanical construction.
What are the advantages of a radial flux permanent magnet motor?
The radial flux motors has a significantly reduced volume of permanent magnets in comparison to Axial flux motors and are therefore more sustainable. They are also easier to install and they are cheaper. The radial flux motors at Magnetic Innovations utilize the outer rotor topology and incorporate a number of optimizations which improve the overall efficiency.
The flux paths of the motors are kept short by using concentrated windings. This helps to keep the winding resistance (=losses) low. A second advantage is the relative large distance/arm between the rotation centre of the motor and the air gap in which the tangential forces are generated. Which result in motor torque. The larger the arm at which a certain tangential force is generated, the larger the resulting torque a motor can deliver.
Important parameters when choosing a radial flux permanent magnet motor
To successfully apply a radial flux frameless torque motor in your application, a thorough understanding of the requirements is crucial. When used correctly, our radial flux motor can achieve efficiencies up to 94%. Parameters to carefully consider are:
- Torque and speed requirements of the motor.
- Torque to speed curve, where the required torque at various speeds are specified. These specification sets are also known as workpoints.
- The thermal limitations of the application and the selected cooling means (air/ forced air or water-cooling).
Using the torque and speed data the resulting power can be calculated for each workpoint using P_(mech ) [W] = T [Nm] * ω [rad/s]. Off course this equation can also be used for determination of the torque required, when speed and power are known for a given workpoint.
Power density and heat removal radial flux permanent magnet motor
The power density of permanent magnet electric motors is considerably higher than that of the classic AC induction motors. However, to obtain a decent power density, an efficient heat removal from the motor internals is critical. When considering a radial flux permanent magnet motor, it is important to address the duty cycle and the thermal path from the stator to the environment.
If the heat generated by the coils can not be conducted away to the environment by means of conduction, forced air (by means of a fan pushing air through the stator windings) or a water cooling sleeve may be considered. Water cooling is best capable to handle high heat loads and therefore can be best utilized when high power densities are required.
Selecting a motor drive for a radial flux permanent magnet motor
Last but not least: the selection of the right motor drive. There are two ways to approach this. Which of these approaches makes most sense depends on the required working points (Torque and Speed) of the application. You can choose a drive by:
- Selecting the drive based upon the parameters of the motor.
The motor requires the correct Voltage and Current ranges to support maximum Speed (voltage) and maximum Torque (current) envisioned for the application. The selected drive should match this Voltage and current under all of the application conditions. It also has to be capable of handling the electric frequency of the motor, which depends on RPM and amount of pole pairs used in the motor.
- Matching the motor parameters to the drive.
The winding of the motor can be selected to fit an available motor drive, but there are limitations. The maximum voltage is limited for safety reasons, at Magnetic Innovations most of our radial flux permanent magnet motors are suitable for handling 3-phase 400 VRMS systems (with a max. 600Vdc bus). But they will also operate in low voltage (24 Vdc bus) applications. This includes Lion or other battery power sources.
In this article we have explained the differences, including benefits and drawbacks, of radial and axial flux motors. At Magnetic Innovations we believe that our outer runner radial flux motors have quite a few advantages in terms of construction, manufacturability, rare earth material content, cost and efficiency compared to axial flux motors.
When considering the integration of a radial flux permanent magnet motor in your application be aware of your working conditions as these will determine your torque and speed requirements, cooling principle and choice of the drive. Engineers who manage to check the boxes of all of these parameters will find themselves on a sensible path to gaining impressive power densities and efficiencies with our frameless radial flux torque motors.