Don’t miss this presentation at MAGNETICS 2018, February 8-9 in Orlando.
Lowell Christensen, Consultant, Lowell Christensen LLC
Axial flux motors are a very good candidate for remote controlled battery powered applications. This includes the autonomous vehicles used in land, sea and air applications. These types of Remote Controlled Vehicles started as hobby vehicles but the commercial applications are becoming one of the fastest growing applications of the future. The aerial drone is currently the most popular or these applications and has become very popular in aerial photography. The land and sea applications are still more of a hobby type but will also become a large part of future autonomous applications. There is a lot of activity in using drones for commercial delivery applications. This article will be based on the drone applications but the information would still apply to the land and sea vehicles. There are basically two important parameters for the autonomous vehicles, load capacity and travel distance. This will mean that the drone motors will need a very low weight and a very low resistance to maximize the delivery distance and load. The weight and resistance tradeoffs will be discussed in this article.
These applications will be battery powered and will operate with a low voltage and high current levels. This will cause the motors to have a low voltage constant and a low winding resistance. Most of the battery powered autonomous applications have a fixed speed range but as the loads increase the number of batteries and battery voltage will increase. The Arial Drone application is a special case since the motor speed needs to decrease as the load capability and propeller diameter increase. This is because the propeller tip has a maximum linear speed meaning that the farther out the tip is the lower the rotational speed must be. The lift the propeller produces is a function of the propeller diameter and power supplied by the motor. As the drone gets larger the lift capacity will need to increase. This will require more batteries and battery weight. Minimum weight and minimum resistance are very critical for maximum flight time and load capacity. The low weight will allow a higher load capacity. A lower resistance will have less winding losses to allow more output torque from the motor. Figure 1. shows an example of the changing input conditions as the drone motor diameter gets larger. The battery voltage, Vb, voltage constant, Ke, and rotary speed, Sp, are plotted as a function of the motor magnetic circuit outer diameter. The rotational speed used here was assuming that the propeller diameter was 9 times the motor diameter and the maximum propeller tip linear speed was 6160 inches per second. The condition when speed is not limited by the propeller is shown as the dotted line in Figure 1 for comparison. In the drone input conditions in figure one, the speed will decrease exponentially toward a nearly fixed value while the voltage constant increases exponentially to a maximum motor design limit. The voltage used in this example is increasing linearly because the number of battery cells used was increased linearly with the increase in diameter. The battery voltage variation is not fixed but was used as a linear increase in this example to make the curves easier to understand. The number of batteries is determined by the application and as the drone size in increased more power will be required. Each battery cell will have a 3 to 4-volt range for the lipo batteries used in most drone applications. The dotted lines for speed and Ke show the curves when the speed is not limited by the propeller tip. Under this condition the curves are linear Instead of the exponential condition caused by the limit of the blade tip.
Currently these applications use outer rotation radial flux motors with a very large inner diameter. Axial flux motors can also be designed with a large inner diameter. The axial flux motor is basically two discs separated by an air gap. The flux path is axially across this air gap and then loops thru the disc into the next pole. This means that the flux lines follow a 3D path instead of the 2D path in the radial flux motors. Laminations cannot easily be used in the 3D path so the magnetic circuit material needs to be an isotropic magnetic material. The Soft Magnetic Composite, SMC, materials can be pressed into the 3D shapes needed. Since the parts can be made 3D, they can have features added to improve the reliability of the motor subassemblies. There are several grades of this SMC material and the Somaloy 700 series has very close parameters to lamination steels. The winding for this motor is a bobbin style winding that is placed around a tooth extending out of the disc for the stator. The magnets are bonded to the other disc. The motor torque is increased by increasing the diameter. The length of the stator is limited to the length that the tooth can extend above the stator base when the part is pressed. The stator and rotor sections can be stacked making the motor longer to get more torque. The Axial flux motor design is a lot simpler in design and the bobbin winding can have a higher copper density with simple tooling and less damage to the magnet wire. The coils are individual parts and can be connected in parallel to give redundancy to the winding. This means the motor can meet the application requirements at a higher reliability and a lower cost than radial flux motors.
The motor winding is usually the highest weight component in the motor. Reducing the amount of copper in the winding will increase the resistance of the motor. This weight verses resistance tradeoff is the biggest criteria of the drone motor design. Since the outer dimension and height of the motor is a given parameter, the weight reduction is done by removing volume from the inside of the motor. The motor performance and efficiency increase as the number of poles is maximized. Increasing the ID is needed in axial flux motors to increase the number of poles. This is because the bottom width of the tooth will determine the number of teeth possible. Increasing the ID will reduce the area for copper and increase the resistance. This increase in resistance will reduce efficiency in low weight motors. The high efficiency would be more of a requirement in land and sea applications. Figure 1 shows a range of input parameters. Column C shows the nominal value used for each parameter. Each row is the parameter variation used to plot each curve.
The curves in figure 2 show the effects of each parameter on the winding resistance. The resistance will drop and flatten out as the outer diameter is increased. This shows that the Axial flux motor will be very effective in diameters of 2.5 inches and above. The resistance also increases exponentially toward a maximum value as the ID to OD ratio is increased. This is because the total area available for copper wire is reduced as the tooth radial length is reduced. As the ratio is increased the number of slots can also increase. These curves show that this effect has the same shape for the two parameters. The ID circumference will determine the number of slots based on a slot width and the minimum width of a tooth.
Figure 3 shows the effects on the weight of the magnetic circuit components as each parameter changes. Again, the outer diameter has the largest effect on the weight. The weight starts at a low value and increases exponentially as the diameter increases linearly. The OD/ID curve will decrease as the value for this ratio increases. This is because There is a smaller volume of material as the ID increases. As the number of slots increases the weight will decrease. This is because as the slots increase less copper will be in the end-turns since each individual slot will be narrower. The curve for the changes in the slot width is almost flat. This is a very small difference in the weight of copper in the slot and the added weight when the slot is made narrower. The high wire fill factor times the density of copper will reduce this density to near the value of the density of the SMC material so it doesn’t matter which material is removed.
The purpose of this article was to show that Axial Flux Motors are a suitable candidate for battery powered autonomous vehicles. This discussion concentrated more in the Arial Drone applications where minimizing weight is more important than high efficiency. The land and sea vehicles are more dependent on the efficiency and interpretation of the curves would be more toward minimizing the winding resistance than the motor weight. This would mean that the only change required for these motors would be to design the axial flux motor with fewer poles and a smaller ID. Doing this would also make this a very good motor for higher voltage fluid control applications such as fans and pumps. The high percentage of wire fill from the bobbin type of wind will give a lower resistance and a better-quality winding. This is probably the most important advantage of the axial flux motor over the radial flux motor. There are a lot more parameters needed in the motor design process. This is just an introduction and overview of the axial flux permanent magnet motors using the SMC powered metal materials. The 3D magnetics capability is a big advantage over the 2D designs of radial flux motor but does give axial flux motors some limitations. In a radial flux motor torque capability can be increased by making the magnetic circuit longer. In the Axial flux motor, the axial height of the stator is limited by manufacturing limits of powdered metal materials. This would mean that the torque capability would be increased by either making the motor larger in diameter or by stacking the magnetic circuit components to make the axial flux motor longer. I will be making a presentation at the Magnetics 2018 Conference in Orlando Florida in February 2018. This presentation will be in comparing a 3.5-inch diameter 1.5-inch long axial flux motor to both inner and outer rotation radial flux motors in the same overall size. This would include more of the design specifics. I would like to thank Symmco Corp and Hoganas Corp for the help in understanding the use of Soft Magnetics Composite Material in Axial Flux Motors. This was very helpful in coming up with the graphs of the Axial Flux motor designs.