A motor is a machine which transforms electrical energy into mechanical energy. Based on the direction of the magnetic field, motors are classified into two categories : First is the radial flux motor (Induction motor), which is used in 99% of applications today, and second is the axial flux motor. In this article I am going to focus on the axial flux motor and its differences with the radial flux motor. so let’s start. Axial flux motors are considered the ultimate future of electric vehicles and most importantly of electric aviation because they have a high torque-to-weight ratio, which is ideal for aircraft.
In a radial flux motor, the magnetic flux is perpendicular to the axis of rotation. In an axial flux type the axis of rotation is parallel to flux lines as shown in fig 1.
When I energise the coil, they become electromagnets. The axial motor’s operation depends on the interaction between the permanent magnet and the electromagnets. In the design of axial flux motors have fixed coils and freely rotating permanent magnets are the Axial flux motor’s coil arrangement, as shown in fig 2
When I energised coil A with DC current, the S pole of the rotor is attracted to the stator’s opposite N pole(refer fig 3a). Simultaneously, the like poles repel. The tangential force components make the rotor rotate(refer fig 3b). When the rotor aligns with coil A, the net force acting on the permanent magnet becomes zero.
So now what do you think(refer Fig 3b)? Because of zero net force Will the rotor motion stop at this point? No.The rotor’s speed or the inertia effect causes it to travel ahead of the perfect alignment angle. During this time, the next coil B gets energised(refer fig 4). The rotor then reaches near to coil B because of the same forces of attraction and repulsion.
Later, coil C gets energised. After that, in the next half-rotation, coil A energises again, but this time with the opposite polarity, just by changing the supply directions. The process constantly repeats and the rotor continues to rotate. However, in this operation two coils are always dead(example coil B energised in Fig 4). These dead coils drastically reduce the motor’s power output.
To solve the issue, just energise one more coil pair by simply passing the opposite polarity current through the second coil. at the stator side two south poles are together. Here again a net tangential force is developed (refer Fig 5). The combined effect produces more torque and power output from the rotor. Interestingly, this process also ensures that the motor has a constant torque output.
How will you decide which coil to energise to get continuous rotation?For this purpose, I use a smart electronics controller. The sensor determines the rotor’s position, and based on this information, the controller decides which coil to energise.
The first differentiating factor is the flux flow path in the machines. When I compare the magnetic flux pattern of both the motors, I observe that the axial motor’s flux flow path is much more dense and shorter when compared to the induction motor.
To confirm the flux paths let’s take help from FEA. Here I have taken an FEA result in EMWorks 2D software of induction motor as well as axial flux motor. As you know, the interaction between the stator magnetic field and the rotor generates torque. In radial flux motors some field loops rarely interact with the rotor as I have shown in fig 6. On the other hand, in the axial motors the majority of the flux lines lie in the useful work area, to generate torque.
For this reason, the axial flux motor gets a higher density of flux, thus generating a higher torque output from the same size of the motor.
The second differentiating factor is the larger diameter. In axial flux motors, the rotor magnets can be located further away from the central rotating axis. a larger radius allows the motor to generate more torque(refer fig 7).
Made it clear by this simple torque equation.
Torque = Radius X Force
However, if I try to increase the diameter of the induction motors, the rotor’s inertia will increase, which can lead to the motor sniffing up huge currents during the start.
Axial flux motor’s keep this issue under control, as the rotors already have less inertia. These lightweight motors are the best choice for electric aeroplanes. So because of an incredible efficiency level and their compact size axial flux motors can even become a good choice in electric cars.
I hope you enjoyed this Article. Thanks for reading!