Why Choose Axial Flux Motors for EVs?

Introduction

Axial flux permanent magnet (AFPM) motors offer superior power, efficiency, and torque density with their unique flat rotor and stator arrangement. Originally developed for vehicle applications, these motors have seen significant advancements in materials, making them even more powerful and compact. Ongoing research is focused on improving their performance, reducing their costs, and expanding their use in various applications. Consider the benefits of using an AFPM motor in your electric vehicle, with its excellent performance, efficiency, and unique design. AFPM motors are quickly becoming the go-to choice for electric vehicle manufacturers due to their flat rotor and stator arrangement and ongoing research into improving their performance and cost-effectiveness.

Magnax’s Axial Flux in-Runner Motor Design [1]

Design and Construction

Types of AFPM Motor Designs

There are several types of AFPM motor designs, each with its unique features and advantages. Here is a comparison of some of the most common designs:

• Single-rotor design

  o      Simplest and most common design
  o      Typically used in smaller and less powerful applications
  o      Lower cost and easier to manufacture
  o      Limited in terms of power output and efficiency

• Dual-rotor design

  o      Has two rotors on either side of a single stator
  o      Allows for a more powerful and efficient motor
  o      Higher torque and power output than a single-rotor design
  o      More complex and expensive to manufacture
  
  
  
  

Dual Rotor AFPM Design Using EMWorks Electromagnetic Simulation Software [2]

• Segmented design:
  

  o      Rotor and stator are both divided into segments
  o      Permanent magnets are placed in the spaces between them
  o      Allows for more flexible control over the magnetic field and reduces the risk of magnetic flux leakage
  o      Better heat dissipation and more resistance to demagnetization
  
  o      Higher cost due to the segmentation of the motor

• Halbach array design:
  

  o      Uses a specific arrangement of magnets to create a stronger magnetic field on one side of the rotor and a weaker magnetic field on the   other side
  o      Can be more efficient than other designs
  o      More complex and expensive to manufacture

Each design has its trade-offs in terms of performance, cost, and complexity. The selection of the design depends on the specific requirements of the application.

Materials used for the Construction of AFPM Motors

AFPM motors are typically constructed using the following materials as shown below:

AFPM Structure Diagram [3]

• Permanent magnets:

• Laminations:
  

• Winding materials:
  

• Structural materials:
  

• 3D printing: is being increasingly used in manufacturing motor components such as stators, rotors, and end caps as shown in the figure below. This technique allows for highly precise and complex geometries to be created, and can also reduce manufacturing time and costs.

• CNC Machining: Computer numerical control (CNC) machining is a highly precise manufacturing technique that is commonly used for the production of motor components such as stators and rotors. This technique can be used with a variety of materials, including steel, aluminum, and copper.
• Casting: Casting is a manufacturing technique that involves pouring molten metal or plastic into a mold to create a component. This technique can be used for the production of motor components such as end caps, housings, and frames.
• Assembly Techniques: AFPM motors are typically assembled using techniques such as gluing, clamping, and press-fitting. Adhesives and epoxies are often used to bond motor components together, while clamps and presses can be used to ensure proper alignment and fit.
• Automated Manufacturing: In large-scale production, automated manufacturing techniques such as robotic assembly lines and automated testing can be used to improve efficiency and reduce costs.

• Direct torque control (DTC): DTC is a control method that uses hysteresis controllers to regulate the torque and flux of the motor. This method provides fast and accurate control of the motor but can be less efficient than other methods.
• Field-oriented control (FOC): FOC, also known as vector control, is a control method that controls the magnetic field of the motor using a mathematical model. This method provides accurate control of the motor and can be more efficient than DTC.
• Model predictive control (MPC): MPC is a control method that uses a mathematical model of the motor to predict its behaviour and adjust the control inputs accordingly. This method provides accurate and efficient control of the motor, but can be more complex and require more computation power than other methods.
• Sensorless control: Sensorless control methods use advanced algorithms to estimate the position and speed of the rotor without the use of sensors. This can reduce cost and complexity, but can also be less accurate than sensor-based control methods.

Challenges

1. Advanced materials and designs: Researchers are exploring the use of advanced materials such as high-strength composite materials, nanomaterials, and high-temperature superconductors to improve motor performance. Novel motor designs, such as double-sided and modular stator configurations, are also being investigated to improve power density and reduce manufacturing costs.
2. Integration with power electronics and battery management systems: To optimize the performance of AFPM motors, they need to be closely integrated with the vehicle's power electronics and battery management systems. Researchers are developing new control strategies, such as model predictive control, to optimize motor performance and improve efficiency. They are also exploring the use of advanced power electronics such as silicon carbide (SiC) and gallium nitride (GaN) devices, which offer improved switching speeds and lower power losses.
3. Magnetization and demagnetization techniques: Researchers are investigating novel magnetization and demagnetization techniques for AFPM motors. For example, pulse magnetization techniques can be used to improve magnetization uniformity and reduce the need for magnetizing fixtures. Demagnetization techniques are also being developed to reduce the risk of magnetic field interference in sensitive electronic equipment.

Future Developments

The prospects and potential of AFPM motors in the field of electric vehicles are very promising. AFPM motors offer several advantages over traditional radial flux motors. Ongoing research and development, the performance of AFPM motors is expected to improve even further, making them a preferred choice for electric vehicle applications. The demand for electric vehicles is growing rapidly, driven by increasing concerns about the environment and rising fuel costs. AFPM motors are well-suited for electric vehicles because they offer high efficiency and reliability, which is critical for achieving long driving ranges and reducing operating costs. Additionally, AFPM motors can be designed to operate at high speeds, making them suitable for a wide range of vehicle applications, from electric cars and buses to electric motorcycles and scooters.

As the electric vehicle market continues to grow, the demand for AFPM motors is expected to increase significantly. According to a recent report by Allied Market Research, the global market for AFPM motors is expected to grow at a compound annual growth rate (CAGR) of 12.6% from 2021 to 2028, driven by the increasing demand for electric vehicles and growing investments in renewable energy. Overall, the future of AFPM motors in the field of electric vehicles looks very promising. With the ongoing advancements in materials, designs, and control strategies, AFPM motors are expected to offer even greater performance, efficiency, and reliability, making them a key technology for the continued growth of the electric vehicle market.

Case Study – EMS

• No-load analysis:

The double-sided rotor coreless axial flux generator with 24 poles made of N42 permanent magnets, known as the axial flux permanent magnet machine was analyzed using EMS simulation software. This device has the potential to be utilized for both transportation and energy production. The back electromotive force (back emf) is a voltage that is generated in an electric motor or generator as a result of the rotation of the rotor. It is opposite in polarity to the applied voltage and is proportional to the speed of the rotor. As the speed of the rotor increases, the back emf produced by the generator also increases.

Three-Phase Back EMF of AFPM at 1200 rpm

Relationship Between the Back EMF and the Speed of the Rotor

The first figure above shows the three-phase back EMF of AFPM at 1200 rpm while the second presents the relationship between the output voltage and the speed of the rotor. It indicates that this relationship is linear and direct. In other words, the voltage output of the generator increases in proportion to the increase in the speed of the rotor. This is a fundamental concept in electrical engineering and is essential in understanding the behavior of electric motors and generators.

The linear relationship between the back emf and the rotor speed is a critical factor in the design and operation of electric machines. It is used in the control and regulation of motor and generator systems, and it also helps in optimizing the efficiency of the system. By understanding this relationship, engineers can develop more efficient and effective electric machines that can meet the demands of modern technology.

• Electromagnetic loss analysis:

When an electrical machine operates under a load, it experiences electromagnetic losses. These losses include iron and copper losses, which can have a significant impact on the efficiency of the machine and increase maintenance costs. Therefore, it is essential to study these quantities carefully during the design phase of any electrical machines, including motors and generators.

EMS software is utilized to predict the electromagnetic losses in the machine. The software allows engineers to calculate the various losses in the machine and analyze the performance of the device under different load conditions.  

Winding losses are one of the types of losses that can occur in a machine. They are caused by the resistance of the winding and the current flowing through it. The graph below displays the results of the winding losses calculated by EMS for the machine under consideration.

Winding and Core Losses of AFPM

The core loss is another type of loss that can occur in the machine, which is due to the hysteresis and eddy current losses in the iron core. The core loss results are shown in the figure above, and they indicate that the core loss is high in this machine due to the relatively high-frequency voltage generated by it. Understanding and analyzing the electromagnetic losses in a machine is crucial in the design and optimization of electric machines. By minimizing these losses, engineers can improve the efficiency of the device and reduce maintenance costs, making it more economical and sustainable.

• Mechanical Stress Analysis: