Paradigm Shift: Yokeless Axial Flux Motors & Soft Magnetic Composites.

We’ve discussed the advantages of axial flux motors over other types of motors in detail, particularly in relation to the benefits soft magnetic composites (SMCs) have over traditional materials, such as laminations. Now, we’d like to introduce the advantages of  yokeless axial flux motors and how pairing them with SMCs can bring your electric motor design to a whole new level. 

What Is the Difference Between Axial Flux & Yokeless Axial Flux Motors?

Axial Flux Motor

In an axial flux motor, the gap between the rotor and stator – or the flux between the two – is parallel to the axis of rotation. It has a greater use of the copper windings and no end-turns, which leads to an increase in the number of turns and decrease in heat generation. 

As you can see below, the yoke connects all the stator teeth together. The flux goes up one tooth, down the other, and through the yoke.

Yokeless Axial Flux Motor

As the name implies, a yokeless axial flux motor doesn’t have a stator yoke. Due to the absence of a yoke, the stator teeth need to be connected together in an alternative way. In a yokeless axial flux motor, the stator is made of individual stator teeth that are not connected to any back iron. Magnets are then placed on the top and/or bottom of a rotor which interacts with the flux created by each individual segment. 

In a yokeless axial flux motor, the flux jumps from the stator tooth to the magnets and back to the next stator tooth. Advantages of this design are the ability to have the flux interact with the two sets of magnets, thus significantly increasing the power density of the design – meaning less weight, less current, less losses, and more power.

Why Choose Yokeless over Regular Axial Flux Motors?

If you follow electric motor trends, you’ve probably heard a lot from YASA and Magnax about yokeless axial flux motors. So what makes them superior?

The design of a yokeless axial flux motor allows for higher torque density, with reduced weight, space volume, and electric consumption. Some real-life applications of this are:

• In a drone, higher torque density means reduced weight in your motor, which means higher potential payload of the carried goods.
• The reduced weight in an electric car motor would allow for further distances without charging.
• Hand Tools made with yokeless axial flux motors would be more lightweight than their current radial flux design, creating a hand tool that can be held without fatigue.
• In refrigerators and other household applications, such as HVAC units, a yokeless motor would use less energy, leading to lower electric bills. 
  

Yokeless axial flux motors may also have other advantages, such as:

• Easier fabrication and winding 
• You could eliminate the bobbins and wind directly onto the stator tube to achieve a higher winding density and eliminate the potential air gap effect of the bobbin.
  
  

Are Laminations Your Best Alternative?

So the question is, can you make a yokeless axial flux motor with laminations? The answer is technically yes – but is it really worth it?

• Laminations don’t allow a 3d shape or 3d flux path.
• Using laminations in a yokeless design can be very expensive. Imagine having to stamp out a series of differing shapes and assemble them to form the body and caps – a manufacturing nightmare!
• To hold the laminations together, you’d have to weld or stake them, which would generate more heat in the motor – which you’re going to have to cool to get rid of. As you’ll see in the next section, with SMCs, this wouldn’t be a problem.
• Assembling laminations create inherent air gaps, thus lowering the saturation induction of the assembly compared to the performance of an individual sheet.
  
  

Are SMCs a Better Alternative?

Using soft magnetic composites in a yokeless axial flux motor drives down cost while elevating performance. Let’s explore some of the unique advantages that SMCs offer in this motor design:

Higher Saturation = Higher Torque

As you can see below, using SMCs can provide your axial flux motor with higher saturation, leading to higher torque:

Shape Making Capabilities

The net-shape making capabilities of SMC materials are unparalleled by other processing alternatives. Your unique shape is easily created through powder metallurgy/SMC, which enables 3-D flux carrying capabilities and rounded corners. 

Reduced Need For Cooling

The torque density of your electric motor is strongly related to the performance of its cooling system. In a yokeless axial flux motor, the cooling inherently occurs through self-ventilation by the rotor discs. We’ve eliminated any end-turn effects, further reducing a source of heat.

The inherent reduction of eddy current core losses with SMCs can further reduce the need for large and costly cooling systems – reducing overall heat while increasing overall performance. 

Simplified Supply Chain

Another benefit of using SMCs for your yokeless axial flux motors is the abundance of available materials in the powder metal market. Unlike lamination materials, which are currently in short supply, powder metal materials are readily available at extremely high quantities with simplified supply chains. 

Ready for Innovation?

When you want high torque, low weight, and effective cooling, think yokeless axial flux motors. The addition of SMCs to your design opens the door to innovation and freedom of design only dreamed about previously. 

If you’d like to learn even more about how soft magnetic composites can improve your electric design, check out our Resource Hub: