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Double-rotor or Double-stator: a Matter of Efficiency

If you’re familiar with electric motors, you’ve probably noticed that not all axial flux motors are the same. Traxial opted for a single-stator double-rotor (SSDR) configuration, while other axial flux motor developers use a double-stator single-rotor (DSSR) disc-type structure. Maybe you’ve even read about a double-stator triple-rotor set-up?

Outrunner vs inrunner

Before diving into the differences, I’d like to mention that just having the rotors on the outside doesn’t make it an outrunner. Inrunners and outrunners refer to whether the housing (the outside) of the motor is spinning. Thanks to their formfactor, axial flux motors are perfect for both traditional motor-gearbox as in-wheel car concepts. For in-wheel, the housing of an outrunner motor is attached to the wheel directly in this case. Both the housing of the motor and the wheel will be moving simultaneously. On the other hand, Inrunners have static housing and will make a shaft turn instead of the housing. Usually, outrunner motors rotate at lower speeds, in many cases the same speed as the wheel, while inrunners have the ability to rotate at higher speeds (>10.000 rpm) which means that a (small) gearbox is required to reduce the speed to wheel speed.

Magnax Motor Render April 2021Traxial Inrunner

Double-stator vs double-rotor

The setup of the axial flux motors (double-stator or double-rotor) has nothing to do with inrunner or outrunner configuration. Both double-stator AF motors and double-rotor AF motors can have both an inrunner or outrunner configuration.

In this article we’ll focus on double-stator and double-rotor disc-type, sometimes referred to as I-type and H-type. The first difference you’ll spot when looking at the image below is that the double-stator motor has a yoke on every stator. This is different from the dual-rotor configuration. What basically happened is that in order to create a yokeless AF motor, only one stator is used and the of yoke (which fixes the teeth) is removed.

A yoke:

  • Fixes the cores to the windings and is usually made in one piece during manufacturing
  • Guides the magnetic flux
  • Usually has an integrated cooling channel to cool the stator

The yoke is a massive piece of iron that ceases to exist when switching to an H-type (double rotor). But wait. Don’t you always need to close the flux loop somehow? Sure, a yokeless motor only refers to the stator being yokeless. It doesn’t tell you anything about the rotor discs. They can have an iron plate that fulfills the function of a yoke and guides the flux.

How does the magnetic flux path look for both set-ups?

In a dual-stator configuration (or a yoked motor) the magnetic flux flows from one stator to the other and crosses the rotor.

double-stator DSSRYoked AF or dual-stator

In this set-up, with a spinning rotor and two static yoked stators, the alternating magnetic field in the stator causes iron losses.

Advantages of yokeless axial flux motors

With a yokeless axial flux motor, the spinning rotors and their iron plates are moving in the same direction and at the same speed as the magnetic field. This lowers the iron losses significantly. Less iron loss means greater efficiency. This adds up to the lower iron losses because we don’t have a stator yoke.

double-rotor SSDRYokeless AF or dual-rotor

There’s a second reason why yokeless axial flux motors experience a higher efficiency. The coils have concentrated windings. Distributed windings are often used for dual stator motors and connect multiple coils. Hence, there’s more copper involved, which not only increases the weight but also the copper losses.

distributed windings

Distributed windings of JY Stator

In addition, the absence of a stator yoke reduces the weight. This results in higher power density.

So why are double-stator motors still being manufactured?

Because they’re easier to manufacture. It’s hard to find a solution without a yoke that holds together the different elements of the stator and still allows effective cooling of the coils.

At Magnax, we’re developing our motor so that the production process allows for automated assembly. We use a patented mechanical system that makes a very robust and strong stator without yoke possible, and where the coils can be cooled directly. The oil flows along the copper wires of the coils.

This enables the most effective evacuation of heat in the same amount of time, making it possible to send more current through the windings. And this has a positive impact on both power density and torque.

In addition, the modularity of our motor allows for stacking which easily doubles or triples torque – at the same footprint of a radial flux motor!

Interested in testing our AF Inrunner motors soon? Leave your info here and we’ll let you know as soon as we have our B-samples ready.

This Post Has 2 Comments

  1. Hi Jeff. Enjoyed your paper on double axial flux motors. I’m developing a vertical axis tidal stream energy extractor turbine, called the Hales water turbine. This turbine sits under the water and the reaction blades(x 6) rotate in the vertical axis and has a slow speed, high torgue. The diameter of the cylinder that holds the blades are 1 metre on the smallest turbine and up to 4 metre on the largest (so far). I was wondering if you would be interested in doing a preliminary study into an axial flux electrical generator to go on top of the turbine ?

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