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Patent of the Month – Hysteresis Bearing-less Motor

Patent of the Month –  Hysteresis Bearing-less Motor

| On 09, Jun 2019

Darrell Mann

It was a close-run thing for our Patent of the Month award this month. We nearly gave the honour to US10,177,688 from a joint university/industry foundation in Korea. It offers the world the potential for a really simple, very elegant tidal energy collection device. The core of the invention is the deployment of a smart ionic polymer-metal composite that eliminates the need for moving parts. Always a good idea in a device that has to survive hostile environments for long periods of time without any maintenance. So far so good. The only problem is that, while the inventors have solved the difficult problem, all of the peripheral energy conversion stuff makes use of conventional non-seaworthy kit which kind of negates the value of the main inventive steps. Hopefully, if/when they go to the commercialization stage, the transition out of academia and into the ocean, it becomes a very resilient technology. In the meantime, it felt like we ought to award the Patent of the Month to a different team, from another university we’ve featured several times in this part of the ezine, MIT. Their patent, US10,177,627, was granted on 8 January.

A recent paper published by the inventors is probably more readable than the invention disclosure (you can find a copy here: https://core.ac.uk/download/pdf/141472834.pdf). That said, the background section and its description of the problem is kind of coherent for the non-lawyer:

As is known in the art, bearingless motors levitate and drive a rotor with a single stator unit. This approach can eliminate mechanical bearings in a compact form factor.

As is also known, bearingless motor technology has drawn international research efforts, which has led to developments of bearingless motors of various types. Bearingless motors are found to be particularly useful in applications such as blood pumps and pumps for high-purity chemical processes. Bearingless slice motors are particularly suitable for such applications. Bearingless slice motors levitate a pump impeller passively in axial and tilting directions and actively in two radial directions. The passive levitation is realized with reluctance forces generated between a soft-magnetic stator and an impeller comprising a permanent magnet. Active levitation, on the other hand, is realized with feedback control. Gruber et al. (2015) developed a bearingless slice motor that drives a reluctance rotor. In such an embodiment, the magnet is eliminated from the rotor and placed on the stator to create a homopolar bias flux for passive stabilization of the rotor in axial and tilting directions.

Replacing a reluctance rotor in bearingless slice motors with a hysteresis rotor enables the advantages from hysteresis motors, such as robust and simple rotor construction, smooth torque generation, and smooth transition from asynchronous to synchronous operation. These advantages make the homopolar flux biased, hysteresis bearingless motor described herein suitable for a wide variety of applications including, but not limited to: high-speed rotary applications, ultraclean pumping systems and/or blood pumps that require disposable impeller replacement. The operating principle of a homopolar hysteresis bearingless motor as described herein can be best understood as a combination of a flux-biased magnetic bearing and a hysteresis motor.

With this particular arrangement, an electric drive to pump a fluid by rotating a magnetically-levitated hysteresis rotor is provided. Utilizing homopolar flux biasing decouples the force and torque generations, provides force generation independent of rotor angular position, and provides force/current for suspension which are higher than prior art approaches, and suspension force linearization. Such a pump finds use in a wide variety of applications including, but not limited to, blood pumps, ultra clean pumping systems and high speed rotary applications. Since no mechanical connections (such as bearings and shafts) are involved for the impeller suspension and torque generation, pumps provided in accordance with the concepts described herein impose less stress and heat on a fluid than in prior art approaches. Therefore, pumps provided in accordance with the concepts described herein are particularly advantageous for pumping delicate fluids such as biological samples. Such pumps can thus be used as blood pumps to reduce the level of hemolysis and thrombosis.

If we summarise this by saying reluctance motors solved the ‘eliminate lubricant’ problem by evolving (per the Dynamization trend) from fluid to field, the new ‘hysteresis’ motor solves the contradictions that emerge from that reluctance motor technology. Which is that the reluctance motor, in order to deliver the required torque, currently needs a complex arrangement of sub-systems and therefore finds it difficult to achieve good robustness. Here’s how that combination might best be mapped on to the Contradiction Matrix:

And here, from the paper rather than the patent, is how the invention solves the conflict:

[The figure at the top of this article] shows the design of our bearing-less motor and the prototype system. The stator mainly consists of a bottom plate, twelve stator teeth (Principle 1), twelve combined windings, and a flux biasing structure comprising a permanent magnet and a flux collector (Principle 4). The stator teeth and the flux collector form an annular groove (Principle 3, 14), in which a ring-shaped rotor is inserted. The permanent magnet provides a bias flux through the hysteresis rotor in the radial direction. The level of bias flux can be adjusted with an iron core inserted into the permanent magnet (Principle 24). The ring-shaped rotor is made of D2 steel (Principle 35), which has some favorable characteristics for our design: First, its permeability is relatively high and therefore advantageous to generate reluctance forces. Second, D2 steel exhibits some level of magnetic hysteresis (Principle 4), which can be utilized to generate a hysteresis torque. The operating principle of a homopolar hysteresis bearingless motor can be best understood as a combination of a fluxbiased magnetic bearing and a hysteresis motor. We next discusses the force generation mechanism of a fluxbiased magnetic bearing as applied to our bearingless motor design:

[The next figure, below] shows across-sectional side view of our bearingless motor. Here, the ring-shaped rotor forms two air-gaps with the stator: an outer air-gap with the stator teeth, and an inner air-gap with the flux collector. The homopolar bias flux (dashed blue lines) from the permanent magnet traverses through the hysteresis rotor radially outwards and returns via the stator bottom plate. As the stator superimposes 2-pole suspension flux (solid red lines), the net flux density in the outer air-gap weakens in area (L) and strengthens in area (R), thereby generating a differential reluctance force on the rotor to the positive x-direction. The direction of the suspension force is determined by the 2-pole flux direction, and the magnitude of the suspension force is determined by the strength of the 2-pole flux. Therefore, by controlling the direction and magnitude of 2-pole suspension flux based on the position measurements, we can actively stabilize the rotor translations in any radial direction. Due to the homopolar bias flux, other degrees of freedom, i.e., translation along the z-axis and tilts about the x- and y-axes, are passively stable.

As ever, ‘the field’ wins. Asymmetric fields win bigger.

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