Magnetic Torque Tunnel  

 

 

 

 

 

 

Authors: Fred Hunstable, Dr. Andrei Popov, Michael Van Steenburg   

November 5, 2019  

 

   

Key Takeaways  1. The magnetic tunnel combines axial and radial flux magnetic flux patterns  2. All magnetic flux is concentrated within motor to maximize torque  

production  3. Enables a 4 rotor machine  4. Surrounds a toroidally wound stator maximizing slot fill factor  5. Rare earth or ferrite magnet options  6. Air or liquid cooling options  

ABSTRACT  Traditionally engineers have developed motors that harness the magnetic fields in  2D produced either radially or axially from the rotation of the rotors. It wasn’t until  3D flux path motors that exotic topologies were created to maximize torque  production. This paper provides a description of a 3D flux path magnetic tunnel  design combining axial and radial magnetic flux patterns. This design  concentrates the flux within the motor to maximize torque production, which is  particularly effective in producing high torque at low speeds. Various magnetic  tunnel configurations have been developed which combine axial and radial  magnetic flux patterns that surround a toroidally wound stator that can use both  rare earth or ferrite magnets with air or liquid cooling options.  

INTRODUCTION  A conventional motor design in a micro-mobility or EV application can be either in  a radial or axial flux topology. The radial flux motor is more common due to its  familiarity of manufacturing. These motors are designed as an outer rotor type  which is suited for wheel hub motor applications or as an inner rotor type which is  suited for EV applications due to easier thermal management. The best thermal  management a motor can have is in the axial flux topology which allegedly  produces the highest torque density on the electric motor market today, although  these motors have challenges in the manufacturing process which increases the  production cost.  

 

 

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DESIGN & CONSTRUCTION  Linear Labs uses a different type of design which is a combination of both radial  and axial flux topologies in the same machine. This new topology is called a  “magnetic tunnel” which engulfs a toroidal stator with discrete wound coils  arranged in a ring-arc rotor configuration.   

Significantly greater torque production is achieved by surrounding the stator coils  with magnetic material of the same polarity direction. This topology is comprised  of two radial motors: one inner rotor and one outer rotor in which the stator is  located in the center. Figure 1 shows a 28 pole design (left) and an 8 pole design  (right). The motors share the same stator winding wire gauge with the same  number of turns, in which the same current flows, producing the same slot fill  factor at the same current density. In the front and back of the stator, the two axial  rotors are added.  

 

Linear Labs Motor  28 poles  

Linear Labs Motor  8 poles  

  

Figure 1 - Side view comparison of 28 pole (left) versus 8 pole motor (right)  

 

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Figure 2 - (Left) Magnetic Polarity, (Right) Magnetic Field Density  

The coil assembly is located in close proximity to the four permanent magnet  rotors which reduces the volumetric area taken up by the coil assembly compared  to a conventional machine, but more importantly all of the magnetic field  generated by the coils contributes to torque production.  

IMPLICATIONS ON STATOR DESIGN  Another difference between motors incorporating the magnetic tunnel versus  conventional designs is the discrete coil winding topology. Although this topology  is not new, the radial wound coils or axial capstan coils were preferred in  conventional manufacturing due to familiarity and the existing manufacturing  infrastructure. In conventional designs the coils can be wound as “distributed” for  motors with low pole numbers or “concentrated” for motors with high pole  numbers. In conventional designs the low pole numbers are preferred for high  speed low torque motors applications such as EVs where gears are used, while  high pole numbers are preferred for low speed high torque motors applications  such as industrial motors and micro-mobility where these motors are used in  direct drive applications.  

The energy losses in all motor designs happen in two predominant areas. The first  area is in the windings which is called “copper loss”, “ohmic loss” or “i2R loss”, and  the second area is due to hysteresis and eddy current losses in the stator material  called “iron losses”. In a distributed winding low pole number motor, the copper  loss is dominant, while in the concentrated winding high pole number motor, the  copper loss and iron loss are similar.   

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The discrete windings of a motor using the magnetic tunnel have such flexibility in  connection that they may function as both distributed or concentrated windings.  In other words, the toroidal discrete windings can be used for low pole or high pole  count motors with two big advantages. The motor can perform as a distributed  winding configuration but have significantly lower phase resistance, with less  copper material being necessary for the same output. The motor can also perform  as a concentrated winding configuration with the same performance, but can also  be used in a multiphase configuration without rewiring the motor.  

Another difference in a motor incorporating the magnetic tunnel and a  conventional design is in the manufacturing process of the stator. Conventional  designs never adopted the toroidal wound stator due to its complex  three-dimensional flux path which was difficult to manufacture with conventional  stamped and stacked laminated electric steel. Solutions used in axial flux stator  manufacturing utilized soft magnetic composites which also have some  challenges in manufacturing. The proposed magnetic tunnel design uses a unique  invention of combining laminations with soft magnetic composites and thermal  potting compound to produce stator structures capable of handling  three-dimensional electromagnetic flux paths in all radial and axial directions.  

PERFORMANCE  The magnetic tunnel motor operational principles are the same as both a radial  and axial flux motor, but the performance is different. Until now all the classical  machines have used either a single or double air gap where the torque is  produced between the stator and the rotor. In some university research labs there  has been work in three air gap motors which combine the axial and radial  topologies into the same motor, but the overall efficiency of these motors has a  tendency to decrease with the number of air gaps.   

This loss in efficiency of other motor types is because in all of these configurations  the air gaps are open to the exterior of the machine (no magnets in the way) and  therefore magnetic flux leakage occurs. In other words, the electromagnetic flux  between the stator coils, poles and the rotor magnets are not entirely linked. The  portion of flux being leaked is especially important when higher torque is needed  which increases the current in the coils. Once the magnetic flux has leaked, the  machine has lost energy which cannot be recovered.  

 

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