Technology

Transverse flux is not a single well defined concept, but rather a class of different models where the magnetic flux varies in 3 dimensions.

The transverse flux models has been developed mainly the last 20 – 30 years. However, products are few.

This study focus on one of the transverse flux models.

The Advantages

In traditional electric engines the (copper) windings are typically wrapped around the iron core. In transverse flux engines the iron core is instead wrapped around a single core containing the windings. This means that each turn of copper winding is longer on transverse flux engines than on traditional types. This is solved by using fewer turns and a stronger magnetic field. Flux concentration enables use of less powerful magnets, but may also cause cogging problems. Another difference is that transverse flux engines have a separate core for each phase.

To achieve low rotational speed while keeping the motor power at the same level it is required to increase the number of poles. For transverse flux engines the copper windings stays the same independent of the number of poles. For traditional engine designs more poles means more windings spread across smaller iron cores with the total effect of decreasing the engine power.

  • Transverse flux engines allows the number of poles to be adapted according to the optimal rotational speed and best combination of operational frequency, performance and weight.
  • Higher engine power, lower loss, less volume and weight compared to traditional motor designs.
  • Often a gearless design is possible for lower price, lower loss, less maintenance, less volume and weight.

In brief: For a low number of poles (6-12 for a 3-phase motor) there are no advantages of using transverse flux. For a high number of poles (100 – 1000) transverse flux motors are very clear winners.

Disadvantages

  • requires a new design, new construction and production methods
  • currently little experience is available
  • requires a new way to calculate vital parameters
  • stator pole shape and position must be tuned to get sinus shaped output voltage
  • cogging could be a problem

The Studied Model

Transverse flux models may use either axial flux or radial flux:

  • axial flux: the magnetic flux varies in parallel to the axis of rotation
  • radial flux: the magnetic flux varies vertically on the axis of rotation

Radial flux motors and generators are most frequently used because of the very strong forces acting between the rotor and the stator. Radial flux designs means that these forces are self-balancing with minimum mechanical stress.

The transverse flux model studied uses radial flux. The model focus is on simplicity and symmetry. The model focus on using the shortest possible route for the magnetic flux. Flux concentration is used at the poles which often allows ceramic magnets to be a choice. The model is an “outrunner” type where the rotor spin around the stator to enable shortest possible copper winding turns. The air gap between the rotor and stator is smallest possible to enable strongest possible magnetic flux using moderately sized magnets. The model is in this study termed TFM1.

Inrunner versus Outrunner Configuration

The studied model will work well for both inrunner and outrunner configurations.

Outrunner means that the rotor runs outside the stator. This configuration enables shortest possible turns of the stator windings for lowest possible resistive (copper) loss. Also, the space for rotor magnets will be largest possible which enables smaller magnets.

Inrunner means that the rotor runs inside the stator. In this configuration each turn of the stator windings will be longer and thus the resistive (copper) loss will be higher. Also, the rotor magnets requires space which may mean that the total diameter may increase compared to outrunner types. However, inrunner configurations may have mechanical advantages.

Laminations versus Powder

Thin plates of laminated silicon steel are often used for guiding the magnetic flux. Using silicon steel plates for transverse flux engines has some challenges due to the 3-D nature of the transverse flux. The magnetic flux tend to cross the laminated plates and this creates some energy loss. However, the extra loss is not expected to be significant.

Iron powder is an alternative to laminated plates. Iron powder can be pressed together to form most of the required shapes for the engines. This means simpler production compared to using laminated silicon steel plates. The energy loss is identical in all directions making iron powder interesting for transverse flux engines. Iron powder has higher energy loss compared to laminated silicon steel.

Constructing Laminations

Silicon steel laminations for engine production can be constructed very similar as for the prototype.

The radial part vertically on the rotational axis can be fragmented and “glued” together with overlap from layer to layer forming a complete ring.

The axial part in parallel to the rotational axis can be formed by a single sheet that is rolled to form a ring.

The axial and two radial parts are then “glued” together to form a complete magnetic flux conductor for the stator.

This procedure is then repeated 3 times to make a 3-phase engine.

Windings Automation

The stator copper windings on traditional motors represent a real challenge. Copper windings are often too complex for automation in radial flux designs.

Transverse flux windings are on the other hand exceptionally simple.

Instead of using a copper wire it will be easiest to use a wide flat copper wire that is rolled together. Using a flat wire covering the entire winding area means that the voltage drop between the layers will be uniform and low.

Magnets

Neodym magnets are the most powerful magnet types but has some disadvantages. The main problem is that it requires the rare earth metal Neodymium. Another disadvantage is a low maximum termperature, often as low as 80 °C.

Ceramic magnets are less powerful but have price and temperature advantages. The flux concentrating properties of the TFM1 design makes ceramic magnets very interesting for TFM1 engines.

 
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