|Left: Structure of original TF motor. Right: new TF motor. -32% is the relative reduction in axial length compared to a conventional motor. Takizawa et al. Click to enlarge.|
Honda has proposed and is developing a Transverse Flux motor (TF motor) in order to shorten the axial length of the motor for hybrid electric vehicles (HEVs). At the 2013 SAE World Congress, Honda engineers described their progress in improving the new type of three-dimensional magnetic circuit motor.
In contrast to conventional stators composed of a stator core (made from magnetic steel sheet) and winding wires, the TF motor’s stator is composed of a soft magnetic composite (SMC) core and a coil. While reducing axial length and achieving a simple stator architecture comprising only five parts, the new motor raises issues including the need further to improve motor efficiency (currently at 83.7%) and the development of techniques for the manufacture of rectangular wave-shaped coils.
Background. Honda’s front-wheel drive IMA hybrid powertrain system features a motor located between the internal combustion engine and a continuously variable transmission (CVT); as a result, the width of the powertrain increases according to the axial length of the motor. To use the hybrid vehicle system on various vehicle types with different width requirements, the axial length of the motor needs to be shortened.
In an effort to shorten the axial length of the motor, Honda engineers focused on eliminating the coil end, which does not contribute to the generation of torque, by proposing a transverse flux motor, reported in a paper in 2011 (Aoki and Takahashi). The original TF motor featured internal ring-shaped coils and new type of flux pat. The design was later improved to counter magnetic saturation and magnetic “short circuits”.
While the stator of a salient-pole concentrated winding motor, in which each tooth is a separate part, is composed of 110 parts, the TF motor uses a simple stator made by stacking the three 3-dimensional stator cores and two rectangular wave-shaped coils.
However, efficiency in the original design was relatively low (79.1%) due to iron loss characteristics of the soft magnetic composites (SMC) used to make the stator). Too, manufacturing methods for producing rectangular wave-shaped coils from ring-shaped coils was also an issue.
In the new SAE paper, Honda engineers describe methods for reducing iron loss, the development of a method for manufacturing rectangular wave-shaped coils, and the creation and testing of a prototype TF motor that incorporates these elements.
Efficiency. To improve efficiency, the Honda engineers conducted a parametric study on the constituents of the SMC core and manufacturing conditions, and developed SMC specification conditions that reduce iron loss.
Specifically, they needed to reduce the hysteresis loss and the eddy current loss to bring the TF motor iron loss close to their target.
To reduce hysteresis loss, they needed to reduce the coercivity of the iron powder. Principal factors here are the grain boundary of the iron powder and the strain of the iron powder during the compacting process. They therefore sought to increase the grain size through annealing of the iron powder, as well as means of preventing strain by increasing the heat treatment temperature after compaction.
To reduce eddy current loss, the insulation coating of the iron powder must remain intact. The insulation coating can be destroyed during either the compacting or the heat treatment process, and must be able to withstand the heat treatment temperature of 650 °C.
Compacting destroys the MgO coating because the membranes split through contact with each other. This can be prevented by reducing the roughness of the iron powder surface and improving its fluidity during compacting. The engineers therefore examined using a centrifugal mill to reduce the iron powder surface roughness, and increasing the volume of resin binder and using a lubricant to improve iron powder fluidity.
|Three-dimension moveable press. Takizawa et al. Click to enlarge.|
Manufacturing. Honda engineers developed a coil film that fulfills the requirements for rectangular wave-shaped coil formability and insulation, while also developing winding wires and press-forming processes required to manufacture rectangular wave-shaped coils.
The coil film was produced using an extrusion process that allows a thick film to be made, while PFA (perfluoroalkoxy resin) was selected as the film material due to its elongation properties, which aid forming. For the winding wires process, both lead wires are wound on the outer circumference side, and a three-dimensional movable press-forming method was developed to bend ring-shaped coils into rectangular wave-shaped coils while reducing the coil diameter.
|TF motor prototype. Takizawa et al. Click to enlarge.|
Prototype. The engineers built a TF motor prototype using the improved SMC core and the new coil-forming method; maximum torque was 140 N·m (103 lb-ft).
Tests on the prototype showed that average motor efficiency when driving in the JC08 mode improved by 4.6 percentage points from 79.1% to 83.7% due to a reduction in iron loss.
Takizawa, D., Takahashi, T., Shimizu, H. and Kato, R., “Development of Transverse Flux Motor with Improved Material and Manufacturing Method,” SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 6(1) doi: 10.4271/2013-01-1765
Aoki, S. and Takahashi, T., “Development of Compact Transverse Flux Motor with a New Magnetic Circuit Configuration,” SAE Int. J. Engines 4(1):314-322 doi: 10.4271/2011-01-0348