Over the past two years engineers at Honda R&D Co Ltd in Japan have been developing a Transverse Flux Motor (TFM) using a three-dimensional magnetic circuit motor, composed of SMC stator cores and a coil, in order to shorten the axial length of the electric motors used in the company’s hybrid electric vehicles (HEVs).
Honda uses a front-wheel drive IMA system in its HEVs which locates the electric motor between the internal combustion engine and a continuously variable transmission (CVT). However, to use the IMA system on various vehicle types with different width requirements, the axial length of the electric motor needed to be shortened.
Honda engineers focused on developing the Transverse Flux Motor (TFM) and in 2011 found that by eliminating the coil end, which does not contribute to the generation of torque, they could successfully shorten the axial length of the motor. The TFM featured internal ring-shaped coils and a new type of flux path. It uses a simple stator made by stacking the three 3-dimensional SMC stator cores and two rectangular wave-shaped coils (Fig. 1).
However, the efficiency in the original design was found to be relatively low (79.1%) due to iron loss characteristics of the soft magnetic composites (SMC) used to make the stators. Additionally, manufacturing methods for producing rectangular wave-shaped coils from ring-shaped coils was also an issue. In order to improve efficiency of the TFM motor Honda engineers looked primarily at ways of reducing the hysteris loss and the eddy current loss to bring the iron loss closer to their target.
In a paper presented by D. Takizawa , etal at the SAE International Congress in Detroit, in April 2013, the author reported on methods for reducing iron loss, the development of a method for manufacturing rectangular wave-shaped coils, and the production and testing of a prototype TF motor that incorporates these elements.
Honda engineers studied the constituents of the iron powder composite used in the SMC core and manufacturing conditions, and developed SMC specification conditions that reduce iron loss. 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 curing (heat treatment) temperature after compaction.
They state that to reduce eddy current loss, the insulation coating of the iron powder, which can be destroyed during either the compacting or the heat treatment process, must remain intact even at temperature of 650°C.
Compacting was found to break up 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 used a centrifugal mill to reduce the surface roughness of the iron powder. They also increased the volume of resin binder, and used a lubricant to improve iron powder fluidity.
Honda engineers also succeeded in developing a coil film that fulfils 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 result is a TFM prototype shown in Fig. 2 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% from 79.1% to 83.7% due to a reduction in iron loss.
 From paper: Development of Transverse Flux Motor with Improved Material and Manufacturing Method, by D. Takizawa, etal Presented at SAE International Congress. SAE Technical Paper, 2013, doi:10.4271/2013-01-1765