Optimal design of a middle motor for a pedal electric cycle

Pure electric bikes or pedal electric cycles have been welcomed as a kind of new personal transportation. This paper proposes a novel design of a middle motor for pedal electric cycles. This middle motor is an axial-flux permanent magnet brushless dc motor with a stator plate, facing which is a rotor plate as part of a cycloidal reducer next to the stator. The whole set of middle motor and cycloidal reducer is easily installed along the same axle of the crankset of bike.
In the preliminary design, driving requirements are used to make specifications of the middle motor. The numbers of stator slots and magnet poles on rotor are then chosen for a best winding factor and the winding layout is determined accordingly. A one-dimensional magnetic circuit model of the middle motor is built and its shape optimized by using a multifunctional optimization system tool. Finally, the resulting design is verified and refined by the finite element analysis.
A prototype middle motor is fabricated. A new material, soft magnet composite, is used to replace traditional silica steel for stator. The neodymium magnets (NdFeB) are mounted on the rotor surface for its high magnet energy density. Experimental results show that the maximum torque is about 60% of what was designed because of the significant friction exerted from mechanical assemblies. However, the back electromotive force is sinusoidal and the motor is easily and efficiently controlled by sinusoidal current inputs.