Electromagnetic Performance Analysis of a Dual-rotor Ironless Stator PM Machine

This paper investigates the electromagnetic performance of a dual-rotor ironless stator permanent magnet machine for flywheel energy storage system. Since the stator is made of nonmagnetic material, the armature winding is exposed to the magnetic field directly and becomes the object of electromagnetic force. The air-gap flux density is not constant along the radial direction and the phase of the flux density is different along the circumferential direction. This makes the electromagnetic performance calculation, such as back electromotive force (back-EMF) and electromagnetic force, different from the common iron-core machine. Additionally, the uneven flux density distribution will lead to circulating current in the armature winding. In this paper, the expressions of back-EMF and torque are derived. In addition, this paper gives the condition for eliminating the circulating current between the strands of the ironless machine. This work provides reference for the analysis and design of ironless machine.