Reducing cogging torque of interior permanent magnet synchronous motor for electric bicycles

Reducing the cogging torque is important for electric bicycle motors. This paper describes a motor configuration for reducing the cogging torque in an interior permanent magnet synchronous motor (IPMSM) by focusing on the tangential stress in the magnetic saturation regions. First, it is confirmed that expanding the gap at the edge of stator teeth reduced the tangential stress on the rotor surface near the teeth edge so that only the tangential stress in the magnetic saturation regions was large, based on the electromagnetic field analysis. Next, a rotor configuration for reducing the cogging torque is considered by focusing on the tangential stress in the magnetic saturation regions. The generation of a balanced tangential stress was confirmed by adjusting the gap length and the magnetic saturation regions, and a low cogging torque was achieved. Moreover, testing using various prototype motors confirmed that the cogging torque was minimum when the tangential stress on the rotor was well balanced. This does not mean that simply increasing the gap length will reduce the cogging torque as the magnetic flux is reduced. Moreover, a low cogging torque of 2.5% or less of the rated torque was achieved.