Axial-flux PM machines with variable air gap

Laminated soft magnetic steel is very often used to manufacture the stator cores of axial-flux PM machines. However, as the magnetic flux typically has main components parallel to the lamination plane, different magnetic flux density levels may occur over the radial direction: High flux densities near the saturation level are found at the inner radius, while the laminations at the outer radius are used inefficiently. To obtain a leveled magnetic flux density, this paper introduces a radially varying air gap: At the inner radius, the air gap is increased, while at the outer radius, the air gap remains unchanged. This results in equal flux densities in the different lamination layers. As the total flux in the stator cores is decreased due to the variable air gap, the permanent-magnet thickness should be increased to compensate for this. The effect of a variable air gap is tested for both a low-grade non-oriented and a high-grade grain-oriented material. For both materials, the redistribution of the magnetic flux due to the variable air gap results in a significant decrease of the iron losses. In the presented prototype machine, the iron losses are reduced up to 8% by introducing a variable air gap. Finally, a prototype machine is constructed using an efficient manufacturing procedure to construct the laminated magnetic stator cores with variable air gap