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Hybrid excitation synchronous machines (HESMs) are permanent magnet machines which allow the control of the air-gap flux density. In this study, three new axial field HESM structures, each having two rotors and one stator, are proposed. The operation principles of these machines, considering weakening or supporting the air-gap flux density, are described. The magnetostatic and transient analyses of the proposed models have been carried out by using finite element method. To have reliable analysis results, the stator length and input and output diameters of the models have been chosen to be identical. Results are compared in terms of efficiency, power density and range of flux control between each other and another axial field HESM in the literature. In conclusion, the proposed HESM models demonstrate satisfactory performance for the generator mode and they are suitable for wind energy applications. Furthermore, because of their field weakening operation ability they can be used as motors in the vehicle industry.

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Accurate and fast calculation of the self and mutual inductance of coils is an important factor in system design and optimization for many applications. The methods of calculation mutual inductance between two spiral disk coils include elliptic integral solutions and coefficients in literature. In this study, a new semi-analytical method is proposed to calculate the mutual inductance between two thin disk coils without any coefficient and complex integral solutions. For this purpose, the circular winding is treated as polygonal winding with multiple edges. Unlike Grover's average diameter approach, the gaps between the turns of the coils and the diameter of the wire are included in the calculation. To observe the reliability of the method developed here, the mutual inductance between identical disk coils with an inner diameter of 10 and 20 cm were calculated using both methods. To confirm the accuracy of the proposed method, the experimental results are compared with two methods using the same coils. It is observed that the new method gives acceptable results (maximum error of 2.83%) especially at the distances equals to the inner radius. Consequently; this method is useful for the design of loosely coupled systems, such as wireless power transfer and pressure sensors.