Axial flux permanent magnet (AFPM) motors are usually controlled by drive; therefore, in the design of these machines the feeding voltage and frequency are completely independent parameters. Undoubtedly, the values of these parameters have significant effects on the performance characteristics of the machine. The main objective of this paper is to investigate the effects of these parameters on the efficiency of a double-sided TORUS-type nonslotted (TORUS NS) AFPM motor and propose a formula for selecting the optimal values of the feeding voltage and frequency at the beginning of the design process. To fulfill this goal, different machines with various speeds, powers, and feeding voltages are designed based on a proposed design algorithm. Then a formula for calculating machine efficiency is presented based on the frequency, number of pole pairs, output power, and feeding voltage of the machine. In order to confirm the validity of the proposed formula, several fabricated machines with different powers and speeds are selected as case studies. The efficiency of these machines is calculated using the proposed formula and is measured in the laboratory. Comparing the results confirms the excellent accuracy of the proposed formula.