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While the design method has been reported for surface mounted permanent magnet synchronous machines which have an infinite constant power speed ratio, the control method about this type of machine is not widely discussed yet. This paper proposes a novel flux weakening algorithm for the synchronous machines with an infinite constant power speed ratio. The proposed algorithm does not use the voltage magnitude as a feedback signal, but utilizes the voltage differences between the input and the output of the over-modulation block, and to detect the maximum torque per voltage line, the q-axis voltage, which is the output of the q-axis current regulator, is used. The available maximum output torque by the proposed algorithm in the flux weakening region is closed to that by the six step operation, while the ability of the current control is still maintained. The effectiveness of the proposed method is verified by the experiments.