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This article proposed a position signal-free field current estimation method for brushless synchronous starter/generator (BSSG). Generally, the estimation of the field current of BSSG depends on the position signals provided by a physical sensor or position estimation algorithm. However, there are risks in using the traditional method when the position sensor is broken or using estimated position information to evaluate the field current. To overcome this problem, this paper proposes a position signal-free estimation method. In this method, the mathematical relationship between the field current of the main machine (MM) and the current vector modulus (CVM) of the main exciter (ME) rotor currents are analyzed. The theoretical findings reveal that the average field current of the main machine (MM) equates to the minimum or √3/2 times the maximum of ME's rotor CVM in both rotating and stationary coordinates. Since the ME's rotor CVM is independent of the rotor position information, the estimation of the field current of BSSG can be achieved without the rotor position signal departing in. Finally, an experiment platform is established to emulate the BSSG, and experimental results validate the theoretical findings.