This paper presents the proximate standing wave feature of the resultant magnetic field in a brushless doubly-fed machine (BDFM). To investigate the variation pattern of the resultant magnetic field and to predict its influence on machine performance, a wound rotor structure with low harmonic contents is designed. The variation law of resultant MMF is found to be similar to standing wave through summation of analytical equations for two magnetomotive forces (MMFs) with different pole pairs and opposite revolution directions. Based on this magnetic field of proximate standing wave, a new restriction for pole number combination is derived to eliminate asymmetrical magnetic forces. The torque angle of the BDFM is also accurately expressed by axis positions of MMFs. The finite element (FE) simulation verifies the proposed proximate standing wave feature of the air-gap magnetic field. New constraint on pole number combination is proved to be valid by comparing FE simulation results about magnetic force. Results from no load experiments and full load simulations are compared with data calculated by analytical methods. The good agreement validates the proposed torque angle expression and the torque angle curve.