Due to the unique flux distribution, biased flux machines (BFMs) could be driven by biased current to realize flexible flux weakening or strengthening control. In this paper, the electric machines with biased flux are investigated. The key techniques such as constructing biased flux in coils, exploring topologies, developing coil connection patterns, and designing drive circuits are presented. Based on that, the general design guideline for double stator BFMs is concluded. To verify this analysis, one 12/11 (stator teeth/rotor teeth) double stator BFM prototype is built and tested. The proposed double stator BFM has good torque and efficiency performance. Meanwhile, the biased current including dc zero sequence is used to adjust the exciting field. The simulation and experimental results verify the validity and effectiveness of this machine design method and control strategy. The flexible flux modulation characteristic makes this kind of machines suitable for applications with wide speed range.