This paper presents a design approach for interior permanent magnet (IPM) machines with variable-flux characteristics using low coercive force magnet material for improved efficiency and extended operating speed range. A flux-intensifying interior permanent (FI-IPM) type having Ld > Lq is used in the design due to positive Id operation and reduced load Iq effects. Design consideration of machine structures and variable-flux machine attributes are discussed. In addition, leakage flux in a rotor is specially designed to obtain another degree-of-freedom in variable-flux control as well. Evaluation of the designed machine is provided using FEA simulations and experimental evaluation of a proof-of-principle prototype machine. The designed machine shows benefits in improving efficiency and extending the range of the torque-speed map when magnetization level of the low coercive force magnet is operated at the optimal levels.