Implementation of maximum power point tracking (MPPT) control is of great help to improve the efficiency of variable-speed wind turbine below the rated wind velocity. Theoretically, once the MPPT control is activated, the operating point will eventually reach the theoretical maximum point precisely. But the actual operating point cannot be controlled to reach the target value due to the losses of new system, which consists of planetary gear, generator and auxiliary motor. This study presents a way to roughly estimate iron loss in auxiliary motor, using bisection method to obtain iron loss without so many detailed parameters needed and compensate it in the torque MPPT control loop. Finally, the theoretical analysis method is verified by simulation and experiment results.