Summary form only given. A torque sharing function (TSF) method is one of the torque control methods that drives each phase of an SRM so that the overall torque becomes a constant torque. The TSF method produced in previous works defined the TSF only at a positive torque production region. This definition leads to a good performance at low speed. As the speed increases, however, the positive torque near the unaligned position is reduced and the negative torque near the aligned position is increased due to the delay of the current rising and falling time. This paper extends the definition region of the TSF to the negative torque region to have enough time to raise or decrease the current at the positive torque region. This extended TSF enables to increase the positive torque even in relatively high speed operation. Optimal determination of the TSF is investigated for various control objectives in mathematical and experimental ways. The comparison study of the control objectives is also presented.