PURPOSE: To make it possible to perform accurate speed control by providing a square root extraction calculator for creating a linear relation between a torque command signal and a motor output torque in an input stage of a current controller.

CONSTITUTION: A torque command signal 11 output from a speed controller 1 is sent to a square root extraction calculator (OA) 2. OA2 extracts the square root for torque command signal 11 and gives its square root value to a current controller 3 as a new current command signal 12. Now, OA2 will be explained. In a variable reluctance type step motor 7', an output torque 7 is proportional to the square of current. That is, if a motor current 15 is I, a current command signal 12 is I*, a torque constant is Kt, an output torque is τ, a torque command signal 11 is τ* and a gain K2 of the current controller 3 is sufficiently large and I*=I can be considered, then equation I can be obtained. However, equation II occurs by providing OA2, and the relation between torque command signal 11 and the output toque becomes equal to equation III and can be expressed by proportional relation.