Vector control is widely known as a method that can highly-accurately control an AC rotary machine. In the vector control of the AC rotary machine, since the AC rotary machine is rotated with an intended power or number of rotations, its control has been usually performed such that a rotor position (rotor's magnetic-pole position) or rotor speed of the AC rotary machine is acquired using a speed sensor or a position sensor. However, these sensors are disadvantageous from the aspect of their fault-tolerance or maintenance. Accordingly, a method is used in which the rotor's magnetic-pole position or rotation speed of the AC rotary machine is detected without using any sensor. As this method, an induced-voltage based method is publicly known, which is advantageous mainly in operations in a high speed region where the induced voltage is large. On the other hand, with respect to a speed region including a zero speed or a low speed region, where the induced voltage is difficult to be used, such a technique is known in which a voltage or current with a frequency different to the fundamental frequency is overlapped to the AC rotary machine, and using inductance saliency obtained thereby, the rotor's magnetic-pole position is estimated (for example, Patent Document 1).
However, in general, according to the method in which the rotor's magnetic-pole position is estimated using inductance saliency, there is a problem that, when a large current flows through the AC rotary machine, an error occurs in the estimated rotor's magnetic-pole position due to effect of magnetic saturation in inductance. Influence of this positional error results in degraded controllability, such as deteriorated accuracy in the determined position at the position determination control, or deteriorated responsivity for controlling the speed.
Methods for solving this problem are described in Patent Document 2. In Patent Document 2, there are disclosed: a method in which a high-frequency alternating voltage is applied to an AC rotary machine, a high-frequency current value obtained thereby is converted into d-q-axis coordinates according to the phases shifted by 45 degrees from an estimated angle, and using high-frequency impedances Zdm and Zqm obtained thereby, a rotor's magnetic-pole position is estimated so that these impedances become matched to each other; and a method in which, in order to make correction under high load, a compensation angle θr^ resulting from multiplying a torque component iδ* as a current command value by a proportional constant Kθ, is subtracted from a magnetic-pole position θ^ having been estimated, to thereby calculate an estimated position θc^.
Further, in Patent Document 3, there are disclosed: a method in which a correction angle θcr is calculated so that a length vh* of a voltage-command spatial vector becomes matched to a target vh** of a voltage vector length determined by a function fv(T*) related to a target torque T*; and a method in which, in order to make correction under high load, the correction angle θcr is added to a calculated and estimated rotor's magnetic-pole position.