In the case of controlling the output torque of an AC rotary machine with high accuracy, it was necessary to provide a rotor position sensor for applying current based on the rotor position of the AC rotary machine.  However, the rotor position sensor has restriction of placement due to its relatively large volume, and there is trouble on wiring of control transmission lines for transmitting sensor output to a control apparatus, leading to increase in failure factors such as disconnection.  Meanwhile, the rotor position can be indirectly obtained by detecting induced voltage occurring during rotation of the AC rotary machine, and so-called sensorless vector control which performs torque control based on the rotor position with high speed and high accuracy is already being put into practical use.  In the sensorless vector control, generally, the induced voltage is estimated and calculated from an inverter voltage instruction applied to the AC rotary machine and a current detected value flowing in the AC rotary machine.  However, before the operation of the inverter is started, the motor rotor position cannot be obtained.  Particularly, in the case where the AC rotary machine rotates at a high speed and the induced voltage amplitude is large, when the inverter is reactivated, unnecessary torque occurs due to unstable current control, or in the worst case, overcurrent protection operation is performed, whereby reactivation may become impossible. 
To solve such problems, a technique of Patent Document 1 is proposed. 
In the technique of Patent Document 1, during a certain period from reactivation of an inverter, sensorless control is not performed but only current feedback control is performed.  An approximate value of a motor rotation rate is estimated from the rotation rate of a voltage vector or a current vector obtained during the above certain period, and then sensorless control is started with the estimated value set as an initial value.  Thus, even when a permanent magnet motor is rotating at a high speed, the inverter is smoothly reactivated.