Until now, a control apparatus for an AC rotating machine, when output torque from the AC rotating machine is accurately controlled, has needed to be equipped with a rotor position sensor in order to allow a current to flow into the AC rotating machine based on a rotor position of the machine. However, the rotor position sensor is relatively bulky, which has imposed restriction on its arrangement, and caused bothersome work of connecting control transmission wires for transmitting the sensor output to the control apparatus and an increase in troubles such as wire breaking. In contrast to this, what is called sensorless vector control has already been put into practical use, which enables the rotor position to be indirectly estimated by detecting an induction voltage generated during rotation of the rotor of the AC rotating machine, and based on which, executes high-speed accurate control.
In the sensorless vector control, the induction voltage is generally estimated from an inverter voltage command applied to the AC rotating machine and a detected value of the current flowing through the AC rotating machine. However, the rotor position of the AC rotating machine cannot be found before starting up the inverter; in particular, when the AC rotating machine is rotating at high speed, thereby generating a reverse voltage of high amplitude, undesired torque sometimes has been produced due to unstable current control when restarting the inverter, and in the worst case, the restart has also become unable due to overcurrent protection function.
In order to solve the problem as described above, the sensorless vector control is not executed during a certain period after restarting the inverter, but only current feedback control is executed instead, an approximate value of rotation speed of the AC rotating machine is estimated from rotation speed of the voltage vector or current vector during the period, and following that, the sensorless vector control is started using the estimated value as its initial value. Thereby, the inverter is smoothly restarted even when the AC rotating machine using a permanent magnet is rotating at high speed (see Patent document 1, for example).