<p>Actual instantaneous reactive power value Q is calculated from a voltage applied to a synchronous motor and a current detected value. Instantaneous reactive power estimated value Qhat is calculated from inverter output angular frequency É1 and a current detected value. Inverter output angular frequency É1 is controlled to minimize the difference (”Q) between Q and Qhat. This difference (”Q) contains a component proportional to position error ”¸ of the synchronous motor and a component proportional to its rate of change (d”¸/dt) independently of winding resistances. A frequency control system is provided to minimize this difference down to zero. This frequency control system has accomplished a high-accuracy sensor-less control. The instantaneous reactive power can be substituted by any value which contains a position error (”¸) function and can be calculated without using winding resistances.</p>
