<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 ω1 and a current detected value. Inverter output angular frequency ω1 is controlled to minimize the difference (ΔQ) between Q and Qhat. This difference (ΔQ) contains a component proportional to position error Δθ of the synchronous motor and a component proportional to its rate of change (dΔθ/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.</p><p>The instantaneous reactive power can be substituted by any value which contains a position error (Δθ) function and can be calculated without using winding resistances.
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