<p>Operation of an inverter deviating from the operational region in a low rotational speed region is judged more strictly without interfering with the operational region of a motor thus protecting an inverter carefully. Two current thresholds I<SUB>TH1</SUB> and I<SUB>TH2</SUB> larger than the upper limit I<SUB>U</SUB> of a current in the operational region D are employed. The current threshold I<SUB>TH1</SUB> is employed as a criterion for judging abnormality of an input current i<SUB>INV</SUB> to the inverter when the rotational speed (Rot) is smaller than a speed threshold (RTH1) otherwise the current threshold I<SUB>TH2</SUB> is employed.</p>
