Abstract: A hermetic box contains a motor and a self reset type protector which interrupts the supply of electric power to the motor when an abnormally heated condition takes place in the hermetic box and automatically supplies again the electric power when the abnormally heated condition ceases. The motor is energized by an inverter having AC output terminals connected to the motor outside of the hermetic box. A current detector which detects that no current is flowing into the motor is provided on the output side of the inverter outside for a hermetic box.

Abstract: A PWM inverter provided with a sinusoidal waveform generator circuit which produces modulated waves to switch the asynchronous operation into the synchronous operation when the output of a frequency commander commands a frequency greater than a predetermined value and when the sinusoidal waveform generator produces an output of a phase that is zero.

Abstract: A control apparatus for producing a pulse width modulating signal to be used for operation of a PWM inverter wherein a modulating wave of a sinusoidal waveform corresponding to the desired AC output voltage of the inverter is sampled at selected periodical time intervals to obtain a level of the modulating wave at each sampling point and the interval is arithmetically determined between each sampling time point and a time point which appears between the sampling point and the next sampling point and at which time point a virtual triangular waveform, which has positive and negative peaks of a predetermined amplitude occurring alternately at the selected periodical time interval, takes the same level as the sampled level of the modulating wave. On the basis of the time interval, the PWM signal is produced.