Abstract: In a state of synchronous operation in which a voltage and a frequency of an inverter output are increased in a stage of starting, when it is decided that a voltage difference signal is not at a specified level by a voltage difference signal level deciding section (41) or when a comparison result in a mode comparing section (42) signifies that a position signal and an inverter mode do not have a specified relation, a V/F pattern setting section (44) outputs a voltage command signal for lowering the inverter output voltage to a PWM section (54).

Abstract: When a brushless D.C. motor 3, 13 is driven by voltage-fed inverters 2, 5, 12, 20, a conducting interval of the voltage-fed inverters 2, 5, 12, 20 is determined to be greater than 120.degree. and equal or less than 180.degree. by electrical angle so that improvement in efficiency and enlargement in operating range of the brushless D.C. motor is achieved with simple and cheap controlling.

Abstract: In a brushless DC motor system, an RMS value of an input current of a voltage-fed inverter (702) is detected by a current transformer (705) and an RMS value detector circuit (706). A rotation speed of a brushless DC motor (703) is detected based on a cycle of a position signal outputted by a position sensor circuit (704). Then, in response to the RMS value of the input current and the rotation speed, the phase of the inverter output voltage relative to the phase of the motor counter-electromotive voltage is set to a phase at which the motor efficiency comes generally to a peak, and a switching command to the voltage-fed inverter (702) is generated. With this constitution, when the applied voltage waveform is controlled only by the voltage-fed inverter in response to the rotor-position detection signal, the motor attains high-efficiency operation without involving any increase in cost and yet regardless of load conditions.

Abstract: A first insulation layer (3), a first conductive layer (1), a second insulation layer (4) and a second conductive layer (2) are laminated, and a larger spacial gap between either one (2) of the first conductive layer (1) and the second conductive layer (2) and the opposite layer (3,4) thereto (2) than that between the other conductive layer (1) and the opposite layer (3,4) thereto (1) is made, and high voltage is applied across both conductive layers (1,2), and when charged dusts pass therethrough, the charged dusts are attached on only one (2) of the conductive layer. These charged dusts are electrically neutralized, and thereby, it becomes possible to prevent weakening of the electric field and declination of the dust-collecting ratio through lapse of time.