| 摘要 |
<p>1,108,571. Inverting, control of A.C. motors. BROWN, BOVERI & CO. Ltd. 22 April, 1965 [24 April, 1964], No. 16944/65. Headings H2F and H2J. An inverter circuit for supplying power to single or polyphase motors comprises a plurality of switching units each consisting of a " halfbridge" of controllable rectifiers which are controlled by a waveform derived by comparison of a sinusoidal waveform whose frequency is the desired output frequency and a higher frequency sawtooth waveform, the phase of the sawtooth waveform being different for different switching units. The invention reduces harmonics in the output. Fig. 1 discloses a known arrangement in which a " half-bridge " 5 comprises controllable rectifiers 5a, 5b each shunted by a diode. Their junction forms a terminal for connection to one phase of a star connected motor winding. A sawtooth waveform u<SP>1</SP> and a sinusoidal waveform u 2 are compared at 3 and depending on which is the greater so the rectifier 5a or 5b is rendered conductive. The waveform at the motor terminal is therefore that shown by U Pho , and this is also the phase voltage if the star point is earthed. Another waveform (U PhM ) shows the voltage across the winding when the star point is not earthed and all phases are considered. This arrangement leads to harmonics which are minimized by an arrangement according to the invention shown in Fig. 2. In this a further " half-bridge " 5(2) is provided, and the two half bridges are interconnected through an interphase transformer whose centre tap is connected to the motor phase terminal. The control voltage for the second unit 5(2) is derived from the comparison of a common sinusoidal wave with a sawtooth waveform which is 180 degrees phase displaced from that used for the first unit. In a further embodiment (Fig. 4, not shown) four half-bridges and sawtooth waveforms of 0, 90, 180 and 270 degrees displacement are utilized. Another embodiment (Fig. 5, not shown) disclosed an arrangement wherein the phase windings of the motor are separate, and are each connected in series between two half bridges. Fig. 6 (not shown) discloses three motors fed from a common supply, the sawtooth voltage used in the three controls being phase displaced from one another. The invention may be fed from a rectfier thereby forming a frequency changer.</p> |
