TREFASIG VEXELRIKTARE MED TOLVPULSIG, I HUVUDSAK SINUSFORMIG UTGANGSVEXELSPENNING

1483201 Inverting SIEMENS AG 12 Sept 1974 [21 Sept 1973] 39911/74 Heading H2F A three-phase inverter arrangement comprises six single-phase inverters arranged to produce periodic voltages of the same frequency and substantially identical trapezoidal waveforms differing in phase, the trapezoidal waveforms having rise and fall times in the range 25/360 to 36/360 of the period and produced by the relatively slow charge and discharge of the commutating capacitor of the individual inverters from the D.C. supply. The trapezoidal waves from the individual inverters are combined to provide a substantially sinusoidal three-phase output. As shown in Fig. 2, each individual inverter 1 ... 6 includes the centre-tapped primary of a respective transformer a ... f having the secondaries connected as shown to terminals x, y, z of a synchronous motor or data-processing installation. Fig. 4 (not shown) depicts a graph relating the total harmonic distortion in the substantially sinusoidal combined output of the individual inverters to the electrical angle occupied by the linear rise or fall times of the trapezoidal waveform from the individual inverters and shows that if the angle is 15 degrees the distortion is at a minimum of approximately 1%. Each individual inverter is as shown in Fig. 5 and comprises a pair of thyristors n1, n2 arranged to be alternately conductive so as to switch the current I L in inductor v alternately through the two halves of the centre-tapped primary of transformer Tr. The turning on of either thyristor n1 or n2 discharges the commutating capacitor C substantially linearly through the conductive thyristor and thus provides the linear substantially slow rise and fall of the trapezoidal output waveform. Circulating currents are controlled by free-wheeling diodes n11, n12, n21, n22 connected to tappings A1, A2 on the transformer Tr. The currents on the A.C. side are monitored and combined to provide a signal Ia which is compared at S with the inductor current I 1 so that in the ease of an overload surge, triac T is turned on, connecting C1 to change the value of the commutating capacitor C.