GB1376758A - Adjustable frequency current source power supply of the inverter type - Google Patents

Abstract

1376758 Inverting; frequency changing; control of A.C. motors LITTON INDUSTRIAL PRODUCTS Inc 21 March 1972 13082/72 Headings H2F and H2J A power supply for an A.C. load (e.g. motor 22), comprises a source (56 or 260), of regulated direct current feeding an invertor 82 via an inductor 80, the invertor 82 comprising at least two parallel paths, each path comprising a controlled rectifier 90, 94, 98 and a diode 92, 96, 100 connected between the current cource 56, 260 and the load 22. A commutating capacitor 102, 104, 106 is connected between the junction of controlled rectifiers and diodes in each of the parallel paths. Fig. 20, shows an inverter 82 in which the frequency is controlled by variation at 114 of the frequency of a relaxation oscillator 112, the output of which is distributed by means of a ring counter 110 and logic circuits (Fig. 4, not shown) to the gates of thyristors 90, 94, 98. A precharge circuit 108 is provided to facilitate starting. The current source 260 is energized by a battery 250, the output of which is adjusted by means of a chopper comprising a bridge of two transistors 264, 266 and two diodes 262, 268 which are made conductive at a frequency determined by variable resistor 72. The current is monitored by means of resistor 74 and fed to a summing junction 78 so as to maintain constant current 21. The base drive frequency for transistors 264, 266 may be synchronized with that of the inverter 82. Regeneration.-Fig. 2, shows a pair of inverters 82A, 82B supplying current to the stator windings 26A ... 26C of a synchronous or induction motor. The constant current 21 supplied to the inverter is derived from the supply mains 52 by means of a three-phase bridge of thyristors 156 ... 166 having their conduction angles controllable by means of a variable resistor 72. The current is monitored by resistor 74 and fed to summing junction 78 so as to provide automatic regulation. A transformer 116 provides an output in secondary winding 122 which is a function of the rate of change of output voltage from the bridge of thyristors 156 ... 166 and also a function of the input voltage to inverters 82A, 82B. Transformer 116 has the effect of reducing the beat frequency ripple in the system. When the motor is feeding an overhauling load, regeneration is achieved by means of the second inverter 82B. Rectifying arrangements with leading and lagging power factors.-Fig. 16, shows an alternative form of constant current supply 56. A single phase supply is transformer-coupled to a pair of thyristors 60, 62 which operate at lagging power factors. A further rectifying arrangement comprising thyristors 601, 621 and diodes 602, 622 with commutating capacitor 623 operates to give leading power factor Fig. 7 ... 14 (not shown) are graphs showing the operation at various power factors. In Fig. 5 (not shown), the rectifiers for leading and lagging power factors have separate transformers and separate firing control arrangements. The thyristors in the current source 56 and in the inverter 82 may be triggered in such a way that there is always a conductive path from the A.C. source to the load. Modifications.-The motor may be reversed by changing connections in the ring counter 110. The thyristors may be replaced by gasfilled tubes. The invention is also applicable to induction heating and HF lighting.