Patent GB-1027298-A (GB)
Title: Inverting apparatus
IPC: H02M7/538
Filed: 1964-05-26
Abstract: 1,027,298. Inverting. WESTINGHOUSE ELECTRIC CORPORATION. May 26, 1964 [June 3, 1963], No. 21753/64. Heading H2F. in a transistor inverter comprising an oscillator 6, a controlling stage 4 and power stage 2 means are provided to deal with reactive current due to inductive loads, these means comprising a transformer 46 connecting the controlling stage to the power stage and including additional windings 62, 66 which are connected in the emitter-collector paths of the main transistors. The oscillator 6 includes transistors 16, 18 and a transformer 12 having a square loop core 31. Positive D.C. is derived from a bus 20, which is also connected to a common line 30 through a resistor 32 to effect starting. The base-emitter circuits of the transistors are completed to earth through a resistor 34 and a diode 36 poled to prevent direct current flow from the resistor 32. Square wave output is furnished on the windings 38, 40. The control stage 4 includes transistors 8, 10 and the transformer 46 whose core is of two different magnetic materials, and whose output windings 64, 68 and 62, 66 are in the base-emitter and collector emitter paths respectively of main transistors 84, 86 in the power stage 2. The output transformer also has " two-material core " and furnishes the outputs at 80, 80. In operation, assume an oscillator output so that transistor 84 is conductive and dotted ends are positive (time t 0  in Fig. 2, not shown). Assume also a load such that current lags by 90 degrees. After a time the current flowing in the additional winding 62 will induce in the primary winding 48 a voltage greater than and in the same direction as the supply voltage, and in consequence transistor 10 although " conductive " does not conduct by virtue of diode 52. The ampere turns of windings 62, 64 are substantially balanced. At time t 1  the oscillator output reverses, transistor 8 is rendered conductive and 10 non-conductive. A large current pulse I 54  flows through 48, 8 and 54, and the drive current to transistor 84 supplied by 64 decreases rapidly. This causes the transistor current I 64  to decrease rapidly. Current then flows from 22, through 56, 53, 48 and 8 to earth reversing the polarities of the windings on transformer 46 (time t 2 ). When the current I84 ceases, primary current still flows-due to the inductive load-through the diode 90. A reverse voltage is thereby maintained between 76 and 22 preventing transistor 86 conducting even though the drive voltage on winding 68 is now correctly poled. When I 90  ceases transistor 86 conducts (t 3 ). In the period t 2 -t 3  base-emitter current flows in transistor 86, but base-collector current is prevented by the additional threshold voltage of diode 96. At time t 4  the oscillator output again reverses, causing transistor 10 to conduct and the sequence to be repeated. In a further embodiment (Fig. 3, not shown), the power stage is modified by the elimination of networks 91, 95, and the connection of the diodes 88, 90 to the ends of additional turns on the winding 72. A bridge power stage is also disclosed (Fig. 4, not shown).