GB1172301A - Improvements in or relating to Phase Control Arrangements for Inverters - Google Patents

Abstract

1,172,301. Frequency changing. ELIN-UNION A.G. FUR ELEKTRISCHE INDUSTRIE. 17 Jan., 1967 [25 Jan., 1966], No. 2508/67. Heading H2F. In a frequency changer for supplying an inductive heating load H, Fig. 1, comprising a winding a tuned by a. capacitor b, the current and voltage zero-crossings are sensed by detectors c and d with pulse shaping circuits A and B and used to control the timing of ignition pulses fed to thyristors in the inverter G which is supplied from a rectifier F. In this way the frequency of the inverter can be matched to the resonant frequency of the load, and the phase of the ignition pulses may be controlled by a counter-voltage time generator C to ensure maximum efficiency. Control circuit. Fig. 3. At a voltage zerocrossing of the output, negative pulses are applied at terminals 3 and 4 and change over the bi-stable circuit T5, T6 so that T6 blocks and T5 conducts, thereby blocking T7 and allowing C4 to charge through R18. At a current zero-crossing, i.e. a quarter-cycle later, negative pulses are applied at terminals 1 and 2 and change over the bi-stable circuits T5, T6 and T1, T2 so that T6 and T2 conduct. T3 is now conductive and charges C3 through a voltage-dependent resistance R5, the voltage across which corresponds to the voltage on C4. When C3 reaches the ignition voltage of unijunction transistor T4, the latter conducts and causes T9 to deliver an output pulse on line 10 and to restore the bi-stable circuits to their initial condition when T7 conducts and discharges C4. This cycle is then repeated. The output pulses are fed to the thyristors via an output circuit E, Fig. 1, which includes a bistable circuit (FF, Fig. 5, not shown); this is also supplied with pulses at the voltage zerocrossing times so that it remains in phase with the inverter if there is a fault in the control circuit. Automatic phase control.-The actual time for which counter-voltage is applied to the thyristors is detected by means not described and compared with the period of a monostable circuit (T11 and T12, Fig. 4, not shown), this period being manually adjustable. If the actual time is greater the voltage applied to terminal 6 in Fig. 3 is increased and if it is smaller an additional initial charge is added to C4 through terminal 5 (from terminal 21, Fig. 4, not shown), so that in either case an appropriate adjustment is made to the charging time of C3.