Patent GB-1085364-A (GB)
Title: Improvements relating to inverters
Inventor: GURNETT KEITH WILLIAM
IPC: H02M7/517
Filed: 1963-05-21
Abstract: 1,085,364. Inverters. FERRANTI Ltd. May 19, 1964 [May 21, 1963], No. 20075/63. Heading H2F. A main inverter (T1, SCR1, SCR2) is provided with commutating capacitors C2, C3, which are independently charged via a secondary inverter SCB5 ... SCR8, T2 and diodes D3, D4, the level of charging being increased in response to excess output current to ensure proper commutation, firing signals for the SCR's being provided by an oscillator 9 and transformer T3, the feedback control circuitry ineluding transistors TR2, TR3, TR4 and diode bridge 18, and output short-circuit protection being effected by a relay A. The main inverter includes a tuned output circuit 2, C1, and a current transformer T4. Reverse diodes D1, D2 shunt SCR1 and SGR2, and a choke winding 3 is included in the common cathode circuit. Windings 6, 7 are coupled with the winding 3 and are in series with SCR3 and SCR4 across the capacitors C2, C3, so that these are discharged on firing SCR3 or SCR4 to induce in winding 3 a reverse voltage to shut off the conducting main rectifier, this firing being simultaneous with the application of a square wave firing signal to the other main rectifier. The secondary inverter has a secondary winding connected with diodes D3, D4, for charging C2, C3, and a primary winding with ends connected to SCR5 and SCR6, and intermediate tappings connected to SCR7 and SCR8, the firing of these giving an increased output voltage and a higher charge on C2 and C3. The oscillator 9 has a winding 10 which provides via differentiating circuits firing signals for SCR5 and SCR6, and also via diodes D8, D9, a rectified output which is applied to transistors TR2, TR3, TR4. Windings 11 and 12 connected with differentiating circuits provide signals for SCR7, and SCR8, these signals being of insufficient amplitude for firing unless a transistor TR1 is rendered-conductive to short a common resistor 10. Winding 13 provides signals to SCR3 and SCR4. Winding 14 provides control for the chain SCR's (SCR1 and SCR2) through a controllable magnetic amplifier device. Operation.-Under normal conditions no signal is applied to the base of transistor TR1 therefore SCR7 and SCR8 are not fired and normal charge is applied to C2 and C3. If the output current rises such that the voltage derived at T4 and rectified by bridge 18 breaks down a Zener diode DZ1, a signal is applied to transistor TR4 shorting R15, cutting off TR3, rendering conductive TR2 and applying a signal to TR1 so that R10 is shorted and increased charge is applied to C2, C3. Similarly, with zero output voltage in T1 and hence in winding 17 a similar train of events occurs. Regulation.-Control current for the magnetic amplifier 15 is derived from a winding 16 on the output transfer via diodes D12, D13. Under reduced power, i.e. with delay between the "shut-off" triggering of SCR3 or SCR4 and the "'firing" " triggering of SCR1 or SCR2, L1 goes positive and an overswing diode D5 conducts maintaining current flow in L1 ready for the next half-cycle. Protection.-A sample of the output voltage is rectified by a bridge 19 and applied to hold closed relay contacts A. If the output voltage falls due to a short-circuit, the relay contacts open introducing a current limiting resistor R2. A thermal relay may be included in the D.C. supply leads.