CONTROL RECTIFIER CIRCUIT INCLUDING AN ARRANGEMENT FOR RENDERING A CONTROLLABLE RECTIFIER NON-CONDUCTING

摘要

1,062,736. S.C.R. switching circuits. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. Aug. 10, 1965 [Sept. 28, 1964], No. 39363/64. Heading H3T. [Also in Division H2] In a controlled rectifier circuit wherein a commutating capacitor C in series with a switching device S is connected in parallel with a controlled rectifier SCR, so that operation of the switch discharges the capacitor to shut off the rectifier, a blocking means-as shown a saturable reactor SR-is included in series with the rectifier in order to delay the subsequent application of forward voltage to the rectifier and hence give a longer time for shutting off. The capacitor is initially negatively charged as shown. When the switch S is closed at t, the capacitor discharges and the flux in the saturable reactor (previously set by the load current i) is reversed. When the capacitor starts to become positively charged this positive voltage appears across the unsaturated reactor SR and is not applied to the rectifier SCR until the reactor SR is again forwardly saturated at time t 3 . Thus the total turn-off time available for the rectifier is t 1 to t 3 . This is ideally twice the time available in the absence of the saturable reactor. The ideal time is not quite met in practice since (a) there is significant magnetizing current for the saturable reactor; (b) the sudden saturation of the core causes a high rate of voltage rise on the rectifier; (c) the turn-off time is modified by the presence of the saturable reactor. To overcome the effect of (a) a reverse diode may shunt the rectifier (Fig. 3, not shown), and a source of negative voltage may be connected in series with the rectifier. Other methods include a current absorbing resistor and a bias winding on the saturable reactor. A D.C. chopper is disclosed (Fig. 4, not shown) which incorporates a further controlled rectifier as the switch S, a reverse diode across the main rectifier and a divided saturable reactor to provide biasing. Inverting circuits are also contemplated.