Patent GB-626219-A (GB)
Title: Improvements in or relating to field excitation arrangements for electric alternators
IPC: H02P9/34
Filed: 1946-09-06
Abstract: 626,219. Automatic voltage control systems; automatic regulation. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S. Sept. 6, 1946, Nos. 26829 and 37128. [Class 38 (iv)] The voltage of an alternator is controlled by supplying the field and/or exciter field from a rectifier which is supplied from a transformer, the transformer input voltage being made dependent on the voltage across either the inductance or the capacitance of a series resonant circuit which is connected across the output of the alternator and which resonates at the normal frequency of the alternator. The transformer output voltage is mainly determined by a saturable reactor which may be integral with the transformer), the D.C. winding of this reactor being supplied from the alternator output by a further rectifying circuit having a non-linear characteristic so that as the alternator voltage varies, the voltage across the field rectifier circuit is correctively changed. The field 2, Fig. 1, is excited by a bridge rectifier 4 supplied from a series resonant circuit 5, 6 across the alternator output. The winding 6 acts as the resonant inductance, due to the air gap in core limb 12, and also acts as the primary of a transformer in which the secondary 10 feeds the rectifier. A D.C. saturating winding 11 is supplied from a pair of directly heated rectifying valves 13 forming a full-wave rectifier fed from an autotransformer 15 which is connected across the alternator output. The rectifier filaments are fed from a winding 18 so that the filament current varies with alternator voltage, and as the normal anode voltage is such that the valves are saturated, the anode current varies considerably with change in filament temperature, so that the control current in winding 11 changes by an amount appreciably greater than the change in current in the windings 6, resulting in a corrective change in the voltage across rectifier 4. A saturable reactor 19 may be connected in series with the rectifier filaments to increase the change of control current for a given change of output voltage, and a damping circuit 20, 21 may be included' in the control circuit to prevent hunting. In a modification, Fig. 2 (not shown), the filament winding is connected in series with a further transformer in the anode circuit to provide a still greater change in filament current for a change in output voltage. In a further modification, Fig. 3 (not shown), the control winding on the transformer is fed from a resonant circuit comprising, in series, a capacitor, a reactor saturated at the normal operating voltage, a resistance and a second rectifying bridge. By suitable adjustment of the resistance, the circuit will produce large changes in control current for small changes in alternator output voltage. In another modification, Fig. 4, the primary winding 30 of the transformer feeding the rectifier bridge 4 is connected across the capacitor 5 of the resonant circuit 5, 29. The D.C. control winding 11 is supplied from a rectifier 28 connected either as in the previous embodiment, or as shown, connected across the capacitor 25 of the resonant circuit 25, 26, this circuit in turn being connected across the capacitor 35 of a further series resonant circuit 35, 36 connected across the alternator output. A further D.C. control winding 32 is connected in series with the alternator field, and a winding 33 is connected in series with a resistor 34 across the field. Under steady conditions windings 33, 34 balance each other, but under transient conditions the resultant flux gives a stabilizing effect. A similar effect may be produced by omitting winding 32 and replacing resistance 34 by a capacitor. In a further modification, Fig. 5, the secondary winding 40 is shunted by two saturable reactors 42, 43 connected in a bridge arrangement in which the voltage of the rectifier 28 is connected in series with a voltage across resistance 37 (proportional to field current) and a voltage across resistance 38 derived from an auxiliary rectifier 59 (proportional to field voltage). In another embodiment, Fig..6, a three-phase transformer 46 has two sets 51, 52 of three interstar connected secondaries, set 51 and anodes a, b, c of the mercury arc rectifier 47 supplying the field excitation ; set 52 being connected directly across cathode and anodes d, e, f, which are grid controlled in dependence upon the output voltage of the alternator. An additional set of windings on the transformer connected in series with the output may be used to overcompound the alternator.