GB935483A - Improvements in or relating to excitation systems for alternators - Google Patents

Abstract

935,483. Alternators; automatic voltage control. MULLARD Ltd. Aug. 24, 1961 [Aug. 29, 1960], No. 29720/60. Classes 35 and 38 (4). An alternator excitation system comprises two sources connected in parallel with each other and supplying current to the field winding, one source providing a rectified current proportional to the output current and the other providing a rectified current dependent upon the output voltage of the alternator, the output voltage of the alternator being automatically regulated by adjusting the voltage of the second source in dependence on changes in the output voltage wherein the adjusted voltage of the second source is augmented by a voltage component dependent upon the voltage drop across the field winding. The field current Ie is compounded from two currents Iv and Ic. Fig. 1 (not shown) and Fig. 3, the former current being derived from a silicon controlled rectifier bridge regulator 14 controlled by an error voltage produced in a comparator 17 and the latter current from the output current of the alternator via rectifying bridge 12. In order to overcome the back bias across diode D1 due to the voltage drop across the field winding 5 caused by the current Ic, the output voltage of the regulator 14 is augmented by a feed forward source 19 providing a voltage drop equal to the voltage drop IcR produced by current Ic across the field, with the result that the gain of the D.C. amplifier 18 can be reduced with increase in stability. Since in practice the field resistance R is not constant and can vary considerably with temperature it is necessary to monitor the voltage across R, subtract from it the contribution due to current Iv and use the resultant to control the feed forward voltage source 19. In order to eliminate the necessity of having an additional source 19, the regulator 14 may be controlled by an error signal, Fig. 4, comprising a first signal (1) due to the alternator output voltage and a second signal (2) equal to a fraction of the voltage across the field minus a voltage equal to the maximum contribution from Iv under any condition (Iv max R max) which may be carried out using a battery. The circuit of signal (2) also includes a potential divider for reducing the initial voltage to the desired fraction. Specification 935,482 is referred to.