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The regulation curve of a double 3-phase mercury-arc rectifier is characterized by a sharp fall in the output voltage as the load current increases from zero to a value about twice that of the peak magnetizing current of the inter-phase reactor. Over this current range the mode of operation of the rectifier changes; at no load each anode fires in turn, in a manner typical of a 6-phase rectifier, but when the load current is just sufficient to magnetize the inter-phase reactor, double 3-phase operation of the rectifier commences. To reduce the no-load voltage rise, which in some rectifier applications cannot be tolerated, one of the more satisfactory methods is to excite the inter-phase reactor from an auxiliary source. It is shown in the paper that, by using a frequency tripler as a source of auxiliary excitation, a tenfold reduction is obtained both in the no-load voltage rise and in the critical load current at which double 3-phase operation commences. The variation of the rectifier no-load voltage rise with the phase and magnitude of the frequency-tripler output voltage and with the alternating supply voltage is also deduced.
L.R. Blake (Mon,) studied this question.