Patent GB-271061-A (GB)
Title: Improvements in or relating to electric systems comprising an asynchronous machine cascaded with a commutator machine
IPC: H02K17/38
Filed: 1927-04-27
Abstract: 271,061. Siemens - Schuckertmerke Ges. May 15, 1926, [Convention date]. Alternating-current commutator and induction machines. - In a group consisting of an asynchronous machine cascaded with a commutator machine which is excited at the supply frequency directly or through voltage or frequency transformers, substantially magneticallly closed iron inductances are connected in or in parallel with the excitation circuit of the commutator machine and are so dimensioned that at normal supply voltage their magnetizing current is not proportional to their induced voltage; further, additional inductances are employed excited by the magnetizing current of the saturated inductances, directly or through converters, and the voltage induced by the magnetizing is introduced into the excitation circuit of the commutator machine in opposition to the supply voltage applied thereto. In the form shown in Fig. 4, the asynchronous motor or generator 1 supplying wattless current is coupled with a commutator machine 2 excited from the line through a transformer 3. The transformer has strong saturation at the normal network voltage and choking coils 4 in series with the transformer carry the relatively large magnetizing current and cause a corresponding drop in the excitation volts. If the network volts drop, the excitation current of the machine 2, and therefore the excitation of the main machine 1, do not fall much at first as the magnetizing current in the transformer falls considerably owing to the decrease in the saturation. As a result, the action of the choking coils becomes less and the variation in the network voltage when the wattless current fluctuates is thus reduced. The saturated component of the excitation circuit may. alternatively, be in the commutator machine itself, in which case this machine is excited either at network or slip frequency. Or a frequency transformer may be used for the excitation and for producing the saturation, or choking coils, which are saturated at normal network frequency, may be connected in parallel at any desired point in the excitation circuit. Other methods of producing saturation consists in using for the excitation a frequency transformer having closed slots in that portion which carries network frequency and in providing the transformer 3 with a third " leg " or a separate air leakage gap. Fig. 5 shows a form in which the choking coils 4, Fig. 4, are replaced by a transformer 5 the primary coils of which carry the magnetizing current producing the saturation while its secondary coils are connected between the transformer 3 and the commutator machine in such a manner as to reduce the excitation of the machine 2. By sufficiently increasing the saturation, the excitation of the commutator machine mav be increased when the network volts fall. The arrangements described, when applied to machines which supply only or chiefly wattless current, prevent an undue rise in the stator voltage of such machines in the event of their being switched off without previous reduction in their excitation.