GB210097A - Improvements in and relating to alternating-current electric generators - Google Patents

Abstract

210,097. Contell, F. July 25, 1922. Exciting; high-frequency machines; rotary transformers.-In an alternating-current generator excited by alternating-current in which the irequency is increased or decreased by half of the product of the pole number and the speed per second in accordance with the direction of rotation of the exciting field, for the higher frequency the generating coils are grouped in mesh and for the lower frequency they are grouped in star and the exciting current is increased in order to give approximately equal terminal volts in both cases. The exciting current may be derived from the machine itself, from mains, or from another machine on the same shaft as the main generator. Fig. 2 shows a group which gives a fixed frequency independently of the speed of the set and consists of an auxiliary alternator N excited with alternating-current and exciting a main alternator M on a common driven shaft P. If f be the frequency supplied to the machine N, for example through leads 1, 2, 3, this machine excites the machine M with a frequency of f - n p / 2 where n and p are the speed and pole number of the-machine N. The machine M has the same pole number p and is so arranged as to give a frequency of f n p / 2 + n p / 2 i.e. it gives on the bus bars 4, 5, 6 the original frequency f independently of the speed of the set. By means of two such sets of two generators and a series of alternator connected so that the current generated by each one excites the next one to give a cumulative frequency increase in the series in known manner, currents of still higher frequency may be generated as follows :-The current from the last machine of a series of sav ten generators is used to energize one set of two generators, as shown in Fig. 2, and the constant high-frequency current from this set is used to excite the first of the series of ten machines again. This gives a further increase of frequency which is taken up on the second set of two generators which remains running as a selfexciting constant high frequency machine, the series of generators being no longer required. The cores of the high frequency machine are made of non-magnetic materials to reduce hysteresis losses. Specification 23396/08 is referred to.