Patent GB-868400-A (GB)
Title: Alternating current generating system
IPC: H02P9/42
Filed: 1959-08-28
Abstract: 868,400. Induction generators. JACK & HEINTZ Inc. Aug. 28, 1959 [Oct. 6, 1958], No. 29429/59. Classes 35 and 38 (2). An induction generator 11 is enabled to supply current at a predetermined frequency from its stator windings 14 even though its shaft S is driven at varying speed, by exciting the rotor windings S with slip-frequency current from a static frequency changer 17 fed from a synchronous exciter 18 on the main machineshaft. The exciter 18 is preferably of frequency twice or three times the main generator frequency and may absorb or generate power depending on the slip. The exciter 18 is excited in turn by a permanent magnet generator 22 through a rectifier and regulator 23. The static frequency changer is controlled to convert the current a<SP>1</SP>, b<SP>1</SP>, c<SP>1</SP> from the high-frequency generating exciter 18 into the desired slip-frequency excitation a, b, c for the main machine by impulses from a pulse generator 25. This pulse generator receives slip-frequency signals a<SP>11</SP>, b<SP>11</SP>, c<SP>11</SP> from a commutator frequency changer 27 whose rotor is excited at the desired output frequency from a source 28, e.g. having tuning fork control, and the slip-frequency is taken from a commutator 31 connected to the rotor winding. There are no stator windings. Another type of frequency-changer may be used, if desired. In a modification, the slip-frequency generator is dispensed with, and instead the proper control signals for the frequency changer are derived directly from the pulse generator from the shaft frequency and a reference signal of the desired output frequency by suitable internal connections. The main machine may be further excited by a synchronous condenser 15. The frequency changer 17.-The frequency changer 17 is of the cycloconverter type and consists of three sections 17a, 17b, 17c, one for each phase of the excitation current required. Each section is connected to a corresponding section 25a, 25b, 25c of the pulse generator. Only phase (a) is described, the others being identical. The frequency changer preferably comprises six controlled silicon rectifiers 1-6 connected in back-to-back pairs to phases a<SP>1</SP>, b<SP>1</SP>, c<SP>1</SP> of the exciter 18 and triggered to conduct by impulses applied at their gate electrodes 48. Rectifiers 2, 4, 6 conduct when the slip frequency current is positive, and rectifiers 1, 3, 5 conduct when it is negative. Current from the positive rectifiers 2, 4, 6 passes to line 41, through windings on the pulse generator, and through a choke 42, while current from the negative rectifiers 1, 3, 5 passes to line 39, through windings on the pulse generator, and through a choke 42. Both of these chokes, which actually constitute a centre-tapped choke feed into the phase a of the main machine excitation. More rectifiers or more phases may be employed to improve the output waveform. The pulse generator.-The rectifiers 1-6 are triggered by impulses from transformer units 1<SP>1</SP>-6<SP>1</SP>, respectively. The cores of these transformers are of square-loop material having thus two steady-state states, an impulse being generated at change-over. The cores are in pairs corresponding to the rectifier pairs and have input windings 50-53 and output winding 54, and input windings 60-63 and output winding 64, respectively, output rectifiers 55, 65 being employed if desired. Coils 50, 60 on all cores are fed in series from phase all of the slip-frequency generator. Coils 51, 61 on all cores are fed between phases a<SP>1</SP>, c<SP>1</SP> of the highfrequency supply from exciter 18, through a choke 67 to produce a phase lag of 120 degrees with respect to phase a<SP>1</SP> fed to rectifiers 1, 2. The other cores are symmetrically connected. The windings on each core-pair are reversely connected so that the cores fire 180 degrees apart. Windings 53, 63 carry the output current of the positive and negative rectifiers, respectively, and so prevent simultaneous conduction of both rectifiers of a pair. Further feed-back windings 52, 62 across shunts 70, 71 may be provided to facilitate commutation. In the modification of the invention in which the slip-frequency generator is dispensed with, the core windings 50, 60 are fed across phases a<SP>11</SP>, b<SP>11</SP>, c<SP>11</SP>, respectively for each core pair instead of from phase a<SP>11</SP> alone.