Patent GB-283957-A (GB)
Title: Arrangements for exciting commutator machines cascaded with asynchronous machines
IPC: H02K17/38
Filed: 1928-01-17
Abstract: 283,957. Siemens - Schuckertwerke Akt.-Ges., (Assignees of Siemens-Schuckertwerke Ges.). Jan. 22, 1927, [Convention date]. Alternating-current induction machines. - In an arrangement in which the commutator machine cascaded with an asynchronous machine is excited with a voltage proportional to the slip frequency, there is inserted in the excitation circuit of the cascaded commutator machine an auxiliary commutator machine which runs synchronously with the synchronous machine and is excited through its stator with constant voltage (preferably the mains voltage) notwithstanding the variable slip of the synchronous machine. The arrangement is particularly applicable to asynchronous machines exposed to recurrent load shocks, such as the motors driving rolling mills or compressors or asynchronous generators driven by reciprocating engines, and it results in the synchronous machine always taking up or giving off the same electrical power independently of the mechanical power taken from or given to it. Fig. 1 shows a form in which the commutator machine 2, cascaded with the asynchronous machine 1, is rotorexcited with mains frequency and proportionately to the slip by means of a 3-phase shunt commutator exciting machine 12 mechanically coupled to the machine 1 and stator excited with constant. voltage from the mains through a regulating transformer 13. By adjustment of the excitation of the machine 12, the secondary voltage of the main machine 1 may be cancelled at any slip. The voltage introduced by the transformer 14 is, in such a case, for giving or taking up constant power independently of the slip. If the machine 12 only cancels a proportion of the secondary voltage of the main machine, the no-load speed and phase compensation of the main machine mav be controlled by means of the transformer 14. If the stator of the machine 12 has salient stator poles its rotor voltage, which will change with change of speed of the main machine, may be completely or partially cancelled by the transformer 14. The machine 12 may give single instead of polyphase voltage. In the form shown in Fig. 2, the commutator machine 2 is only connected to the main machine 1 electrically. It is mechanically connected to an asynchronous machine 15 and is rotor-excited at approximately mains frequency by means of a frequency transformer 16, on its own shaft, fed from the mains through a transformer 17 and supplying the rotor of a machine 18 on the shaft of the main machine. This machine 18 gives the excitation for the cascaded commutator machine 2. The voltage of the machine 12, which increases proportionately to the slip of the machine 1 is introduced into the excitation circuit of the machine 2 by means of .a transformer 19. The size of the machine 12 may be reduced by connecting it as shown in Fig. 3, into a cascaded generating circuit. This circuit consists of an asynchronous motor 20, an asynchronous generator 21 with its stator winding connected to the slip-rings of the cascaded machine 2 and its rotor cascaded with an auxiliary commutator machine 22 rotor-fed from the machine 12. The commutator machine 2 may also be excited through its stator, as shown in Fig. 4, where the stator coil 23 carries excitation at slip frequency. This excitation of the machine 2 is derived partly from the mains through a frequency transformer 24 and partly from the stator-excited auxiliary machine 12 through a transformer 25. In place of acting directly in the excitation circuit of the cascaded machine 2, the machine 12 may control a quick-action regulator which, in turn, regulates the excitation.