GB684415A - Improvements concerned with the speed and voltage control of stator fed a.c. commutator motors and frequency converters - Google Patents

Abstract

684,415. Control of A.C. motors. SCHWARZ, B. Feb. 1, 1951 [Feb. 7,1950], No. 3122/50. Class 38 (iii) [Also in Group XXXV] Relates to a system for the speed and voltage control of an A.C. commutator machine which has a stator winding connected to a source of supply and whichmay be used as a motor or as a frequency converter. In Fig. 1 the commutator machine M is used as a selfdriven frequency converter supplying motors 1M1, 1M2, through slip-rings SL which are connected to tappings in the rotor winding of the machine. The machines 1M1, 1M2, may be of the induction squirrel cage or wound rotor type, or they may be synchronous motors. Secondary windings S of a supply transformer T are connected to the separate stator windings St of the machine M and also to the separate phase windings WS, WR, of a single core induction regulator IR. Three of the commutator brushes B are each connected to a common connection between stator winding WS and rotor winding WR, the other brushes being connected to tappings t in windings S or tappings t<SP>1</SP> (shown dotted) in windings St. The variable and adjustable voltage applied to each phase of the rotor circuit of the machine M by positioning the induction regulator has a vertical position which is substantially at right angles to the voltage of the corresponding phase winding of the supply transformer. The machine M may be operated at speeds above and below synchronism, so providing reversible operation of the motors 1M1, 1M2. Suitable positioning of the tappings t or t<SP>1</SP> produces a constant voltage component for phase compensation of the rotor current of the machine M, and power factor correction. In order to obtain a fixed low frequency output, the machine M may be driven by a separate constant-speed motor, the stator winding St being disconnected during such operation. The machine M may be utilized as a phase converter fed from a single-phase supply, started by means of a phase-splitting device or by an auxiliary motor, and providing a multi-phase output when it has reached a speed sufficient for self-propulsion. The frequency converter may be used, for example, in its subsynchronous speed range only, the introduction of a fixed speed-reducing voltage into the rotor circuit of the machine allowing a reduction in size of the induction regulator. For this purpose the transformer secondary winding S may have a zig-zag connection Fig. 2 (not shown), or additional transformer secondary windings may be provided which are connected to tappings in the main secondary windings Fig. 3 (not shown). An arrangement for the simultaneous voltage control in both the stator and rotor circuits of the machine M may be provided when it is desired that a higher voltage/frequency ratio is obtained at low output frequences than at high frequencies. Fig. 4 (not shown) illustrates such an arrangement wherein the secondary winding of the transformer is connected in zig-zag formation and has a tapping connected to one brush of the commutator machine, the other corresponding brush of which is connected to a tapping in the machine stator winding. Two single core induction regulators are provided, their rotors being mechanically coupled. The rotor primary windings of the regulator are each connected to a constant voltage supply and their secondary windings are each connected to the supply transformer and to the machine stator winding. The induction regulators may be replaced by other variable voltage transformers or by additional secondary windings on the main supply transformer, these windings having a plurality of taps, Fig. 5 (not shown).