GB629586A - Improvements in and relating to wattless current regulating systems for a.c. circuits - Google Patents

Abstract

629,586. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. July 3, 1944, No. 12643. Convention date, July 1, 1943. [Class 38 (iv)] In a wattless current regulating system, for polyphase circuits, the exciter of a synchronous motor has buck and boost fields which are respectively energized by voltages proportional to the line voltage and current of the motor supply circuit so as to maintain the power factor at a predetermined value. As shown, the regulator is applied to the control of a synchronous motor 4, the excitation of which is varied by buck and boost field windings 14, 15 of an amplidyne generator. A transformer 19 is connected through transformer 23 to lines 1 and 2 of the main circuit, its secondary having tapping points 27 connected across two rectifiers 17 and 18. Two current transformers 25 and 26 are connected to a variable resistor 24 which is itself connected between rectifiers 17, 18 and a tap switch 28 on the transformer 19. The current circulating in the resistor 24 is a function of the vector difference of the currents in lines 2 and 3 of the main circuit and is 30 degrees out of quadrature with the voltage across the transformer 19 which is proportional to the voltage between lines 1 and 2. Fig. 3 shows the relation between input voltages of the rectifiers 17, 18 when the tap switch is at the mid-point of the transformer 19. As the voltage across the boost winding is greater than that across the buck winding the amplidyne generator will boost the motor excitation advancing the phase of its current until buck and boost voltages are equal when the resistor volt drop will be at right angles to the line voltage component. This balance condition represents a phase advance of line current by thirty degrees. This current may increase or decrease without unbalancing the regulator. Fig. 5 shows the vector diagram when the transformer tapping point is displaced so that a balance can only occur when the phase of the resistor volt drop is displaced thirty degrees thus retarding the phase of the line current and establishing unit power factor. When the load varies the regulator holds the current to such magnitude and phase that the end of the resistor volt drop vector has its locus on the perpendicular bisector of the voltage vector. As the current decreases it will become more and more lagging until at no load a definite amount of zero power factor wattless current will be maintained in the synchronous motor. In modifications the transformer 19 may have two separate secondary coils opposed in phase ; it may be energized by a rotary phase shifter, in which case only one current transformer may be used ; a phase shifter may also be used for supplying the resistor 24 ; the outputs of the rectifiers may be passed through individual resistors in series, a single field winding being connected across them. If the current transformers were omitted the system would automatically maintain unity power factor with the tap switch on the mid-tap. The system may be otherwise arranged to maintain constant reactive power at all loads.