GB923340A - Speed control system for electrically powered mechanism - Google Patents

Abstract

923,340. Automatic speed control. SENECA FALLS MACHINE CO. July 23, 1959, No. 25339/59. Class 38 (4). [Also in Group XXXVI] A circuit for sensing the load on an induction motor has a first input signal derived from the phase-to-ground voltage V 1 , a second input signal (voltage V,) being obtained from a current transformer in the line from which voltage V 1 is derived. Voltages V 2 , V 3 represent the transformed values of V 1 , and the rectifier D i accepts the algebraic sum of V 2 , V 4 to produce voltage V 5 , whilst voltage V 6 is the sum of voltages V 3 , V 4 . Voltage V 7 is then the sum of voltages V 5 , V 6 and has a magnitude directly proportional to the current in phase with the voltage V 1 . The combined action of potentiometer P 1 and capacitors C 1 , C 2 , filter the voltages V 5 , V 6 , and provide a smooth direct voltage V 7 , the potentiometer providing means for balancing V 7 to zero for a particular load. An A.C. tachometer generator supplies a signal Vt whose phase reverses with reversal of the motor shaft. This signal is amplified and has its magnitude limited to a constant value VtL to provide a constant voltage V 9 acting as the carrier input to a ring modulator 20, V 7 being the D.C. input to the same modulator. The output voltage V 8 is thus an A.C. signal with the same phase as V 9 and whose magnitude is proportional to V 7 . In a modification, the voltage V 9 is connected to a demodulator and is compared with a reference voltage to provide a resultant voltage whose magnitude is proportional to V 9 and whose polarity reverses as the phase of V 9 reverses. This resultant voltage operates a polarity-sensitive relay to control the D.C. load signal V 7 , whereby a signal of the correct polarity, in accordance with the direction of rotation of the motor, is provided, Fig. 6 (not shown). The A.C. motor may be the prime mover of a mechanical amplifier with an integrally coupled servomotor and tachometer to provide low-level torque and speed. A speed reference potentiometer provides an A.C. signal to an electronic amplifier which also receives a signal from the tachometer. The amplifier output forces the servomotor and tachometer to reach a no-load speed corresponding to the setting of the potentiometer. By adding a load-responsive signal, the speed may be held constant with changing load, or may be varied in a predetermined manner, Fig. 5 (not shown). The load-responsive circuit may provide a speed boost when the load exceeds a predetermined value so that speed-drop may be corrected for variable loads above any fixed losses. For three-phase systems without an earthed star-point, the voltage V 1 is derived from two phases of the supply through a resistance-capacitance network, Fig. 3b (not shown). Specification 686,650 is referred to.