Patent GB-768749-A (GB)
Title: Improvements in electrical circuits for measuring power in an alternating current load
IPC: G01R21/00
Filed: 1954-12-20
Abstract: 768,749. Power measurement. CANADIAN GENERAL ELECTRIC CO., Ltd. Dec. 20, 1954 [Dec. 19, 1953], No. 36793/54. Class 37. An electric circuit producing a direct voltage representing power in an A.C. load comprises a balanced detector system with input means producing and applying to the two inputs thereof respective voltages representing the voltage across and current in such load so as to produce direct output voltages proportional respectively to the vector sum and difference of the input voltages, there being provided non- linear resistance means in the outputs of the balanced detector system which produce the power representative direct voltage proportionally to the algebraic sum of voltages derived by the non-linear resistance means in dependence on and varying with the squares of the respective output voltages. An electric motor load L is connected to A.C. source A through switch S while a voltage transformer T1 is connected across the supply lines and a current transformer T2 in series therewith. The voltage transformer secondary is centre tapped to the slider 9 of potentiometer R2 in series with resistance R3 and the cathodes of opposed diode rectifiers V1, V2 whose anodes are capacitance coupled to the ends of the secondary, while the current transformer secondary is connected in parallel with resistance R1 to the extremities of the potentiometer R2 and resistance R3. The diode anodes are connected through non- linear resistances T3, T4 of law i + KV<SP>2</SP>, where i = current, V = voltage, K = constant, to the ends of resistances R4, R6 in series with a load balancing potentiometer R5 whose slider 10 is connected to the diode cathodes while the opposite ends of the resistances R4, R6 are respectively connected to the grid of negative feedback amplifier triode 3 (capacitance bypassed to cathode for A.C.) and to the slider 12 of potentiometer R11 in a resistance chain R9, R10, R11, R12 across the H.T. supply, the triode anode being connected to the junction of R9, R10 and to output terminal 7, and the cathode being connected to H.T.-and earth over resistance R7 and variable resistance R8. Initially with switch S open, resistance R8 is adjusted to develop at terminal 7 a unidirectional control or measurement voltage representing zero power, and with switch S closed and power supplied to the motor, voltages E proportional to line voltage are supplied in antiphase to the diode anodes, while a voltage I proportional to the line current and leading or lagging voltage E by the phase angle 6 of the line current is connected between the diode cathodes and the centre point of the voltage transformer secondary. Voltages E and I are vectorially combined and rectified to develop at the respective anodes unidirectional voltages #2(E+I cos 6) and - #2(E - I cos #), where 6 is small. By reason of the non-linearity of resistances T3, T4, the voltages at the junctions of T3, R4, T4, R6 referred to the slider 10 of potentiometer R5 are respectively proportional to (E+I cos #)<SP>2</SP> and (E - I cos #)<SP>2</SP>, so that their algebraic sum is proportional to (E+I cos #)<SP>2</SP> - (E - I cos #)<SP>2</SP> = 4 EI cos 6, i.e. to the power consumed in the load, and the corresponding amplified voltage at terminal 7 is applied to deflect a D.C. meter or operate a control circuit regulating the motor load. Slider 12 of potentiometer R11 is adjustable to calibrate the device to give a desired voltage for a particular value of power input, and slider 9 of potentiometer R2 is adjustable to regulate the sensitivity of response to the current transformer output. The diodes are replaceable by germanium or selenium rectifiers, while the current and voltage transformers may be functionally transposed. The device is also applicable to the measurement and/or control of the power supplied by a generator to a load, and may be applied to a balanced polyphase system, while the measurement and/or control voltage may be derived directly by across resistance chainR4, R5, R6 without amplification.