GB398838A - Improvements in or relating to apparatus for measuring or indicating the power factor of an alternating current supply system - Google Patents

Abstract

398,838. Power-factor measurements. ELLIOTT BROS. (LONDON), Ltd., Century Works, Lewisham, London, and SHOTTER, G. F., 14,' The Ridgeway, Friern Barnet, Middlesex. March 19, 1932, No. 8230. [Class 37.] Power-factor is measured or indicated by two electromagnet systems acting differentially on respective conducting elements mechanically connected, the rate of change of driving force of one electromagnet. system being different from that of the other for the same change in power-factor. One of the conducting elements is so shaped that its electromagnet system urges it more into or out of the field produced thereby. The magnet systems comprise current and voltage cores, the latter being provided with short-circuit rings or being energized through resistances, inductances or capacitances to produce a phase displacement of 90 degrees between two voltage fluxes. In a balanced three-phase system, the phase displacement is obtained by connection to different phases. A four-wire three-phase balanced system, Fig. 1, has voltage and current cores 3 .. 6 acting upon a circular rotor 1 and shaped rotor 2, the driving torque on disc 1 being proportional to the sine of the phase angle and that on disc 2 to the cosine. As the disc 1 rotates disc 2 into or out of the fields due to cores 4, 6 the torque on the shaped rotor varies until it balances that on disc 1 in which position power-factor is indicated. In a three-phase unbalanced system, two shaped rotors are employed, the series magnets being energized from two lines and the voltage magnets from midpoint tappings on autotransformers across two of the phases. A threephase four-wire unbalanced system has three shaped rotors, the current and voltage connections being made to all phases. In the threewire system the circular rotor is acted upon by two driving magnet systems and in the fourwire arrangement by three driving magnet systems. The electromagnet system acting on the shaped disc or discs may be duplicated to give increased control torque and readings at all power factors, the voltage core windings being connected in parallel and the current windings in series. Instead of employing common cores for the voltage and current magnets, they may be separated and employed in conjunction with disc rotors having central portions 52 cut away, Fig. 6, this arrangement preventing eddy current reaction between the magnet systems. The voltage pole tips may be provided with shading-rings 55. The circular rotor may have radial slots to prevent eddy current interaction. A single disc rotor may be employed to give torques depending on the sine and cosine functions. Drum rotors, or plates moved rectilinearly in guides may be employed. Instead of displacing the flux due to the voltage magnets, these may be connected to the same phase and the fluxes due to the currents displaced, or both fluxes may be relatively displaced. Specification 373,120 is referred to.