Patent GB-760515-A (GB)
Title: Improvements in or relating to alternating current electrical measuring instruments
IPC: G01R11/36
Filed: 1954-04-21
Abstract: 760,515. Power measurement; electric supply meters. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. April 21, 1954 [April 30, 1953], No. 11448/54. Class 37. An electric wattmeter or watt hourmeter for an A.C. circuit comprises a current transformer having a magnetic core and means for compensating ratio errors in said current transformers; such compensating means comprising auxiliary windings of said magnetic cores which are energized from a source of substantially constant potential for producing in the cores magnetomotive forces substantially equivalent to the exciting magnetomotive forces required to supply the core losses and core magnetization flux components. Fig. 1 shows apparatus for measuring the energy and power demand of a load 6 on a generator 4 over a 3-phase A.C. circuit 12, 14, 16 wherein a detachable integraing watt hourmeter 8 is connected into the circuit over plural jaws 20 engaging contact blades 24, and comprises two identical single phase watt hourmeter elements 28, 30 having respective voltage windings 32, 41 ; and current windings 34, 42 rotating armature members 36, 44 mounted on a common shaft 38, under the influence of adjustable damping magnets 39, 46 for heavy, and adjustable short-circuited turns 40, 48 for light, load calibration adjustment. The current windings are energized respectively from current transformers 49, 50 whose primaries are the phase conductors 12, 14 (phase conductor 16 being direct) while voltage windings 32, 41 are energized across phase conductors 14, 16 and 12, 16 respectively (through voltage transformers if required) so that the combined meter 8 registers the total energy in the load according to Blondel's Theorem. An associated thermal demand meter 54 comprises a rotary shaft 55 driving a pointer 62 and biased in opposite directions by bimetallic thermally responsive spiral springs 56, 57 respectively heated by coils 64, 66 and 68, 70. Coils 64, 68 are series connected across the secondary 74 of a voltage transformer 76 energized from phase conductors 14, 16, and are also energized by the output of current transformer 50 in, series with current winding 34, the junction of heaters 64, 68 and the centre tap of the secondary 74. Similarly heaters 66, 70 are series connected across secondary 92 of voltage transformer 94 energized from phase conductors 12, 16, and are also energized by the output of current transformer 49 in series with current winding 42, the junction of heaters 66, 70, and the centre tap of the secondary 92. The phase senses are such that heaters 66, 70 are respectively energized proportionately to the sum and the difference of a current proportional to the voltage between phase conductors 12, 16 and the current in phase conductor 12, while heaters 64, 68 are similarly energized proportionately to the sum and the difference of a current proportional to the voltage between conductors 14, 16 and the current in conductor 14, so that the pointer indicates the power demand of the load. Auxiliary windings 108, 112 respectively coupled to voltage windings 32, 41 illuminate lamps 110, 114 to indicate correct operation of voltage windings 32, 41 and voltage transformers 76, 94 in the manner set forth in U.S.A. Specification 2,571,938; a first degree of illumination indicating correct operation; a second degree of illumination failure of a transformer and a third degree of illumination failure of an auxiliary winding. Current transformers 49, 50 each comprise double secondaries 99, 81 wound on respective pairs of high permeability closed circuit nickel-iron cores 116, 118 through which pass the phase conductors 12, 14, and having compensating windings 120, 122 opposedly wound on the corresponding cores as set forth below. The light-load response of the separate meter elements for a predetermined load power factor is corrected by identical short-circuited turns 40, 48 of conductive non-magnetic material adjustably positioned adjacently to the voltage coil poles to intercept a variable proportion of the flux therefrom whereby a shaded pole torque is applied to the associated meter disc in dependence upon the energizing voltage and the position of the compensating device, adjustable at a light load at unity power factor, whereby the calibration remaining constant for all subsequent conditions of power factor and loading of the associated single phase meter element for constant voltage between the associated phase conductors. The ratio error (which is large at light loads compared with its value at full load) is corrected at full load by adjusting permanent magnets 39, 46 relatively to their associated meter discs to impose a correcting torque and compensating the current transformers 49, 50 at light loads by means of the windings 120, 122 wound in opposition on the respective pairs of cores 116, 118 and energized at substantially constant voltage through current limiting resistors 124, 126 from auxiliary windings 112, 108 (or directly across secondaries 92, 74 of voltage transformers 94, 76). The windings produce magnetizations in their respective cores sufficient to overcome the core losses and initial magnetizing fluxes arising at low loads; the percentage correction decreasing with increasing load so that the differential between ratio errors at light and heavy loads is substantially narrowed, while the windings have substantially zero resultant magnetic coupling with the primaries and secondaries of the associated cores. The ratio errors of the current transformers 49, 50 are thus reduced at light loads, whereby reduced calibrating torques due to short-circuited turns 40, 48 are required for correct light load calibration, and the armature rotational speed is made substantially proportional to the power factor of the load, while the residual ratio error at heavy loads is corrected by adjustment of permanent magnets 39, 46 as above. Phase angle errors are stated to be similarly corrected.