GB695364A - Improvements in and relating to alternating current electrical measuring circuit arrangements - Google Patents

Abstract

695,364. Supply meters; power measurements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Feb. 13, 1951 [Feb. 23, 1950], No. 4719/50. Class 37. In order to measure volt amperes or voltampere-hours, a watt meter or watt hour meter has both current and voltage coils energized in accordance with either the voltage or current of the circuit. The meter M has its voltage coil 3 connected across the mains 1, 2 and its current coil 4 connected across the mains 1, 2 in series with an operating coil 5 of a torque balance relay TR, a ballast resistor 6, and the contact arm 7 of a potentiometer 8 connected across the mains 1, 2. The relay TR includes a second operating coil 9 energized in accordance with the current in the main 2 through a current transformer 10. The relay TR is of the induction type and is driven in opposite directions by the coils 5, 9 so that its moving contact 11 engages one of two contacts 16, 17 according as the ampere turns of the coil 5 are greater or less than those of the coil 9. The contact arm 7 is driven through a reduction gear 12 from a reversible motor RM having a winding 13 connected across the mains 1, 2 and a winding 14 connected between the moving contact 11 and the centre tapping of a resistor 15 connected across the mains 1, 2. The motor RM drives the contact arm 7 in one direction or the other according to the relative senses of the currents in the coils 13, 14, which depends upon which of the contacts 16, 17 is engaged by the moving contact 11. The potentiometer 8 is thus adjusted so that the current in the coil 4 is maintained in proportion to that in the mains 1, 2. Since the currents in both coils 3, 4 are derived from the mains voltage they have a predetermined phase relationship which is unaffected by the phase relation between the current and voltage in the mains. This phase relationship may be modified by a resistance or reactance so that the meter M may operate at any desired power factor. Alternatively the current coil 4 may be energized directly in accordance with the current in the circuit by means of a current transformer, whilst the voltage coil 3 is supplied from the same transformer under control of a motor operated rheostat controlled by a torque balance relay operating in accordance with the voltage of the circuit. Modifications are described for use in balanced or unbalanced polyphase systems. Specification 670,536 is referred to.