Patent GB-243424-A (GB)
Title: Improvements in means for measuring volt amperes
IPC: G01R5/14
Filed: 1924-08-23
Abstract: 243,424. Raworth, J. E., (Lincoln Meter Co., Ltd.). Aug. 23, 1924. Special quantities, registering; phase-differences and power-factors, measuring; power meters.-A meter of the thermal type for measuring volt-amperes or volt-ampere hours on an alternating-current circuit has pairs of heaters arranged in parallel as regards current from the mains and in series as regards currents generated by windings on a power-factor meter, so that a difference in the heat generated is produced proportional to the volt-amperes. The apparatus is shown arranged for measuring the demand in volt-amperes on a three-phase circuit ABC. Current from the current transformer T1 of phase C is led to similar heaters H1, H2, where it divides and flows equally therethrough and through a winding in slots 3, 9 of the stator G of a power-factor meter. The winding in slots 3, 9 is connected at its mid point to a winding in the slots 2, 8 which is connected to the current transformer T1. Similarly the current from the current transformer T2 is led to similar heaters H3, H4, between which it divides equally and from thence passes through a winding in the slots 5. 11 at 120 electrical degrees from the winding in the slots 3, 9. A winding in the slots 6. 12 is taken from the midpoint of the winding in the. slots 5, 11. The current from the transformer T1, after passing through the windings in the slots 3, 9 and 2, 8 is combined with the current of the transformer T2 after passing through the windings in the slots 5, 11 and 6, 12 and the resultant current passes through a winding in the slots 4, 10 and returns to the transformers as shown. The rotor k of the power factor meter is excited from the voltage of the three phase circuit ABC and is free to rotate, the position which it takes up in accordance with the power-factor being determined by the phase of the currents in the windings in the slots 2, 8; 6, 12; and 4, 10. Since the currents in the halves of the windings in the slots 3, 9 and 5, 11 are equal and opposite except for the influence of the voltages generated therein by transformer action, these windings have no position-determining effect on the rotor k. A voltage is induced in the windings in the slots 3, 9 differing in phase by 30‹ from the current in the slots 2, 8. This induced voltage causes a current to flow in series through the heaters H1, H2 which combines with the load current from the transformer T1 so as to cause the difference in heat generated in the heaters HI, H2, to be always proportional to volt-amperes. Similarly the heaters H3, H4 measure the voltamperes in phase A, the sum of these quantities giving the total volt-amperes of the circuit ABC. An indicating-arm may be attached to the rotor k to indicate the power-factor, and may move indexes to indicate its maximum motion in each direction. The torque due to the rotating field produced by the rotor winding may be compensated for by a spiral spring or by strengthening the position-determining windings on the stator. In order to enable a low-voltage meter to be used on a high-tension circuit, the stator may be on one voltage and act as a voltage transformer by having lower voltage windings on the rotor or vice versa. The windings on the power-factor meter may be modified for a two-phase circuit, or phase-splitting means may be employed for producing the rotating field in the rotor when measurements on a single-phase system are required.