GB159148A - Improvements relating to electrical measuring, indicating or relay apparatus for polyphase circuits - Google Patents

Abstract

159,148. Metropolitan-Vickers Electrical Co., Ltd., (Assignees of Fortescue, C. le G., Chubb, L. W., and Slepian, J.). Feb. 13, 1920, [Convention date]. Induction-motor relays.-Apparatus for measuring, indicating, or being operated by either or both of the two balanced polyphase components into which an unbalanced polyphase system of currents or voltages may be resolved, comprises a synchronously running machine with one winding connected directly or indirectly to the system so as to produce a magnetic field having two balanced components, one of which is stationary with respect to the other winding of the machine and the other of which rotates at a double-frequency speed with respect to this other winding, one or more measuring, indicating or relay instruments being connected to one of the windings of the machine. In the form shown, the stator 7 of an induction motor 8 is connected in star to the mains 2, 3, 4 carrying a three-phase unbalanced load. The rotor 9 of the motor 8 is driven in the direction opposite to that in which it would normally rotate by a synchronous motor 10, the rotor windings 9 being connected to a voltmeter 11 of the induction-motor type, which may have relay contacts 12. One component of the field produced by the stator 7 rotates at the same speed and in the same direction as the rotor 9, and therefore does not cut the rotor windings. The other component cuts the rotor windings with double the frequency of supply and induces a voltage which is measured by the voltmeter 11. The other component may be measured by reversing the connections of the stator winding 7 to the mains 3, 4, or by reversing the motor 10. The motor 8 may have a squirrel-cage rotor, in which case the stator windings 7 are connected between the mains 2, 3, 4 and the voltmeter 11. Two induction motors may be employed having squirrelcage rotors driven synchronously by the motor 10, the stator windings of the two induction motor; being connected in series to the system so that one offers high impedance to one component of the voltage system and the other offers high impedance to the other voltage component rotating in the opposite direction. Two voltmeters or relays are connected one with the windings of each machine; for instance, the separate components of the pressure may be separately measured by voltmeters connected across one phase of each stator winding, balancing-reactances being connected across the other phases. The separate components of the currents may be measured by similar means, the stator windings being connected to the secondaries of current transformers, which secondaries may be shunted by resistances. Various factors, such as the components of the power, may thus be found.