Increased speed because of reduced inertia and possible, as will be shown, to polyphase directional regreater torque is obtained from the polyphase dicombine the polyphase funclays in relay protective rectional relay described in this paper. A tion of 3 elements in a single arrangements is generally recsingle cup-like moving element is used in place element without the use of ognized today. of multiple disks as used heretofore, the polyauxiliary switching. This has The first polyphase direcphase function being obtained by the use of resulted in a large reduction tional relays consisted of 2 multiple poles.in size and weight, together or 3 single-phase elements conwith low inertia and high nected to a common shaft.speed, as illustrated in table I. These single-phase elements were almost identical to existThe relay operating element is similar to the element ing forms of meters, namely, induction-disk watt-hour shown in figure 1, but a magnetic return path is provided meters and indicating wattmeters. During the evolu-for each salient. tion of these devices, many improvements were made, someThe production of torque is like that in the inductionfollowing the advances in meter design, and others being disk directional relay and the unit may be considered to the result of problems met only in the protective relay field. be a number of directional magnets acting upon the same Notwithstanding these improvements, it became in-rotor. creasingly apparent that a relay with higher speed and Referring to figure 2, each coil produces rotor currents greater torque was necessary for many applications. which react with the flux of each other coil to produce Furthermore, such a relay would be convenient even for torque. The torque of adjacent pairs of salients is large, those applications not requiring high speed, providing it relative to that of other pairs. For example, the torque involved no added expense. produced by combination 1 and 3 is approximately 15 per cent of the torque produced by combination 1 and 2, Improved Single-Phase Directional Relay while that produced by combination 1 and 4 is about 2 per cent, a negligible amount. The torques of alternate Improvements in the speed of single-phase and poly-salients (e. g., 1 and 3 or 2 and 4) being appreciable, the phase induction-disk relays have been limited by inher-connections are so arranged that these torques balance ent high disk inertia and low torque efficiency. These out substantially to zero; an adjustment being provided in limitations are largely overcome in the induction cylinder addition, for more accurate balance of the current torques. or cup relay recently developed. A single-phase power directional element of this design is shown in figure 1.

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