Determination of the constants of instrument transformers . to E^ givingtan 0. But as such an instrument does not seem to be available,some indirect method has to be used. The method first tried wasto balance the small quadrature component Q, using as a source ofemf. a second phase from a two-phase machine, and making use ofa telephone as detector. The capacity effects of the various partsof the circuit reduced the accuracy at low frequencies on accountof the greater sensitiveness of the telephone to the harmonicspresent. Either a thermo-galvanometer or a vibration galva-nometer gave greater a
Determination of the constants of instrument transformers . to E^ givingtan 0. But as such an instrument does not seem to be available,some indirect method has to be used. The method first tried wasto balance the small quadrature component Q, using as a source ofemf. a second phase from a two-phase machine, and making use ofa telephone as detector. The capacity effects of the various partsof the circuit reduced the accuracy at low frequencies on accountof the greater sensitiveness of the telephone to the harmonicspresent. Either a thermo-galvanometer or a vibration galva-nometer gave greater accuracy, but neither was satisfactory asregards ease of manipulation. A method considerably more accu-rate is to use a vibration galvanometer and only a single source ofemf., placing variable inductances in the two parts of the highresistance circuit and alternately adjusting the resistances andinductances for zero deflection, and then calculating the differencein the phase angles by which the emf. leads the current in the twoparts of the high Fig. 3.—Two-dynamometer method for test of potential transformers. The method finally adopted as most satisfactory was to use asdetectors two electrodynamometers (Fig. 3). The field coil of one ^g^J^] Instrument Transformers. 283 was placed in series with the high resistance, while the field of thesecond was energized by a current in quadrature from a two-phasemachine. The operation is to throw the switch to the right andadjust i?2 for no deflection. The current in the moving coil is thennot necessarily zero, but is in quadrature with that in R^. Thenthrow the switch left and read the deflection which measures thequadrature component Q. The first setting gives the ratio —^=^ (i) while the deflection of the second dynamometer gives the phaseangle ^ tan6 = ^- The latter instrument draws a very small current from the net-work of conductors, and hence for the phase angle measurementhas to be calibrated as a voltmeter imder th
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