Mutual effect of elements in a three-phase, two-element wattmeter
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The experimental curves (see Fig. 5) show that the nature of the wattmeter error relationship to the power factor is the same as that obtained by computation from (3). This confirms the validity of the assumptions made in deriving the formulas.
The maximum resultant torque due to the mutual effect of elements and the maximum discrepancy in the tested instrument's readings in single-phase, two-wattmeter test circuits occur for cosϕ=1, i.e., precisely for the conditions under which the wattmeters are calibrated and their main error determined. Therefore, although the calibration and testing of three-phase wattmeters according to the circuits in Figs. 2 and 3 are simpler and more productive, they are permissible only for wattmeters whose errors due to the mutual effects of their elements are small as compared with their accuracy class.
The torques due to the mutual effects of elements change their values with the position of the moving part, and they are not at their maximum for the zero mark on the scale. Therefore, the system for normalizing the permissible error specified by GOST 8476-60 for the zero mark on the scale is, in our opinion, insufficiently characteristic of the instrument's metrological properties.
KeywordsPhysical Chemistry Analytical Chemistry Torque Power Factor Mutual Effect
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