D-C Machines - Short-Circuit Calculations and Test Results [includes discussion]

Abstract

THE wide use of d-c machinery in naval and industrial applications and the demand for reliability under all conditions of operation increases the importance of the design and operation of protective e(luipment. For this reason the need for a reliable method to compute the transient currents of motors and generators when a system is subjected to a fault is obvious.The field of rotating d-c machinery includes a very wide variety of types and designs. Thle equations used in this paper, however, are applicable, for the calculation of fault transients, to all machines used for power applications in industry and in the U. S. Navy.A description of the theory and the derivation of equations used in this paper have been given in a companion paper.1 Like other equations given in the literature2 for the computation of peak short-circuit current, the equations used here -are based upon simplifying assumptions. Hence, a discrepancy between calculated and test values is expected. For this reason the accuracy of results obtained by computation is unknown unless test results are available. If test results were available on a large group of machines for comparison with computed results, a degree of confidence could be established in the validity of equations for the computation of peak short circuit of any machine in the normal po-wer range. Reported test results in the literature, however, especially for machines of 1,000 kw and larger, are extremely meager. Confidence in the reliability of any method for the computation of short-circuit current can only be established by the confirming evidence of test results. Test Values and Computed ResultsIn this paper the test results on five machines, which were part of a mock Paper 55-114, recommended by the AIEE Rotating Machinery Committee and approved by the AIEE ASSOCIATE MEMBER AIEE submarine propulsion plant, have been used to indicate the accuracy of computations. In a previous paper3 test results on four of the five machines reported on here were given and were compared with the computed results obtained by the best niethod2 then available.The name-plate data of the five machines used for short-circuit tests are given in Table I, and the basic constants required for short-circuit calculations are given in Table II. A comparison of test peak short-circuit currents and the values obtained by computations are shown in Table III. A comparison of calculated and test transient currents appears in Figs. 1 through 6 inclusive. NomenclatureUpper-case letters denote total quantities. Exception is made for flux and flux linkages where the same size letter is used. Lower-case letters denote perunit (pu) quantities; total quantities divided by their respective base values result in a pu system.A =effective area of armature conductors in each slot, square inches; alh al = width of space taken by copper conductors in slot, inches (average a,= 0.78b5) bi=width of armature slot, inches (average b5 = 0.37Pa/Int) Cs = number of armature conductors per slot, Z/S da = diameter of armature, inches Eo=...