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【正文】 ation broke down between a motor and its grounded enclosure . Under any of these conditions, the net current flowing through the hole of the CT is no longer zero but equal to I or I. A flux is set up and a voltage E is induced, which trips CB. Because an imbalance of only 5 mA has to be detected, the core of the transformer must be very permeable at low flux densities. Supermalloy is often used for this purpose because it has a relative permeability of typically 70000 at a flux density of only 4mT . rapid conductor heating: the It factor It sometimes happens that a current far greater than normal flows for a brief period in a conductor . The Ilosses are than very large and the temperature of the conductor can rise several hundred degrees in a fraction of a second . For example, during a severe shortcircuit, intense currents can flow in conductors and cables before the circuit is opened by the fuse or circuit breaker.Furthermore, the heat does not have time to be dissipated to the surroundings and so the temperature of the conductor increases very rapidly. What is the temperature rise under these condition?Suppose the conductor has a mass m, a resistance R, and a thermal heat capacity c. Moreover, suppose the current is I and that it flows for a period t that is typically less than 15 seconds. The heat generated in the conductor is given by Form ,we can calculate the temperature rise for a given value of :hence from which If follows that for a given conductor the temperature rise depends upon the I factor . It is well known that high temperature damage the insulation that covers a conductor. The I factor is ,therefore ,very important because it determines the temperature rise under shortcircuit conditions. For example , a copper conductor, initially at a temperature of 90,cannot endure an I factor in eccess of 22s if its temperature is to be limited to 250during a shortcircuit.In general, the I factor can be calculated knowing (a) the cross section of the conductor, (b) its position(copper or aluminum), and (c) the maximum temperature it can tolerate. The I factor for copper and aluminum conductors are given by the following equations:for copper conductors, for aluminum conductors, where I =shortcircuit current duration of the shortcircuit A = net crosssection of conductor without counting the empty spaces initial temperature of conductor final temperature of conductor Example 261___________________________________________An overhead line made of aluminum conductor AWG has a crosssection of . Under normal conditions this conductor can continuously carry a current of 160 A. Calculate the manximum permissible I factor during a shortcircuit, knowing that the initial temperature is 80and that the manximum temperature should not exceed 250. A m
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