An elementary book on electricity and magnetism and their applications . eing taken from the secondary, thereis a small current going into the primary which is called themagnetizing or exciting current and is perhaps 5 per cent ofthe full-load current. In a step-up transformer the primarycoil has low resistance. Why does it not short-circuit thesystem when there is no load on the secondary? This is onaccount of the back of the self-induction or inductanceof the primary coil. 256. Transformer losses. The losses of energy in a trans-former due to heating vary from 2 to 5 per cent of the i
An elementary book on electricity and magnetism and their applications . eing taken from the secondary, thereis a small current going into the primary which is called themagnetizing or exciting current and is perhaps 5 per cent ofthe full-load current. In a step-up transformer the primarycoil has low resistance. Why does it not short-circuit thesystem when there is no load on the secondary? This is onaccount of the back of the self-induction or inductanceof the primary coil. 256. Transformer losses. The losses of energy in a trans-former due to heating vary from 2 to 5 per cent of the are about equally divided at full load between the copperloss and the core loss. Since the copper loss is simply thePR loss in the windings, it increases with the square of theload current, and this is true for both transformer coils, becausewe have previously seen that the primary current increaseswith any increase in the secondary current. Therefore thecopper loss may be computed from the equation : Copper loss = IlRp + llR3. 378 ELECTRICITY AND MAGNETISM. Fig. 257. — Eddy currents intransformer core. Thick-ness of laminations muchenlarged. The core loss in a transformer is caused by hysteresis andalso by the eddy currents that are set up in the steel lamina-tions. Energy is required to cause thechanges in magnetic flux in the core;how much, depends a great deal on thequality of the steel and on its treat-ment before it is assembled. For agiven transformer the hysteresis losshas been found to vary directly with thefrequency arid with the power of themaximum flux density. The eddy currents circulate in thelaminations of the core, somewhat asindicated in figure 257. These currentsare useless; they heat the ironand so represent loss in the trans-former. But this loss can bekept low by using thin lamina-tions. For a given transformerthe eddy-current loss varies with thesquare of the frequency and with thesquare of the maximum flux Ways of cooling tra
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
