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2025-05-26 20:27本頁面
【正文】 Lenz’s law. Thus the primary receives its power from the source while the secondary supplies this power to the load. This action is known as transformer action. 3. TRANSFORMER PRINCIPLES When a sinusoidal voltage Vp is applied to the primary with the secondary opencircuited, there will be no energy transfer. The impressed voltage causes a small current Iθ to flow in the primary winding. This noload current has two functions: (1) it produces the magic flux in the core, which varies sinusoidally between zero and ? φm, where φm is the maximum value of the core flux。. It is readily seen that the current ponent Im= I0sinθ0, called the magizing current, is 90186。 φ is therefore in phase with Im. The second ponent, Ie=I0sinθ0, is in phase with the primary voltage. It is the current ponent that supplies the core losses. The phasor sum of these two ponents represents the noload current, or emo III ?? It should be noted that the noload current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core it is assumed that there are no other losses in the transformer, the induced voltage In the primary, Ep and that in the secondary, Es can be shown. Since the magic flux set up by the primary winding， there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, tNE ???? /? . This same flux also links the primary itself, inducing in it an EMF, Ep. As discussed earlier, the induced voltage must lag the flux by 90186。 out of phase with the applied voltage. Since no current flows in the secondary winding, Es=Vs. The noload primary current I0 is small, a few percent of fullload current. Thus the voltage in the primary is small and Vp is nearly equal to Ep. The primary voltage and the resulting flux are sinusoidal。 thus E = Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence Ep = and Es = where Ep and Es are the number of turn on the primary an
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