Wednesday, August 15, 2012

High Frequency Transformer

       The lamination thicknesses, beyond the normal power frequency range at frequencies higher than 50-60 Hz, have to be reduced control the iron losses to an acceptable level. If the operating frequencies extend beyond a few kilocycles per second, it becomes necessary to use iron-dust cores or the ferrites, special oxides which exhibit reasonably good magnetic properties with very small eddy current losses.
In such transformers, variable frequency is the normal operating condition and as the frequency increases, the significances of the elements in the equivalent circuit changes.
- At very low frequencies, for example, the magntising reactance is small enough to be comparable with or even less than load impedance so that this tends to be ''shorted out'' and the secondary terminal voltage is greatly reduced.
- In the intermediate frequency range, is high enough for its effects to be neglected and the leakage impedance absorbs only a moderate fraction of the available voltage.
- With further increase of frequency, however the leakage reactance drop tends to cause the output voltage to fall appreciable again.
        Capacitance between turns and between the windings and earth can no longer be ignored since these are shunted across the circuit in a distributed fashion causing behaviour similar to a leading power factor load.
       The design problem here is to ensure the transfer, from primary to secondary, without excessive distortion, of an input signal having many frequency components ; for example a pulse of voltage or and input having the complex frequency spectrum of a musical note. In the high-power field too, with the advent of thyristor inverters, supply voltage may be far from sinusoidal and may appear as castellated or other compositive waveform.

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