i) Permeability : Ability to conduct flux.
ii) Saturation : No increase in flux, though magnetic field strength increases.
iii) Electrical Resistivity : The electrical resistance of the core responsible for `eddy current losses.
iv) Hysteresis : The area of hysteresis loop is the measure of hysteresis loss.
The silicon steel alloys are commonly used in the core
manufacturing for the transformers at power frequencies i.e. 60 to 400
Hz. But use of such alloys can be extended upto audio frequency range
upto 100 KHz using thin gauge tap wound laminations. At the same time
these cores become more expensive and difficult to handle. In case of
higher frequency range upto 200 KHz and more, the problem of eddy
currents becomes very serious. In such cases a material which is not
really a metal or not even a bonded powered metal product, is used for
the cores. It is a ceramic which is magnetic in its nature called
ferrites. Such transformer using ferrites for the core construction are
called ferrite core transformers. The ferrite core produces a lower price to performance ratio.
The ferrites are very hard, brittle and dark grey or black in
appearence. The ferrites have high permeability. The ferrites appear in
numerous shapes like bar, rods, hollow pots, toroids, cross format, Es etc.
Hence the transformers using such cores can be easily adapted to
circuit board mounting. The ferrites have low leakage inductance, The
property of high resistivity of the ferrites keeps the eddy current
losses very small, at high frequencies. This eliminates the need of the
ferrites keep the eddy current losses, at high frequencies. This
eliminates the need for laminated construction for the cores, in case of
ferrites core transformers.
Following table gives the performance comparison between the ferrites and alloys.
The table shows the values of various at 25 kHz. This clearly
indicates the advantages of the ferrites over the alloys, in high
frequency range.
Ferrites consist of an iron oxide, Fe2 O3.
There are different methods of producing ferrites viz the method of
coprecipitation of hydroxides or carbonates and the composite method.
Audio frequency ferrites are made up of nickel-zinc oxide and
manganese-zinc oxide along with iron oxide. Manganese-zinc ferrites have
low hysteresis loss and smaller temperature coefficient of
permeability, which is advantages compared to nickel-zinc ferrites. Burt
due to lower threshold frequency, manganese-zinc ferrites are used for
low frequency applications. Nickel-zinc ferrites maintain their
permeability over a wide range of frequency upto hundreds of megahertz,
hence are used for high frequency transformer cores. These ferrites are
temperature sensitive. The limiting feature of the ferrites is their low
curie temperature (under 500oC). The magnetic properties of
the ferrites deteriorate gradually as the temperature approaches to the
curie temperature. This fact must be taken into account when considering
the ferrites.
The ferrites cores substantially reduce the size and weight of
the transformers used for high frequency applications. The ferrites are
used for flyback transformers in television sets, carrier telephony,
television receivers, radio receivers and audio range pulse
transformers.
The wide variety of ferrites shapes is shown in the Fig. 1.Fig. 1 Variety of ferrites shapes |
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