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Experiment_7_Ferromagnetic_Core_Characteristics -...

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7-1 Experiment 7 Ferromagnetic Core Characteristics OBJECTIVE To observe and study the hysteresis loop of an iron core transformer DISCUSSION Transformer core characteristics can be observed by applying an AC excitation and observing signals representing flux density and field intensity on an oscilloscope. This is often referred to as a BH loop (B is the flux density and H is the magnetic field intensity). A typical BH curve, as shown in Figure 7-1, illustrates a hysteresis effect. The BH trace over one complete cycle takes different paths and is non-linear. Figure 7-1 Typical B-H hysteresis loop for a ferromagnetic material The value of B when H is zero is given by the symbol B r and is called residual (remnant) flux density. This is the value of flux density that would remain in the material if the excitation were removed from the material. The value of H when B is zero is given the symbol H c and is called coercive magnetic field intensity (historically named coercive force). This value of magnetic field intensity is the value required to “coerce” the flux density in the material back to the flux density value of zero. Field Intensity Flux Density Remanence Coercivity
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7-2 First we will discuss measuring flux density. Voltage applied to the primary winding of a transformer will excite the transformer’s magnetizing inductance and will induce a voltage on the secondary winding. Internal flux is related to the coil voltage and number of turns through a simplified version of Faraday’s law of induction.
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