Basic Electrical Theory MAGNETIC CIRCUITSThe hysteresis loop is a series ofFigure 28 Hysteresis Loop for Magnetic Materialsc u r v e s t h a t s h o w s t h echaracteristics of a magneticmaterial (Figure 28). Oppositedirections of current will result inopposite directions of fluxintensity shown as +H and -H.Opposite polarities are also shownfor flux density as +B or -B.Current starts at the center (zero)when unmagnetized. Positive Hvalues increase B to the saturationpoint, or +Bmax, as shown by thedashed line. Then H decreases tozero, but B drops to the value ofBr due to hysteresis. By reversingthe original current, H nowbecomes negative. B drops tozero and continues on to -Bmax. Asthe -H values decrease (lessnegative), B is reduced to -Brwhen H is zero. With a positiveswing of current, H once againbecomes positive, producingsaturation at +Bmax. The hysteresisloop is completed. The loop doesnot return to zero because ofhysteresis.The value of +Br or -Br, which is the flux density remaining after the magnetizing force is zero,is called the retentivity of that magnetic material. The value of -Hc, which is the force that mustbe applied in the reverse direction to reduce flux density to zero, is called the coercive force ofthe material.The greater the area inside the hysteresis loop, the larger the hysteresis losses.MagneticInductionElectromagnetic induction was discovered by Michael Faraday in 1831. Faraday found that ifa conductor "cuts across" lines of magnetic force, or if magnetic lines of force cut across aconductor, a voltage, or EMF, is induced into the conductor. Consider a magnet with its linesof force from the North Pole to the South Pole (Figure 29). A conductor C, which can be movedbetween the poles of the magnet, is connected to a galvanometer G, which can detect thepresence of voltage, or EMF. When the conductor is not moving, zero EMF is indicated by thegalvanometer.Rev. 0 Page 41 ES-01
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