When current in a coil reverses direction thousands of times per second, hysteresis can cause considerable loss of energy. Hysteresis is defined as "a lagging behind." The magnetic flux in an iron core lags behind the magnetizing force.

The hysteresis loop is a series of curves that shows the characteristics of a magnetic material (Figure 28). Opposite directions of current will result in opposite directions of flux intensity shown as +H and -H. Opposite polarities are also shown for flux density as +B or -B. Current starts at the center (zero) when unmagnetized. Positive H values increase B to the saturation point, or +B_max,as shown by the dashed line. Then H decreases to zero, but B drops to the value of B_t.due to hysteresis. By reversing the original current, H now becomes negative. B drops to zero and continues on to ^-B_max.As the -H values decrease (less negative), B is reduced to -B_t. when H is zero. With a positive swing of current, H once again becomes positive, producing saturation at +B_max. The hysteresis loop is completed. The loop does not return to zero because of hysteresis.

Figure 28 Hysteresis Loop for Magnetic Materials

The value of +B_t. or -B_t., which is the flux density remaining after the magnetizing force is zero, is called the retentivity of that magnetic material. The value of -H, which is the force that must be applied in the reverse direction to reduce flux density to zero, is called the coercive force of the material.

The greater the area inside the hysteresis loop, the larger the hysteresis losses.