The rate of energy conversion into heat in a magnetic material due to the presence of an alternating or pulsating magnetic field. It may be subdivided into two principal components, hysteresis loss and eddy-current loss. See also: Eddy current
The energy consumed in magnetizing and demagnetizing magnetic material is called the hysteresis loss. It is proportional to the frequency and to the area inside the hysteresis loop for the material used. Hysteresis loss can be approximated empirically by using Eq. (1), where Kh
is a constant characteristic of the material, f is the frequency, Bmax is the maximum flux density, and n, called the Steinmetz coefficient, is often taken as 1.6 although it may vary from 1.5 to 2.5. See also: Magnetization
Most rotating machines are stacked with silicon steel laminations, which have low hysteresis losses. The cores of large units are sometimes built up with cold-reduced, grain-oriented, silicon iron punchings having exceptionally low hysteresis loss, as well as high permeability when magnetized along the direction of rolling.
Induced currents flow within the magnetic material because of variation in the flux. This eddy-current loss may be expressed to close approximation by using Eq. (2),
where Ke is a constant depending on the volume and resistivity of the iron, Bmax the maximum flux density, f the frequency, and τ the thickness of the lamination in the core. For 60-cycle rotating machines, core lamination of 0.014–0.018 in. (0.35–0.45 mm) are usually used to reduce eddy-current loss.
Measured core loss
The measured core loss in a rotating machine also includes eddy-current losses in solid structural parts, such as the frame, ventilating duct spacers, pole faces, and damper windings, as well as those due to burrs or other contacts between punchings. A number of precautions are taken to minimize these components, which become appreciable in large machines. Structural parts close to the core, including clamping plates, I-beam spacers, dovetail bars, and shields, may be made of nonmagnetic material. Some of these parts may be shielded from the variable flux by low-resistance plates or by flux traps of laminated, high-permeability, low-loss steel. The core punchings are deburred and are coated with a baked-on insulating varnish. The finished cores are often tested with an ac magnetizing coil to locate and correct hot spots caused by damage during assembly. Pole-face losses are reduced by avoiding excessive slot-width to air-gap ratios, by surface grooving of the poles, and by low-resistance amortisseur windings. See also: Electric rotating machinery