Rotational Hysteresis Loss

The cores of transformers, choke coils, and a-c electromagnets are subject to magnetic flux that oscillates along a fixed axis under steady conditions. However, the rotors of rotary electromagnetic devices, such as generators

Figure 3-40. Pole cores and smooth armature core.

and motors, are subjected to rotating fluxes. Thus, in Fig. 3-40, when the arrow on the rotating armature core is in position 1, it is aligned opposite the direction of the magnetic flux, and in positions 2 and 4, the arrow is at right angles to the flux. In position 3, the arrow is in the direction of the flux. Thus, in one revolution the arrow has rotated through 360° relative to the flux. This would also be true if the armature core were stationary and the field poles rotated about the armature core. At low magnetization the rotating hysteresis loss is greater than the corresponding alternating hysteresis loss, and at very high densities the rotating hysteresis loss actually decreases, becoming quite low at very high flux densities.