A quantitative assessment of the change in magnetic properties of Co-Fe-V alloy when subjected to flux and stress conditions similar to those encountered in an aircraft generator

Machine geometric design often compels magnetic flux to behave in a manner that appears to degrade magnetic core properties. Manufacturers' nominal loss values for magnetic materials can therefore be misleading when applied to built-up cores, and "compensating factors" are often used to arrive at a reasonable estimate of iron losses in assembled machines. An investigation into flux behavior and iron loss distribution in a Co-Fe-V alloy stator core of a typical aircraft electrical alternator has been conducted with the aim of assessing quantitatively the effect of rotating flux and external stress, both axial and peripheral, on stator magnetic properties. The work was based on 1) a model alternator and 2) theoretical flux distributions. Both the stress-free model alternator tests and theoretical calculations show increases in mean back-iron loss of 30 percent, a result directly attributed to rotating flux. Stress had a marked effect on core properties, a radial stress of 24 MPa increases iron loss by 270 percent, magnetizing current by 500 percent, and input voltamperes by 800 percent (400 Hz, 1.8 T). A 3-kHz, 1.8-T exitation results in a mean back-iron loss of 6780 W/kg. Both radial and axial stresses have similar effects on iron loss; a radial stress, however, is far more detrimental to core permeability. Maximum iron loss occurs behind the stator teeth and can be as high as 320 percent of an equivalent alternating flux.