Section 4.3 · Further exercises · 7 of 10
Exercise 4.3.7: The write head, with its yoke
About the further exercises of Section 4.3
Ten further exercises in the spirit of Problems 4.3.1–4.3.3, the applications and the home exercises. They use only
Where a material is nonlinear the – curves of Figure 4.3.19 are used, with the readings given in the statement. Exercises 4.3.1–4.3.4 are about reluctance, flux and inductance, Exercises 4.3.5–4.3.6 about transformers and motors, and Exercises 4.3.7–4.3.10 about recording heads, electromagnets, fringing and leakage. Constant: .
The write head, with its yoke
#cf. the write-head paragraph
A ring write head has a gap and a yoke of mean length of soft material with . The cross-section is the same everywhere. The gap field must reach . (a) Find the mmf needed, and the current for turns. (b) By what percentage does the yoke raise the mmf above the gap-only value? (c) The leakage reluctance across the yoke is : how much more current is needed, and what fraction of the coil flux reaches the gap?
The write head of Section 4.3.7 with its yoke: gap and yoke in series for (a) and (b), and a leakage reluctance in parallel with the gap for (c), where the coil must push the extra leakage flux through the yoke only.
Same area everywhere, so in the yoke too. and :
The gap alone needs . The yoke adds , i.e. : of yoke is worth of air, of the gap.
is in parallel with , and the yoke reluctance is in series with the pair. With as unit: , total . For the gap to carry its flux the coil must push , and
Leakage costs flux, not mmf: the extra flux circulates through the yoke whose reluctance is tiny, so the coil hardly notices. What matters for the head is that only of the flux the yoke must carry reaches the medium. The yoke, not the gap, is therefore the first part to saturate, and the saturation induction of the pole material ( for FeCo) sets the writable coercivity of the medium.