Why is the AC force for my problem about half of the force reported in a DC simulation?
Although this behavior might seem strange, it is actually correct. The trick here is that in a harmonic problem, the field varies sinusoidally, but the force is related to the square of the field, producing a DC component, and a 2-times frequency component, but no force at the drive frequency. To see this effect, consider for example a simple c-core electromagnet acting on a slab of iron. The instantaneous force on one pole of the c-core can be obtained via Maxwell’s stress tensor: F = B2*area/(2*μo) Now, imagine that the problem is harmonic, and the gap flux density, B, is defined as: B = Bo*cos ωt Substituting in the force gives: F = Bo2 * cos2 ωt * area / (2*μo) Now, using the trigonometric identity cos2 ωt= (1/2)*(1+cos 2ωt) gives: F= area*Bo2/(4*μo) + (area*Bo2*cos 2ωt)/(4*μo) So there are 2 components of the force: a DC part of the force, and a part at twice the drive frequency. Now, the DC part is 1/2 of what it would be if the problem were evaluated at 0 Hz, so that the peak fo
