Torque in open-access permanent magnet variable flux sources

A flux source with four rotatable permanent magnet rods having access from two perpendicular directions through gaps exceeding the rod diameter can produce a continuously-variable flux density of up to ±300 mT at the centre when high-grade Nd-Fe-B magnets are used. The trade-off between field, access and torque in this mangle design is discussed. The maximum field, achievable when the magnets touch, is equal to the remanence of the Nd-Fe-B. The torque needed to rotate the rods scales as the square of the remanence, and it increases faster than the inverse square of the separation when the separation is comparable to the rod diameter. End effects reduce the torque significantly, but appropriate gearing is needed to ensure smooth operation. Modifications of the basic design can reduce or compensate the torque by using special end pieces, and improve access to fields > 1000 mT at small rod separation.