In magnetomechanical applications, it is necessary to calculate the magnetic force or torque of specific objects. If the magnetic fluid is involved, the force and torque also include the effect of pressure caused by the fluid. The standard method is to solve the Navier–Stokes equation. However, obtaining magnetic body force density is still under controversy. To resolve this problem, this paper shows that the calculation of the torque of these applications should not only use the magnetic force calculation method, but also consider the mechanical pressure using an indirect approach, such as the virtual work principle. To illustrate this, we use an experimental motor made of a nonmagnetic rotor immersed in a magnetic fluid. Then, we show that the virtual work principle in appropriate approach can calculate the output torque of the nonmagnetic rotor due to pressure of the magnetic fluid. Numerical analysis and experimental results show the validity of this approach. In addition, we also explain how the magnetic fluid transmits its magnetic force to the stator and rotor, respectively.

The shape of a iron bed supporting a permanent magnet in a large-sized motor is a important factor for determining the coupling strength with the yoke. In a large-sized motor, there is a difference in electromagnetic force with the yoke depending on the shape of the iron bed. In this paper, we show the differences and problems by calculating the electromagnetic force between the double bed and the single bed through the virtual air gap, and show that the single bed is superior in terms of the binding force. It is also shown that the binding force between the bed and the yoke is improved by carving the groove shape under the bed.