**Acknowledgements**

The calculated result of Eq. (2), |V| becomes 367 mV. If the contribution rate 36 % of the magnetic flux to this result, |V| become 132.12 mV. It almost coincides with output voltage

In the electromagnetic induction type, the magnetic circuit occurs a braking torque to the permanent magnet. However, the rotational speed of **Figure 17(a)** and **(b)** shows almost equal. Therefore, the braking torque by current is sufficiently small in this turbine structure. The

= *μ*<sup>0</sup> *μ*<sup>r</sup> *NI* (3)

a result, B is calculated as 1.96 mT. The maximum braking torque occurs when the magnetization of the permanent magnet and the magnetic flux density of the magnetic circuit cross perpendicular. If all the magnetic flux contributes, the braking torque is 42.9 pNm. This value

The electromagnetic induction-type MEMS air turbine generator was proposed. In this chapter, three types of MEMS air turbine generators that included the different bearing systems, shape of the rotor blades and shape of the magnetic circuits were discussed to achieve the high output power. In the MEMS air turbine, the purpose was achieving high-speed rotational motion. The magnetic circuit required the miniature structure that had the three-dimensional coil, magnetic core and magnetic flux introduction design. Therefore, the multilayer ceramic technology and the ferrite ceramic were used. One of the developed air turbines employed the fluid dynamic bearing system and flat-type rotor. In the miniature structure, the contactless-type miniature bearing system is advantaged because the friction force is impact issue. Moreover, two types of magnetic circuits for the fluid dynamic bearing turbine generator were compared with the magnetic flux loss. By the power generation experiment, the stepwise shape circuit that had the magnetic material introducing the magnetic flux from the magnet was suitable to the generator. The fabricated MEMS air turbine generator showed the

and *μ*<sup>r</sup>

. As

magnetic flux density from the magnetic circuit can be calculated at Eq. (3).

**Figure 17.** Output waveform at the load resistances were (a) 8 Ω and (b) 1 kΩ.

The vacuum permeability and the ferrite relative permeability are expressed in *μ*<sup>0</sup>

at 8 ohm (**Figure 17(a)**).

186 MEMS Sensors - Design and Application

**4. Conclusions**

is considered as sufficiently small value.

The sample of this study was fabricated by the facility at the Research Center for Micro Functional Devices, Nihon University. Part of this study was supported by the CST research project of Nihon University and by JSPS KAKENHI (16 K18055).
