An analysis of electroquasistatic induction micromotors

Abstract

This paper studies the steady-state operation of the electroquasistatic induction micromotor (IM). A rotary pancake IM compatible with surface micromachining serves as an example. A model is developed to predict the electric potential, field and free charge within the IM. The model also predicts the motive torque and transverse force of electric origin acting on its rotor. The torque is balanced against bushing friction and windage to determine rotor velocity. Here, the bushing friction is modeled as a function of the transverse force acting on the rotor. Finally, an equivalent circuit model is developed, which described important aspects of the electromechanical operation of the IM.

The model is used to study IM performance and its dependence on IM dimensions and material properties. For example, IM performance is predicted to be a complex function of axial IM dimensions and a strong function of rotor conductivity. The study also reveals that IM performance can differ significally from that of the variable-capacitance micromotor (VCM). For example, the dependence of motive torque and transverse force on axial dimensions can be significanly different in some IM operating regimes, allowing the possibility of improved performance over the VCM. IM and VCM dependences on micromotor geometry, velocity and material properties can also be significanlty different. The excitation and control requirements reflect the difference between a synchronous (VCM) and an asynchronous (IM) motor, as well as the possibility of obtaining an axially stable rotor position for certain IM material parameters.