ABSTRACT
Active Magnetic Bearings (AMBs) are being increasingly employed for supporting high-speed rotors because of their contact-less operation leading to almost no friction, and the consequent heating, noise and energy loss. The other inspiration is that these bearings are active elements, or their properties can be changed if required. For high-speed applications, involving rotor speeds greater than 10,000 rpm such as high speed milling of aluminium and precision grinding of small parts, AMBs may be preferred over conventional rolling element or hydrodynamic bearings, as these become either unstable or very inefficient beyond a certain rotor spin-speed limit. However, the AMBs may also face stability issues in high speed rotor applications due to the inevitable time-delay involved in sensing and transporting the rotor displacement signal to the controller and then applying the control action on the rotor shaft. Therefore, the present work addresses the issue of delay on stability of rotors supported on AMBs. Control law (the transfer function between displacement of rotor in the air gap and the force applied on it by the bearing) has a pronounced effect on the stability of rotor supported by an AMB. Conventionally, PD and PID control laws are widely used for designing AMBs. This paper has proposed some novel control laws based on the constitutive relationship of viscoelastic semisolids and compare the stability for the rotor AMB system, controlled with conventional control laws, and novel control laws, when the delay is taken into consideration. To this end, a rigid rotor with centrally placed rotor disk supported on AMB has been modelled as a single degree of freedom system, to keep the work simple. The effect of various parameters on the performance of AMB in supporting the rotor shaft with time delay is analysed. The novel control laws have been shown to hold immense potential to attenuate the influence of delay on stability of high-speed rotors.
