Rotating machines have a wide range of application involving shafts rotating at high speeds that must have high confidence levels of operation. Therefore, the dynamic behavior analysis of such rotating systems is required to establish operational patterns of the equipment, providing the basis for controller development in order to reduce vibrations or even to control oil instabilities in lubricated bearings. A classical technique applied in parameter identification of machines and structures is the modal analysis, which consists of applying a perturbation force into the system and then to measure its response. However, there are mainly two problems in modal analysis concerning the excitation of rotating systems. First, there are limitations to the excitation of systems with rotating shafts when using impact hammers or shakers, due to friction, undesired tangential forces, and noise that can be introduced in the system response. The second problem relies in the difficulty of exciting backward whirl modes, an inherent characteristic from these systems. Therefore, the study of a non-contact technique of external excitation, also capable of exciting backward whirl modes, becomes of high interest. In this sense, this article deals with the study and modeling of a magnetic actuator, used as an external excitation source for a rotating machine, mainly in backward whirl mode. Special attention is given to the actuator model and its interaction with the rotor system. Differently from previous works with similar proposal, which uses current and air gap measurements, here the external excitation force control is based on the magnetic field directly measured by hall sensor positioned in the pole center of the magnetic actuator core. The magnetic actuator design was completely developed for this purpose, opening different paths to experimental application of this device, for example, fault detection analysis based on directional modes. It is also presented a comparison between the numerical simulations and practical tests obtained from a rotor test rig and an experimental evidence of the backward whirl was accomplished based on the numerical simulation results.

Journal bearing supported rotors present two kinds of self-excited vibrations: oil-whirl and oil-whip. The first one is commonly masked by the rotor unbalance, hence being rarely associated with instability problems. Oil-whip is a severe vibration which occurs when the oil-whirl frequency coincides with the first flexural natural frequency of the shaft. In many cases, oil-whip is the only fluid-induced instability considered during the design stage; however, experimental evidences have shown that the instability threshold may occur much sooner, demanding a better comprehension of the instability mechanism. In this context, numerical simulations were made in order to improve the identification of the instability threshold for two test rig configurations: one on which the instability occurs on the oil-whip frequency, and another which became unstable before this threshold. Therefore, the main contribution of this paper is to present an investigation of two different thresholds of fluid-induced instabilities and their detectability on design stage simulations based on rotordynamic analysis using linear speed dependent coefficients for the bearings.

No abstract

Electromagnetic actuators are very important for scientific and industrial applications. Their use may vary within a wide range of possibilities due to their most important feature: the ability to apply known and controllable forces to elements or structures without contact. In this article, an application within this range is analyzed: a proportional-integral-derivative (PID) controller is used with a pair of actuators to control the dynamic forced response of a flexible cantilever metallic beam and to keep it at a given reference position. In order to achieve this objective, both the actuators and the controller need to be adjusted. For the actuators, the main parameters evaluated consider mounting particularities, such as the differential assembly and the influence of the air-gap distance over the magnetic flux density and the magnetic force provided. Next, a brief review of PID controllers is pre- | INTRODUCTIONMagnetic actuators are components with a wide range of possibilities whose application field has been growing year after year. [1][2][3] In fact, the main feature of interest in these components is the ability to apply known and controllable forces over many types of elements and structures with no need of contact and no need of any kind of material medium. In this sense, magnetic actuators are a good alternative for common mechanical components. A simple example is a growing field of research and application on magnetic gears. [1,4] These gears offer clear advantages over their mechanical counterparts such as contactless torque transfer, lower maintenance, inherent overload protection, and physical isolation between input and output shafts, being suitable in a market that desires more efficient and reliable gears. Another example of magnetic actuators competing with mechanical devices occurs with magnetic bearings (both active and/ or passive). [1][2][3] These bearings also have some advantages [5] : besides allowing oil-free machines, magnetic bearings are frictionless (and therefore have low-power losses and no wear), allowing high rotational speed. Besides, the bearings can be remotely controlled and are good monitoring devices (because the bearing forces can be properly measured). Therefore, in the field of magnetic bearings, there are some interest applications. Among these, one may find flywheels for kinetic energy storage as peak power buffer units (i.e., for supplying energy on demand peaks) in vehicle applications, [1,6,7] in oil drilling platforms, [8] and even in electric power network applications. [9] Due to its operation characteristics, it is desirable that flywheels rotate at higher speeds and, if a long-term energy storage is needed, low-energy consumption during storage

No abstract