Ricardo Ugliara Mendes

Analysis of a complete model of rotating machinery excited by magnetic actuator system

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.

On the Instability Threshold of Journal Bearing Supported Rotors

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.

Magnetic Actuator Modelling for Rotating Machinery Analysis

Rotating machines have a wide range of application such as airplanes, factories, laboratories and power plants. Lately, with computer aid design, shafts finite element models including bearings, discs, seals and couplings have been developed, allowing the prediction of the machine behavior. In order to keep confidence during operation, it is necessary to monitor these systems, trying to predict future failures. One of the most applied technique for this purpose is the modal analysis. It consists of applying a perturbation force into the system and then to measure its response. However, there is a difficulty that brings limitations to the excitation of systems with rotating shafts when using impact hammers or shakers, once due to friction, undesired tangential forces and noise can be present in the measurements. Therefore, the study of a non-contact technique of external excitation becomes of high interest. In this sense, the present work deals with the study and development of a finite element model for rotating machines using a magnetic actuator as an external excitation source. This work also brings numerical simulations where the magnetic actuator was used to obtain the frequency response function of the rotating system.

Reduction of Rotor Vibration Amplitude Using PID Tuning Methods

Rotating machines present some common inherent operational problems, such as the critical amplitude of motion that the machine may experience when passing through one or more of its natural frequencies. High vibration levels can be harmful to the machine and its attenuation is important to maintain the system working healthily. In this context, the paper proposes a proportional-integral-derivative (PID) controller acting with two pairs of electromagnetic actuators to reduce the vibration amplitude of a flexible rotor, supported by hydrodynamic journal bearings, crossing its first resonance. The oil film behavior of the hydrodynamic bearings is modeled both through linear equivalent stiffness and damping coefficients and by the complete solution of the Reynolds equation applied to short bearings. Next, the relay feedback test (a frequency response method) is applied to estimate the ultimate gain and ultimate period of the respective PID controllers. Finally, five different tuning methods are proposed to adjust the PID: the Ziegler-Nichols traditional method, three Ziegler-Nichols modifications to obtain less aggressive controllers, and a fifth method, the Tyreus-Luyben method, whose objective is to improve the robustness of the controller.

Modal Balancing Using Parametric Combination Resonance

In traditional modal balancing, a representative response of each mode to be balanced must be present in the measurement data. This is usually achieved by measuring the rotor response close to each critical speed corresponding to the modes of interest. The main disadvantage of this procedure is the time consumed during run-up/run-down cycles: at least one for each test mass configuration for determining the influence coefficients. Besides, several critical speeds are passed, which may cause high levels of vibration or even several run-ups if the resulting unbalance is critical. This paper proposes a balancing method for which the system needs to rotate up to the first critical speed only. It is known that parametric combination resonance has the ability to transfer kinetic energy between the mode shapes of a flexible structure (modal interaction). Therefore, controllable bearings such as active magnetic bearings are used to introduce a parametric excitation at different parametric combination resonances, inducing a modal energy transfer from the first critical speed to higher modes. This energy transfer has a similar effect to that obtained if the rotor is operated near higher critical speeds. This allows for estimating influence coefficients and corresponding correction masses using a procedure similar to the traditional modal balancing. The proposed method avoids the need for spinning the rotor above its first critical speed; thus, saving time and allowing for high-speed rotor balancing at low speed. The theory of the proposed balancing method using parametric excitation is presented along with simulations to illustrate its potential.