Critical speed

About: Critical speed is a research topic. Over the lifetime, 2764 publications have been published within this topic receiving 31365 citations.

PI control of HSFDs for active control of rotor-bearing systems

Abstract: This paper describes the proportional integral (PI) control of hybrid squeeze film dampers (HSFDs) for active control of rotor vibrations. Recently it was shown that the automatically controlled HSFD based on feedback of rotor speed can be a very efficient device for active control of rotor vibration when passing through critical speeds. Although considerable effort has been put into the study of steady-state vibration control, there are few methods in the literature applicable to transient vibration control of rotor-bearing systems. Rotating machinery may experience dangerously high dynamic loading due to the sudden mass unbalance that could be associated with blade loss. Transient run-up and coast down through critical speeds when starting up or shutting down rotating machinery induces excessive bearing loads at criticals. In this paper, PI control is proposed as a regulator for the HSFD system to attenuate transient vibration for both sudden unbalance and transient run-up through critical speeds. A complete mathematical model of this closed-loop system is simulated on a digital computer. Results show an overall enhanced behavior for the closed-loop rotor system. Gain scheduling of both the integral gain and the reference input is incorporated into the closed-loop system with the PI regulator and results inmore » an enhanced behavior of the controlled system.« less

Analytical and FEM design of mixing system in STR biodiesel production.

Abstract: Article history: The on going demand for the improved impeller designs usually comes from the users of industrial mixing equipment. Most of the industrial biodiesel reactors using blade agitator for mixing which creates a main flow in the reaction tank (reactor) with circulation on axial and tangential direction. In current study, design of mixing system used in stirred tank reactor (STR) biodiesel production by analytical method and numerical one (finite element method) has been made. Designs have been done based on a pitched blade turbine down flow with two inclined blades (45°) by assumption of maximum working conditions (using caster oil as base fluid). The mixing power and mechanical design of an agitator such as stress analysis by analytical method and numerical one (finite element method) has been carried out. The operating limitations of mixing shaft deflection and critical speed during the biodiesel production process by using both methods, has been calculated. The FEM results have displayed close values with the results obtained from the analytical method. The critical rotational speed of the mixing shaft obtained from the analytical and modal analysis show similar results 5778.26 and 7000 rpm respectively and found to be significantly higher than the impeller maximum required mixing speed.

Influencing factors on motor vibration & rotor critical speed in design, test and field applications

Abstract: This paper will present various case studies of how the rotor or system natural frequencies can be strongly influenced by its external and internal factors and how small variations in these factors can influence the motor vibration at the manufacturer and in the field. Motors constructed to API 541 standards are required to have a rotordynamic lateral natural frequency that is removed from the operating speed by at least 15%. The location of this natural frequency can depend on many factors such as bearing clearance, bearing type, residual unbalance, oil temperature, oil viscosity, and bearing housing stiffness. Depending on the design, some motors are more sensitive to these parameters than others, and small changes in these factors may cause large variances in the motor natural frequency. As a result, small variations in test setup, manufacturing tolerances, or field installations within critical components can cause noticeable differences between the calculated and measured natural frequencies. Variation in motor vibration may also be seen between the motor operating on the manufacturer's test stand and the motor operating in the field. In the field some apparently minor changes on ambient conditions or set up can significantly change the motor vibration. Additionally this paper will propose a worst case calculation method for motor natural frequencies that will provide greater confidence to the end user that the motor will operate successfully in the field before the motor is installed.

Quasi-continuum approximations for travelling kinks in diatomic lattices

Abstract: We extend the quasi-continuum method of approximation to the case of a diatomic lattice. This is illustrated by a lattice in which both small and large atoms interact with first- and second-nearest neighbours. We show that highly accurate quasi-continuum techniques may be generalized to determine the shape of nontopological kinks. Many previous analyses of such systems have found travelling wave solutions only for one particular speed using a second-order continuum theory; we show (i) how wave shapes can be found for arbitrary speeds, and (ii) how solution profiles can be calculated from fourth-order partial differential equations which approximate the lattice. Alongside the theoretical analysis, we also present numerical simulations of the lattice which demonstrate propagation of the predicted waves through the lattice. We show that the particular speed for which solutions have been found in previous studies is a special speed for waves in the lattice, but waves can travel for long periods of time at faster or slower speeds, whilst slowly relaxing to this critical speed.