Critical speed

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

Some Experiences on Rigid and Flexible Rotors in Induction Motors Driving Critical Equipment in Petroleum and Chemical Plants

Abstract: In this paper, the definitions of rigid and flexible rotors, the rotor-dynamic analysis procedure, and the design criteria defined in the American Petroleum Institute (API) standard 541 are reviewed and compared to API 684. The definition of the quasi-flexible rotor is introduced, as it is not covered by the standards. The influence on critical speeds and response to unbalance of the parameters in the rotor-bearing system including foundation stiffness, bearing-support-structure stiffness, bearing-oil-film characteristics, and shaft design are analyzed for each type of rotor. The advantages and disadvantages of each type of rotor manufacturing, initial cost, maintenance, and possible site problems are also analyzed. Numerical results and test data are presented as study cases. The conclusion shows that it is important to know the operation conditions of the machine, specifically the operating-speed range and how the machine will be installed in order to select the most economical and reliable design for the particular case. If the machine will be constant speed, the question is less complex. The conclusion confirms that, in general, low-speed induction machines using standard plain cylindrical bearings and slower (up to 7000 kW) four-pole and high-speed (below 1000 kW) two-pole machines are well suited for rigid rotor design. Larger machines, particularly two-pole, are more practical to design as a flexible rotor. Adjustable-speed-drive machines introduce special concerns for the operation of flexible rotors when there is a critical speed in the operating range. Several design solutions are presented and discussed in terms of initial cost, maintenance, and performance.

Rotor-Bearing System Stability Performance Comparing Hybrid versus Conventional Bearings

Abstract: New closed-form expressions for calculating the linear stability thresholds for rigid and flexible Jeffcott systems and the imbalance response for a rotor supported on a hybrid bearing are presented. For typical bearings characteristics, expressions yield stability thresholds practically equal to those reported by Lund (1966). The hybrid bearing design has a single injection port whose location is so chosen to stabilize the bearing performance and to reduce the steady equilibrium attitude angle. Rotordynamics coefficients graphs for conventional and pressurized bearings, as functions of bearing equilibrium eccentricity and/or Sommerfeld number, are presented. Using the rotordynamics coefficients into the expressions for the corresponding velocity thresholds and the imbalance response, the system stability and vibration performances are estimated and analyzed. When comparing the Jeffcott flexible shaft supported on two journal bearings of the conventional type with the hybrid type, the results show a clear superiority of the pressurized design as far as stability behavior is concerned. Specifically for cases of flexible shafts with similar characteristics to those used in industry, the analysis shows that this design yields velocity thresholds 25%–40% higher compared to the conventional circular ones. Also this bearing displays nonlinear feeding pressure behavior, and it is capable of reducing the synchronous vibration amplitude in most speed ranges, except around the critical speed; moreover, for certain Jeffcott configurations the amplitude reduction can be substantial.

Modular-belt conveyors with variable-speed drive motors

Abstract: A conveyor system in which a modular belt or chain is driven by a drive element rotated at a predetermined variable angular speed to compensate for speed fluctuations due to chordal action A speed signal generator generates a speed signal generally inverse to the uncompensated linear speed of a belt driven by drive elements rotated at a constant angular speed The speed signal generator detects an encoded pattern rotating in known relationship to the drive element The encoded pattern represents a predetermined speed profile that is used to compensate for belt speed fluctuations or to provide custom belt speed performance

On the Vibrations of the Shaft Carrying an Asymmetrical Rotating Body

Abstract: The authors named the rotor having two unequal principal moments of inertia except the polar moment of inertia, "asymmetrical rotor", and studied the vibratory characteristics of the motions of the shaft carrying an asymmetrical rotor analytically and experimentally. The results are as follows : (1) The rotating shaft is statically unstable near its major critical speeds, and the unstable regions become wider as the asymmetry gets larger and the damping forces smaller. (2) The unstable region of the lower major critical speed vanishes as the mounting point of an asymmetrical rotor on the shaft comes to the center of the shaft. (3) There are 8 natural frequencies pi, p^-i (i=1, 2, 3 and 4) in the free vibrations of a four degrees of freedom system treated in this paper, and the relation p^-i=2w-pi always exists. (4) As the asymmetry disappears gradually, the natural frequency pi approaches that of a symmetrical rotor, and the amplitudes E^- and F^- of free vibrations peculiar to p^-i disappear. (5) The vibratory characteristics mentioned above (1), (3) and (4) except (2) of the shaft carrying an asymmetrical rotor are the same as those of the flat shaft carrying a symmetrical rotor.