International Journal of Engineering Research and Development ISSN: 2278-067X, Volume 1, Issue 11 (July 2012), PP.11-16 www.ijerd.com 11 Evaluation of Critical Speed of Generator Rotor with external load Dr Korody Jagannath, Manipal Institute of Technology, Manipal, India 576104 Abstract––As a result of continuing demand for increased performance, modern generators are sometimes designed to operate near critical speed, which causes difficulty in maintaining the rotor balance required to ensure acceptable vibration levels. Forced vibration due to the external load acting on the shaft line is not considered in evaluation of critical speeds of multi-section shaft. The importance of the current work carried out is to evaluate the critical speed through an automated process. By this process any configuration of rotor can be used to evaluate the critical speed of multi-section rotor. Thus for evaluation of critical speed and to study the rotor behavior in dynamic condition, a script is generated in order to automate the process, thereby reducing the time consumed & increasing the accuracy. The uniqueness of this methodology adopted is that with the help of one script the input like diameter, length of each step of rotor, load acting on the rotor is accessed as variable into the script. The script is generated in ANSYS using Ansys Programming Design Language (APDL). The generated script results are correlated with theoretical results and the measured results, which are well within the acceptable limits. Important conclusion of the study is achieving complete automation for evaluating the critical speed by considering external load & for any configuration or combination of rotors. Keywords––Rotor generator, Critical speed, Ansys programming design language, Natural frequency I. INTRODUCTION Industrial machinery is said to be optimized when it results from a state-of-the-art mathematical simulation and design, comprehensive prototype, preproduction testing and manufacturing with minimal costs. A design is stated to be satisfactory if it satisfies the high technical requirements and low production costs simultaneously. Hence the machinery design involves an element of compromise in the many requirements of secondary importance. The success of a design, is not simply achieved by looking at one parameter in isolation, but is a complex process, with various parameters in interaction. Generally, as the objectives of the design can be formulated in terms of a few global variables, there may be a great many variables also defined and handled within each subsystem. The evaluation of how the variables of primary importance affect the overall quality of the design is of significant importance. Choosing an optimal design based on the particular requirements of the application and at the same time keeping in view the manufacturing costs is a challenge for any design. An analysis of rotor bearing dynamics is critical to the design of any high-speed machinery that has rotating parts. Such analysis has taken a giant leap forward with the development of standard computer programs for determining various system characteristics. When the tedious and error prone modeling process is automated and data input is consistently shared among the analysis processors and graphic postprocessors, the analysis would be significantly improved. Early in the development of rotor dynamics, it was a commonly prevailing notion that operation above the first critical speed was impossible. Nascent stages of rotor dynamic study began with Rankine (1869), whose work resulted in his conclusion that a shaft would be stable under the first critical speed and would always be dynamically unstable above that. Thus, the unbounded increase in the vibration in the vicinity of the critical speed was seen as an unstable condition. This misconception was corroborated by Greenhill (1883), who stated that the shaft inertia contributes to its buckling, thus reinforcing Rankine’s concept. Later Dunkerley (1895), using the Reynolds’s eigen value concept, could calculate critical speeds of wide variety of shaft disc systems. The turn of the century saw the strong endorsement of Rankine’s concepts. P. H. Mathuria [1] presented number of methods to find the natural frequencies of undamped system through simple methods suitable for hand calculations and the transfer matrix method. The transfer matrix technique may be regarded as extension of holzer method. The transfer matrix relates the state vector of one station to the next. Hence the more complex problem can be solved using these transfer matrix method as there exist a recurrence formula. Dr. Heinrich Spryl & Dr. Gunter Ebi [2] presented a paper on finite element method on modal analysis of shaft line of hydro power plant. In this, the shaft line was designed to have a first bending critical speed between load rejection speed and runaway speed. The critical speed is determined by structural properties of the shaft line and the bearings which are independent of speed and the speed dependent characteristics, e.g. oil film elasticity, damping, gyroscopic effect and load due to unbalance magnetic pull to the non-symmetry of rotors. In general, the structural properties of the shaft line are well defined in terms of material elasticity and mass distribution.