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Laffoon slits: a method to compensate unsymmetrical rotor bending stiffness
2002 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This work has been carried out at ALSTOM Power Generation in Västerås, a company which has tradition of manufacturing synchronous generators which dates back to the end of the 19th century when the Asea were the owners. This Master of Science thesis project treats balancing of an unsymmetrical rotor to minimize the risk of double frequency vibrations. The work describes the complete method of balancing the rotor, from determination of the Laffoon properties to analysis of vibration levels. The problem arises when a rotor with unequal stiffness rotates horizontally. Under the rotation the gravity gives rise to bending moment which excites the rotor. There are at least two ways of balancing the rotor due to this vibration: by transverse slits (Laffoon) or by axial slots. The first one is better from a production point of view and is the one that has been studied. There is one more aspect that has to be considered and that is the magnetic material in the rotor poles. A good Laffoon configuration is one that removes less material, but at the same time balances the rotor sufficiently. The stiffness variation was the kernel to this problem since it is the stiffness ratio that gives rise to the vibrations. For a homogenous rotor there is no problem to calculate the stiffness but for a rotor with Laffoon slits it is harder. A parameter study showed how the slit parameter affects the stiffness. To compare different Laffoon configurations to vibration levels and how much material that will be removed a vibration analysis must be done. Such an analysis can be performed when the equivalent rotor stiffness is known. Studying the bending stress for a rotor with Laffoon slits showed insignificant stresses for the area around the slits. According to that result a simplified model that calculates equivalent bending stiffness was assumed. The model, implemented in Matlab, calculates stiffness for three cross sections, which gives an equivalent bending stiffness for the total rotor. There are many Laffoon configurations, which can balance the rotor sufficiently. To find the best of these configurations and make the selection effective, an optimization method called “Genetic Algorithm” has been used. The algorithm is built on a natural selection and is easy to understand. The first step in GA optimization is to set up a population of individuals. Each individual consists of a set of parameters and the population will propagate according to “the strongest individual survives” when the individuals are ranked according to a given function. The best individual is received after a specific number of populations or when an optimization criterion is fulfilled. For a rotor with several diameter changes and notches it is important to know the stress concentration factors. A number of simulations have been done to see how the stress around the Laffoon slit changes with two Laffoon properties. To investigate how the Laffoon slits balance the rotor response, analyses have been performed. Three cases are compared to each other: a rotor without slits, a reference case for a rotor with Laffoon slits and a rotor with a new Laffoon configuration. The vibration level is high for a rotor without slits and the maximal vibrations appear at twice the running frequency. The reference case gives higher vibrations than allowed. The new recommended configuration gives an acceptable vibration level.

Place, publisher, year, edition, pages
Keyword [en]
Technology, Rotor dynamics, Unsymmetrical rotor bending stiffness, unbalanced rotor, gravity excitation, genetic algorithm
Keyword [sv]
URN: urn:nbn:se:ltu:diva-47037ISRN: LTU-EX--02/066--SELocal ID: 4a15eb17-8c18-418d-902c-58d088a2dcebOAI: diva2:1020353
Subject / course
Student thesis, at least 30 credits
Educational program
Mechanical Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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