Practical Balancing of Flexible Rotors for Power Generation

Abstract

Balancing technology is still relatively new. Thirty years ago it was primarily still part of the skilled trade and was often obscured. Today there is enough reference literature printed during the last 20 years alone on general balancing and balancing of flexible rotors, that could fill a room, (Ref: N. Rieger). The majority of papers and other references deal with theoretical derivation of equations based on Jeffcott rotor model. With the growth of rotor sizes specifically of electric generators in power plants, so grew the need to develop not only a theory, but also the way to practically balance these rotors. The economy of manufacturing required pushing the rotors to more and more slender; lower and lower stiffness (∝ EIxx/L3) designs, in relation to its mass moment of inertia (Im), these rotors were more difficult to balance. The first to encounter the problem of balancing these rotors were the OEMs. On two different shores of the Atlantic Ocean, two basic balancing theories known as balancing in “N”, or in “N+2” balancing planes and operating rotor modes were developed. Later, with the development of the microcomputer the influence coefficient method had gained popularity among the power plant community and despite good experiences from both sides the controversy over which one produces better results was left open. In this paper a review of the “N” and “N+2” methods including notes on influence coefficients (IC) is conducted from a practical standpoint. The conclusion by the Authors is that there is no “better” or “worse” balancing method, only the more or less economical in a given situation, and neither gives a unified method to satisfy every rotor. General guidance is also provided over which method to use for best results in balancing large turbo-generator sets.