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.