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

The definitions of rigid and flexible rotors, the rotor dynamic analysis procedure, and the design criteria defined in API (American Petroleum Institute) 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 of low and high-speed machines are presented as study cases. The conclusion shows that it is very important to know the operation conditions of the machine, specifically the operating speed range and how the machine will be erected in the field 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, 4-pole and slower up to 7000 kW and highspeed 2-pole machines below 1000 kW are well suited for rigid rotor design. Larger machines, especially 2-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