Influencing factors on motor vibration & rotor critical speed in design, test and field applications

This paper will present various case studies of how the rotor or system natural frequencies can be strongly influenced by its external and internal factors and how small variations in these factors can influence the motor vibration at the manufacturer and in the field. Motors constructed to API 541 standards are required to have a rotordynamic lateral natural frequency that is removed from the operating speed by at least 15%. The location of this natural frequency can depend on many factors such as bearing clearance, bearing type, residual unbalance, oil temperature, oil viscosity, and bearing housing stiffness. Depending on the design, some motors are more sensitive to these parameters than others, and small changes in these factors may cause large variances in the motor natural frequency. As a result, small variations in test setup, manufacturing tolerances, or field installations within critical components can cause noticeable differences between the calculated and measured natural frequencies. Variation in motor vibration may also be seen between the motor operating on the manufacturer's test stand and the motor operating in the field. In the field some apparently minor changes on ambient conditions or set up can significantly change the motor vibration. Additionally this paper will propose a worst case calculation method for motor natural frequencies that will provide greater confidence to the end user that the motor will operate successfully in the field before the motor is installed.