Flexible shaft versus rigid shaft electric machines for petroleum and chemical plants

It is pointed out that most small electric machines are designed to rigid shaft criteria without any concern for possible effects on operating performance. However, an increasing number of large high-speed motors that operate at constant speed are designed to be flexible shaft machines. For those machines that operate at constant speed, flexible shaft design may be a good approach. However, flexible shaft motors may not be well suited to adjustable-speed operation because excessive vibration may occur at some operating speeds. The drive may have certain ranges of operation speed that cannot run continuously because of the high vibration. Rigid shaft motors have the advantage of being less prone to excessive vibration when operated over a wide speed range. With proper coordination and system design, rigid shaft motors operating on resilient mountings can be made insensitive to vibration. Hydrodynamic bearings can provide the required resilience if the rotor system is properly designed. Experience with such drives has not shown a need to forbid continuous operation for certain speed ranges