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

A background on electric machines is presented covering power/speed rating charts and the history of motors and turbogenerators. Their applications in petroleum and chemical plants are described. Motor construction, speed/power limits, and vibration and balance are discussed. Recent developments and design conflicts are examined. It is noted that the vast majority of electric machines have been produced according to rigid-shaft design criteria, but an increasing number of large high-speed motors that operate at constant speed are designed with flexible shafts. Flexible-shaft motors may not be well suited to adjustable-speed operation because excessive vibration can occur at some operating speeds, barring continuous running. Rigid-shaft motors have the advantage of being less prone to excessive vibration when operated over a wide speed range. Motors larger than 1/4 the largest rating that can be built with a rigid shaft need more engineering coordination than do smaller motors. With proper coordination, rigid-shaft motors operating on resilient mountings can be made insensitive to vibration. Experience with such drives has not shown a need to restrict continuous operation