Superimposing Planetary Gears as Variable Speed Drives for Rotating Equipment

Abstract

There are many high-power and high-speed pumps installed in power plants, oil & gas applications and in petrochemical industry consuming a considerable amount of energy. Significant interest exists within operators to improve their efficiency in order to save energy and operating cost. Most of the pumps are driven by electric motors and many of them are speed controlled since this is the most efficient method to adjust flow to process demand. Motor speed is controlled by frequency converters which are installed in-line and therefore they are of full scale and designed to full power. The complete power goes through the variable frequency drive and is subject to losses. A gear then is used in order to step-up motor speed to the requested speed level for the driven equipment. This paper describes a new method to improve efficiency of variable speed drives by power splitting. The main driver is a constant speed motor and its power is transmitted mechanically using the superior efficiency of an epicyclic gear. The gear is designed as revolving planetary gear where all three shafts can turn. One of those shafts is used as input; a second one as output and the third shaft is used for speed control. This principle allows using only a small percentage of rated power as control power to be generated by servo motors. Their frequency converters are placed in a sideline and hence their losses are subject to a small portion of rated power. Copyright© 2018 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station The control speed of the servo motors is superimposed in the revolving planetary gear to transform the constant speed of the main motor into a variable output speed for the driven equipment. A 7500 horse power prototype of an electrically controlled superimposing planetary gear (ESPG) was built and tested extensively. Efficiency measurements were done and could prove peak efficiency of 97 percent for the variable speed gear including lube oil pump, servo motors, frequency converter and transformer. This is up to 2.5 percent more than conventional variable speed systems with a full scale in-line variable frequency drive (VFD).