David W. Spitzer

Reducing the motor speed of a centrifugal pump reduces both the discharge pressure of the pump and energy consumption of the motor. When the motor is operated at 80 percent speed, what is the approximate pump discharge pressure and motor energy consumption expressed as a percent of full speed discharge pressure and full speed motor energy consumption respectively?

A. 65 percent pressure and 50 percent energy
B. 65 percent pressure and 80 percent energy
C. 80 percent pressure and 50 percent energy
D. 80 percent pressure and 80 percent energy

Variable speed drives are increasingly applied in many applications to reduce energy consumption. Reducing the speed of a centrifugal pump reduces the pump discharge pressure by the square of its speed. Therefore, the discharge pressure will be approximately 64 percent of the full speed discharge pressure when the pump is operated at 80 percent of speed. Answer C and Answer D are not correct.

Reducing the speed of a centrifugal pump reduces the energy consumption by the cube of its speed. Even a small speed reduction such as 5 percent can save almost 15 percent of the energy consumed at full speed. In this case, operating at 80 percent speed reduces the energy consumption to approximately 51 percent of the energy required to operate the pump at full speed. Therefore, Answer A is correct

Additional Complicating Factors
Many variable speed drive applications and installations require attention to detail to ensure that the motor and its mechanical equipment can function properly in operation. In addition, failure to attend to certain details of lower speed operation can damage the equipment.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.