Hydraulic Institute Pump FAQs February 2008

Written by Hydraulic Institute Pump FAQs

Page 2 of 2

Q.

We are installing a multistage pump and plan on using a reduced voltage starter. Is this practical and what precautions should be taken?

A.

A plot of speed versus torque requirements during the starting (accelerating) phase of a rotodynamic pump (any pump type) is sometimes needed to check against the speed-torque curve of the driving motor. Rotodynamic pumps typically have a speed-torque curve characteristic where the torque varies as the square of the speed. The driver must be capable of supplying more torque than required by the pump along the entire curve in order to bring the pump up to rated speed under the conditions present during the starting phase. This is generally attainable for rotodynamic pumps with normal induction or synchronous motor performance characteristics. However, under certain conditions, such as with high specific speed pumps or when motor terminal voltage is reduced below nominal tolerances, a motor with higher pull-in torque may be required to maintain adequate torque margins and ensure expected pump acceleration to operating speed.

Reduced motor terminal voltage will result in reduced motor torque. For induction motors, torque is reduced in proportion to the square of the applied terminal voltage. Thus, a motor whose terminal voltage dips to 80 percent of nominal voltage during start-up will only produce accelerating torque equal to 64 percent of the torque under nominal full voltage conditions. Obviously, available torque is reduced significantly further when starting voltage defined by the end user is lower than 80 percent, and care should be taken to consider these situations. See the figure below.

In this example, a 1500-hp motor rated (full voltage) torque of 2200-lb-ft provides 11 percent greater torque than the pump open valve rated condition of 1980-lb-ft and thus could be considered satisfactory. However, at 80 percent voltage, motor rated torque would only be 1408-lb-ft and the motor could not start the pump. However, at the pump closed valve condition of 1393-lb-ft the motor does have enough torque at 80 percent voltage, but the margin would be considered too small and an adjustment would be needed either in the motor and/or the pump's defined closed valve condition.

If the standard motor design is not adequate to ensure necessary torque margin throughout the acceleration period, alternate designs may be proposed, or the closed valve condition may be specified as the start-up condition instead of the open valve condition if it is only the open valve condition that is compromised. See ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Application and Design for more detailed information on this subject.

For more information about HI, our publications, Pump LLC Guide, Energy Saving Video-based education program and standards please visit www.pumps.org. Also visit our new e-learning portal with a comprehensive course on "Centrifugal Pumps: Fundamentals, Design and Applications," which can be found at www.pumpslearning.org.

Comments (0) Add Comment