Rockwell Automation staff installed Allen-Bradley 1336 Plus II variable frequency drives to control the new motors on Sandusky's two largest high-service pumps.Click here to enlarge imageA 5 mgd pump rebuilt in the project received a soft-starter system for its existing motor. For the city's two largest pumps, 6 mgd and 8 mgd horizontal split case pumps, the rebuild project included replacing the pumps' 250 and 300 hp 1940s-vintage squirrel-cage motors with new high efficiency motors. Rockwell Automation staff specified and designed Allen-Bradley 1336 Plus II variable frequency drives to control the new motors.
The new AC drives provide the city with more precise water pressure control, leading to significant energy savings. Over a 12-month period, Sandusky's water works facility has saved over $31,000 in electrical costs, which can be attributed to reduced demand charges because of fewer pump start-ups, more constant flow to satisfy fluctuating demand and more efficient motors.
By controlling motor speed and torque, the Allen-Bradley 1336 Plus II drives allow the HS pumps to have an adjustable speed range the 8 mgd pump can be adjusted from 4 mgd to 8 mgd and the 6 mgd pump can be adjusted from 5 mgd to 6.8 mgd. Most importantly, the drives allow the pumps to gradually increase system pressure and reduce the number of cumbersome pump changes.
Keller noted that the two large pumps now can serve most of the city's needs and are used the majority of the time.
"The variable frequency drives have decreased the need for pump changes, reduced electrical cost and reduced water hammer, leading to reduced water main breaks," Keller said.
ConclusionThe new drives-centered system has decreased the need for pump changes, and has reduced water main breaks by 76 percent. By reducing main breaks the potential for back flow, system contamination and fire protection interruption has been greatly reduced. Water service interruptions also have decreased, which means service to residential and industrial customers has been more reliable.
"Allen-Bradley variable frequency drives have been a tremendous value-add to the Sandusky Water Department," said Keller. "It also has been important to us to have a supplier that will support us and answer questions as we continue to expand and improve our system. Rockwell Automation has been there for us, and the city of Sandusky plans to incorporate variable frequency drives in all future motor-related applications."
About the Author: Ron Rabbitt is a Drives Specialist for Brohl & Appell, where he is responsible for advising customers on how to specify, install and maintain drives. Brohl & Appell is an affiliated distributor who has been meeting the needs of the industrial automation, PVF and electrical markets for over 110 years