As new subdivisions and businesses were added to the Loves Park water distribution system, the city was faced with how to best serve the growing demands. The community was already served by five water wells and three 1900 m3 water towers that helped to maintain adequate system pressure. But growth along its eastern edge meant that more and more demands were being placed on the system.
Without the introduction of additional capacity into the system, the increased flow would eventually cause a decrease in pressure.
The city’s water department decided to install a new well to provide this additional capacity. The new well is 425 meters deep and is fitted with a submersible pump located roughly 245 meter below the surface. Jerry Janzen at Instrument Control Systems in Plymouth, Minnesota, designed the system required to efficiently operate the pump motor. "In order to maximize the control and energy efficiency of the well and pumping station, we used a 250 kW Danfoss VLT® AQUA drive," says Jerry. The drive is connected to the city’s SCADA system, allowing it to run the pump motor at whatever speed is called for to maintain the appropriate system pressure. By matching the actual demands of the water distribution system and running the pump at less than full speed, the drive is able to provide significant energy savings and reduce water losses. Furthermore, it gives the Loves Park water department the ability to plan for future expansion. As new developments are built and flow increases further, the drive will automatically run the pump motor at whatever speed is necessary to meet the increased demands and maintain the pressure setpoint.
One of the primary advantages of using a variable frequency drive is the capability to provide constant pressure at variable flow conditions. The Danfoss VLT AQUA drive at Loves Park’s Well Number 6 provides between 0.75 and 6 m3 per minute at a pressure of 70 PSI (4.8 bar). In many ways, this is a preferable alternative to using water towers, which are far more costly both to install and to maintain. It was determined that using a medium voltage motor would further reduce overall costs due to the significantly smaller diameter motor cables that it would require. This is due to the fact that as the voltage increases, the amount of current required to perform the same amount of work decreases. The 460 VAC output of the Danfoss VLT AQUA drive is wired to a step-up transformer, which brings the voltage to 2400 VAC. The output of the transformer is then fed through a sine wave filter to ensure that the waveform is exceptionally uniform, prolonging the life of the motor. At 460 VAC, the motor would have required very thick cables to carry the current needed to operate the pump. But at 2400 VAC, the current required is much lower, meaning that a smaller gauge of wire could be used. With 245 meters of cable, this resulted in a significant savings.
The use of a variable frequency drive also minimizes equipment stress. By significantly reducing the number of starts required of the well pump, the drive maximizes the pump motor’s longevity. Controlling the acceleration and deceleration when starting and stopping further lowers stress to the motor as well as reducing water hammer in the pipe system. The drive will also allow the city to work on their water towers without impact to water distribution. "Right now, this pumping station is filling the gap," says Craig McDonald, Loves Park Water Department Supervisor. "But it will also allow us to bypass a tower in order to maintain it while still providing water to the people who rely on it". The variable frequency drive provides the system with the flexibility to meet the needs of the community, both today and in the future.
