The city’s water department decided to install a new well to provide this additional capacity. They contacted Strand Associates, an engineering firm with a wealth of expertise in municipal water systems, who designed the new well system and the building that would house it. The new well is 1400’ deep and is fitted with a submersible pump roughly 800’ below the surface. Strand worked with Jerry Janzen at Instrument Control Systems in Plymouth, Minnesota, to develop 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 350 HP Danfoss VLT® AQUA variable frequency drive," says Jerry. The drive is connected to the city’s SCADA system, allowing it to run the pump motor at whatever speed is necessary 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 city the ability to plan for future expansion. As new developments are built and flow increases further, the drive will automatically increase pump speed 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 VLT AQUA drive at Loves Park’s Well provides between 200 and 1600 GPM at a pressure of 70 PSI. 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. "At 460 VAC, we would need very thick cables to operate the pump," explains Brent Studnicka, an Engineer with Strand Associates. "But at 2400 VAC, the current required is much lower, meaning that a smaller gauge of wire could be used." This is due to the fact that as the voltage increases, the amount of current required to perform the same amount of work decreases. "With 800 feet of cable, this resulted in a significant savings," says Brent. The 460 VAC output of the Danfoss VLT AQUA drive is wired to a stepup 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.
The use of a drive also minimizes equipment stress. By 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 VLT AQUA drive provides the flexibility to meet the needs of the community, both today and in the future.