After installation, the pump speeds were set back using the on-board time clocks that initiate pre-programmed, pre-set speeds in the VFDs. The crew programmed the pumps to run at 90% power during open-pool hours, and to ramp down to 60% for 12 hours each night after the pool closed. These reduced settings were expected to reduce energy consumption by 65,000 kWh per season, a savings of more than $7,000 in operational cost annually.
According to Robinson, “With the centrifugal pumps used at the pool complex, the relationship between reduced pumps speeds and reduced power consumption is based on the nonlinear affinity laws. For example, if you halve the speed of the motors, theoretically at least, you’re using roughly one-quarter the power, so the energy-saving value of any speed reduction is magnified.”
VFDs eliminate hard starts and stops
Since the drives were installed, the pool recognized not only a dramatic reduction in energy costs, but also, the pumps produce far less noise. “Now we can actually stand in the pump house and hear each other talk, whereas before we were never able to do that,” says Friend.
South Windsor, Conn., opens three pools to the public; water now is filtered according to the cycles of actual use to save energy.
Another important benefit is that the drives enable the pool engineers to eliminate hard stops and starts. When the strainers need to be cleaned, or other maintenance needs to be done, the engineers can now ramp the motors down, shut them off, perform the necessary tasks and then ramp the speed back up―all without the high-demand and across-the-line surges from hard stops and starts. The VFDs also serve to protect the pumps and the motors from over-current and under-current, shutting down the system if problems occur, instead of allowing the motors to spin themselves into the ground.
“A major problem occurs when our pumps lose prime,” explains Bruce Lundie, facilities mechanic. “We are stepping the speed down at night to save power, so if the strainer baskets become clogged with leaves or debris, we could essentially not draw enough vacuum from the pump. This happened to us before. Once we lost prime during the night. We came in the next morning to find the pumps red-hot and steam boiling out of the strainer baskets, because we had nothing in place to protect the motor or shut down the pump.”
With the drives in place, an ABB tech was able to develop user load curves using the ABB DriveWindow Light performance calculation program. He first determined the amperage draw when the pumps lost prime using the trend data in the program, padded the number a little and programmed the drives to trip on a fault whenever that low amperage limit is reached. This prevents damage to the pumps from losing prime, especially during the overnight hours when no one is around.
Two-year payback anticipated
With the energy savings the pools complex now is experiencing, Friend anticipates recouping the entire project budget, the cost of the VFDs and their installation, in two seasons.
“We don’t have the final numbers back from this season yet, but between the energy we are saving and State of Connecticut energy grants, we’re thinking the retrofit will have paid for itself by the end of next year. This is by far the best decision we could have made.”
Based on this success, Friend already is evaluating ways to incorporate VFDs into other motor-control applications in the future. Working with FlowTech, he has begun designing plans for upcoming projects, including a large water treatment plant renovation. “I’m so pleased with ABB VFDs that I’m specifying the rest of the treatment plan upgrades around incorporating them,” says Friend. “We’ve had nothing but top-notch results from the drives in every experience, and we are extremely confident using them in future projects.”