Save Energy and Extend Equipment Life with Variable-Frequency Drives

A key feature of variable-frequency drives (VFDs) is their ability to regulate the power going to the pump, which helps to extend the equipment life.

Over the past few years, pool professionals have learned that going ‘green’ can significantly lower a pool’s operating costs. Pumps and filters are among the items most scrutinized due to the vast amount of energy they consume. To reduce costs associated with water circulation and filtration it is important to understand why pumps and filters consume large amounts of energy and what options are available to lower consumption—factors that also extend a pump’s motor life. This article will provide a few easy-to-follow steps to help pool professionals determine the best way to extend the life of their pump motors and reduce operational costs by installing variable-frequency drives (VFDs).

A cost-effective strategy

VFDs offer a significant return on investment (ROI) for aquatic facilities—especially semi-commercial and/or commercial aquatic operations (such as homeowner associations, apartment complexes, etc.). These drives allow the pump to run at its most efficient point on the curve, while also reducing preventative maintenance costs.

VFDs can do several things, but one of their key features is the ability to regulate the power going to the pump it is attached to, thus delivering solid power and voltage to the motor, which helps to extend the equipment life.

Better control of the pump is also enabled by allowing the VFD to dial the pump into the exact flowrate that is required, rather than using butterfly valves to bring the pump to within the given parameter. These drives also house the motor’s starter; without it they would have to be purchased separately at a cost between $500 and $750.

Finally, a VFD will also allow for a ramp-up start of the motor, which reduces wear and tear, and eliminates the potential water hammering effect (pressure surge or wave caused when the water in motion is forced to suddenly stop or alternatively change direction), which can cause damage to the pump’s shaft and impeller.

Why add a VFD to a pump room?

By installing a VFD on a pump, an aquatic facility can save as much as five to 10 percent on energy costs. What aquatic facility manager would not want to put these savings for each pump back into the operating budget?

VFD’s are capable of doing the following:

    • ‘Right-size’ the pump to the exact flow required;
    • Save energy with constant flow as filters get dirty (with optional flow sensor);
    • Offer ‘off-hour’ flow management capability;
    • Provide high efficiency up to 98 percent (adjusts drive input voltage for best efficiency point);
    • Eliminates need for motor starters;
    • Provide two-step ramps (soft start feature);
    • Offer overload trip protection to protect the pump’s motor and drive from voltage spikes and phase unbalance; and
    • Save up to 60 percent or more on a pump’s electricity usage.

To quantitatively work on reducing the operating costs associated with running the pump room, start by determining exactly how much it costs to run the facility’s system. To establish this figure, first determine the liters per minute (lpm) (gallons per minute [gpm]) and total dynamic head (TDH) (the pressure head difference between the inlet and outlet of the pump) for each pump, how long each pump runs each day, and what the cost per kilowatt hour (kWh) the utility company is charging.

After arriving at a clear dollar figure, the ROI of adding a VFD can be easily determined.

Step 1: Choosing a VFD

Choosing a VFD is simple. All one needs to know is the electrical phase going to the pump (i.e. single- or three-phase), the horsepower (hp) of the pump onto which the VFD is being installed, the voltage going from the power source to the drive, and whether it is being installed indoors or outdoors. An aquatic facility will see the greatest potential for savings when using a VFD with Class F motor insulation—be sure to check this on existing motors. If the motor is rated a class B, for example, the VFD can still be used, but additional software may need to be added depending on the incoming voltage.

Step 2: Installing a VFD

Basic knowledge of wiring and power is necessary to install a VFD. However, if unsure, an electrician should be hired to complete the installation. The incoming powerline from the electrical source should be connected to the incoming side of the VFD. Then, the pump motor is wired to the outgoing connection of the VFD. It is important to remember, VFDs require the use of a three-phase motor; therefore, if single-phase power is going to the VFD it will act as a phase converter, thus allowing it to be wired to a three-phase motor.

Step 3: Ensure correct pump selection

Proper pump selection (i.e. sizing) and optimal flowrates are additional ways an aquatic facility can increase energy savings. That said, the size of every pump should be checked as many tend to be oversized by design, which means they are bigger than they need to be. This happens because many architects and engineers look at what is required, then pick the next size up to be sure the pump can handle the job.

Pump selection tips

  • Determine flow rate in lpm (gpm);
  • Calculate TDH to account for friction loss (adding 6.1 m [20 ft]/head to account for a dirty filter is optional);
  • Refer to the pump’s performance curve to select the preferred unit;
  • Determine the required pump horsepower (hp) by plotting the liters per minute (gallons per minute) versus TDH (if the plotted point falls between two pump sizes, the next larger pump size should be selected);
  • Do not oversize the pump—if the preferred pump does not provide a proper fit, consider a different model; and
  • Verify the selected filter can handle the system’s flow rate, and be sure the minimum backwash flowrates can be achieved.

Long-term benefits

The greatest long-term benefit of installing a VFD is that it helps to extend the life of the pump motor, thus significantly decreasing maintenance and replacement issues. Further, pump motors last longer not only because of the VFD-enabled ‘soft start,’ two-step ramp up feature but also because of the overload trip protection it offers, which protects the motor and drive from voltage spikes and electrical phase unbalance.

This article was written by Mike Fowler and originally appeared on Pool & Spa Marketing [link].