Guest Column - September 2015
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Performance Improvement

Save Energy Costs, Extend Motor Life With Variable Frequency Drives

By Mike Fowler

Step 1: How to Choose a VFD

Choosing a VFD is very easy. All you need to know is the phase going to the pump (single or three), the horsepower of the pump onto which you are installing the VFD and whether the drive will be installed indoors or outdoors. You see the greatest potential for savings when using a VFD with class F motor insulation—be sure to check this on existing motors. If they are class B, for example, the drive can still be used but additional software may need to be added depending on the incoming voltage.

Step 2: How to Install the VFD

As long as you know wiring and power, most waterpark maintenance facility managers can easily install a VFD. The incoming power from the power source basically gets wired to the incoming side of the VFD, and then the pump motor gets wired to the outgoing part of the VFD. One thing to always remember is that VFDs require the use of a three-phase motor. So if you have single-phase power coming to the drive, the VFD acts as a phase converter and wires to a three-phase motor!

Step 3: How to Ensure Correct Pump Selection

Proper pump selection (sizing) and optimal flow rates are additional ways a waterpark can increase energy savings. Be sure to check pump size on each as pumps for many waterparks are oversized by design so 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 gallons per minute [GPM].
  • Calculate total dynamic head (TDH) (the pressure head difference between the inlet and outlet of the pump) to account for friction loss. Adding 20 ft/head for a dirty filter is optional.
  • Refer to the pump's performance curve to select the preferred unit.
  • Locate pump horsepower required by plotting GPM vs. TDH (if plotted point falls between two pump sizes, select the next larger pump size).
  • Do not oversize the pump. If the preferred pump does not provide a proper fit, consider a different pump model.
  • Verify the selected filter can handle the system's flow rate and be sure the minimum backwash flow rates can be achieved.

Eliminating Sticker Shock

Even with the advantages explained, some facility operators still find the cost of a VSP hard to justify. However, before ruling out a VFD or a new pump, be sure to compare the additional costs to the cost of doing nothing.

For example, let's say a waterpark facility is using a single-speed pump with operational costs of up to $900 per year. After five years, the facility will have spent $4,500 to operate the pump. Comparable operational costs for a facility with a well-designed circulation system that uses a VSP, however, may be as little as $200 per year. During the same five-year period, it will cost the facility less than a quarter of the amount it would have spent using a single-speed pump. Cost savings will continue to multiply the longer the VSP is used. (Costs and savings will vary by region.) The initial cost of a VSP can typically be recouped during the second year of operation. For larger pumps, adding that VFD even to an existing pump is going to put dollars back into the facility through increased energy cost savings and allow the facilities to put those saved dollars to use somewhere else.

With the substantial energy-use rebates that some local power companies offer, in conjunction with the savings in daily operational expenses, some end users are getting back approximately 50 percent of the VFD's cost in less than a year.

VFDs Extend Pump Motor Life, Reduce Maintenance

The greatest long-term benefit of the VFD is that it helps to lengthen the life of the pump motor so the maintenance requirements and motor replacement need is significantly reduced. Pump motors last longer by having a VFD installed not only because of the "soft start" two-step ramp up feature but also because of the overload trip protection, which protects motor and drive from voltage spikes and phase unbalance. Installing a VFD not only saves you on energy usage costs, but also keeps motors working better, longer, so motor replacement is far less frequent.

Mike Fowler is the commercial marketing and sales manager for Pentair Aquatic Systems in Sanford, N.C. He has been with Pentair since 1992, starting his career in the technical services department at Purex Pool Products. Fowler has held many managerial roles within the company, including marketing, accounting and products. For more information, visit