Variable Frequency Drives

A variable frequency drive (VFD) is a system used to control the rotational speed of an alternating current (AC) electric motor. Through manipulating the frequency of the electrical power supplied to the motor, the VFD is able to reduce energy consumption and increase the longevity of the motor. The ability to precisely control the electric motor of a pump, which results in significant cost and energy savings, is turning the VFD into a commonplace piece of equipment seen in many aquatic facilities.

Before the existence of VFDs, swimming pool pumps were set to run at a constant rate throughout all hours of operation. As one can imagine, this method is highly inefficient. Generally high in horsepower, these pumps move water quickly and in doing so, create a high amount of friction and head pressure. The excess friction and head pressure drastically increase the pump’s energy consumption. Furthermore, swimming pools pumps are oversized to account for worst case scenarios such as dirty/clogged filters or situations when the bather load is at a maximum. When pumps are not operating at “worst case scenario” conditions, there are opportunities for energy savings. As energy costs continue to rise, swimming pool operators and owners continually seek ways to cut back energy usage. Installing VFDs in conjunction with preexisting or new swimming pool pumps is an ideal place to start reducing energy consumption.

By continually adjusting the speed of the pump motor, a VFD can successfully adapt almost any pump to its pool’s unique and variable conditions. One common application of a VFD can be seen during the backwash cycle of a pool filter. After the filter’s backwash is complete, the filter bed is free of debris, and the filter head pressure is minimized. The water is now able to flow through the filters without much resistance. If a VFD is installed, it will recognize that the head pressure in the system is significantly less than before the filters were backwashed. The VFD will automatically reduce the rotational speed of the motor in order to maintain the specified flowrate. A slower motor ultimately leads to a direct cost savings for the owner. While some owners may be weary of the added cost of VFDs, many pools see payback through energy savings within one year of installation.

When installing VFDs, a few critical points should be kept in mind. First off, VFDs require a controlled environment. These units perform best when operating conditions are between 15 and 100 degrees Fahrenheit. Since they will be operating in a pool environment, it is important that they are housed in a watertight enclosure mounted or secured to the building structure. While VFDs are able to function as a stand-alone units without controllers, it is preferred to have the units controlled from either a manual potentiometer or a 4-20ma water flow sensor. Controlling VFDs will allow them to modulate pump motors to maintain constant flow rates. Finally, The VFDs will require additional coordination with the electrical engineer and possibly the controls engineer to confirm the intended sequence of operation (how the pumps are intended to run). It should be noted that the VFDs must be rated for the same voltage (power supply) as the drive motors.

After considering the potential cost and energy savings associated with VFDs, it would be hard to argue that VFDs should be a standard item in every swimming pool mechanical room. Recirculation pumps operating at rates conducive to “worst case scenario” conditions is beginning to become a thing of the past. Instead, more elegant control or the electric motor through the use of a VFD allows operators to more accurately control their swimming pool systems.

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