Pro Tip #18

How do VFD’s (variable frequency drives) make a circulation pump slow down and save energy?

There are many options to reduce energy consumption and save electrical expenditures for the pool circulation pumps.  Each option provides a different level of savings, along with different abilities to integrate with the existing circulation, filtration and chemical control systems. These options include:

  • Standard Variable Frequency Drive (VFD)
  • EkoFlex Smart Pump Control System (SPCS-EF)
  • Full-Drive Smart Pump Control System (SPCS-FD)

Each of these approaches to energy savings also requires a different type of installation, with differing costs.  Following is a description of each system, type of required installation, and benefits of each approach.

VFD Overview

A variable-frequency drive (VFD; also termed adjustable-frequency drive, “variable-voltage/variable-frequency (VVVF) drive”, variable speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used to control AC motor speed and torque by varying motor input frequency and voltage. 

VFDs are used in applications ranging from small appliances to large compressors. About 25% of the world’s electrical energy is consumed by electric motors in industrial applications, which can be more efficient when using VFDs.

Standard Variable Frequency Drive (VFD)

The simplest and lowest cost system is a standard VFD.  It is not coupled with a flow sensor, so a standard VFD cannot maintain a set flow rate required by the Health Department for a public swimming pool.  Unfortunately, a standard VFD also does not consider many other important parameters required to operate and maintain a pool’s circulation, filtration and chemical control systems.  It is important to remember that all equipment in the pump room works together as a system.  A swimming pool circulation system is a dynamic system, unlike domestic hot water systems, HVAC systems, and other recirculating water systems. The swimming pool system incorporates a filter, which requires a varying amount of pump head to maintain the health department-mandated flow rate as the filter loads with particulate. When a sand filter backwashes, it requires a minimum 15 GPM/square foot of filter surface area flow rate to properly fluidize the media bed.  Without this flow rate during backwashing, the dirt trapped by the filter cannot be properly removed, resulting in failure of the filter.  This is why most aquatic engineers and pool designers recommend against the installation of a standard VFD.  This is also why most standard VFD’s were abandoned soon after they were installed during the last big push for energy efficiency in the 1990’s.  Operators and owners found standard VFD’s are unable to meet the needs of the dynamic swimming pool circulation system, causing problems more expensive to remediate than the savings they provided.

It is possible to install a manually-operated “bypass” to allow the pump to operate at full speed during backwash; however this has many drawbacks. First, it will not allow for an automated backwash.  The manual bypass, which is mounted in a separate enclosure than the VFD, requires that an operator be present to operate the switch.  It is also necessary for the pool operator to remember to engage the bypass not only before backwash, but to manually switch the VFD back to “savings” mode when the backwash is complete.

In addition, most pumps are “oversized” to ensure meeting Health Department flow rates at “worst-case scenario” of having a dirty filter fully loaded to a 15 PSI differential between influent and effluent pressures.  This requires that a “rate-of-flow” valve (built into the filter system) and a butterfly valve be partially closed during normal operation and backwash.  The rate-of-flow valves are completely opened with a VFD (and a SPCS system) to take advantage of maximum energy savings.   The manual bypass, when incorporated with a standard VFD, can allow the pump to operate beyond system design parameters while in bypass mode, which results in cavitation (and expensive damage) to the pump.

A standard VFD also does not allow for the interlocking of the chemical control system (which could result in feeding of chemicals when the circulation pump is off), nor the pool heater “fireman’s” switch (which could result in the sudden shutdown of the circulation pump before the heater cools, damaging the pool heater).

Many standard VFD’s include a simple, “real-time” clock that can allow the circulation pump to switch to an even lower flow rate at a set time every day.  This does not allow any flexibility for the changing programming schedules at an aquatic complex.

A standard VFD will have the lowest initial cost. It will also be the easiest and least expensive to install. True costs of this type of system are hard to estimate, especially when long-term expenses of the incorporation of a standard VFD will mean that the pool circulation, filtration, heating and chemical system will no longer be automated, and it would provide the lowest level of energy savings.

EkoFlex Smart Pump Control System (SPCS-EF)

The EkoFlex Smart Pump Control System (SPCS-EF) is a much better option than a standard VFD.  The SPCS-EF has a standard VFD as one of its components, along with an overload block, bypass and output contactors to provide additional protection to the pump motor. The manual bypass in the EkoFlex will allow the operation of the pump even if the VFD is not operating.  In addition, it includes a 5% line reactor to protect the VFD and other internal components.  The SPCS-EF interlocks with the chemical control system, preventing chemical feed if the circulation pump is shut off, and the SPCS-EF is enclosed in a NEMA 12 wall-mounted enclosure.  It comes standard with a manual potentiometer, but the EkoFlex can be provided with a 4-20mA flow sensor, or incorporated with a BECSys5 or BECSys7 controller to ensure that Health Department-required flow rates are maintained.  The SPCS-EF has a manual switch for off, bypass, and normal filtration (energy-savings mode), and is programmed for easy switching between the different modes.  Unlike a standard VFD, the EkoFlex also incorporates an interlock to the pool heater’s fireman switch.  The SPCS-EF EkoFlex has easy-to-see mode indicator lights in its front panel.

It is important to note that the SPCS-EF, when incorporated with a 4-20 mA flow sensor or BECSys controller, will provide for proper backwash flow rates of 15 GPM/sq. ft. of filter area, as long as the flow sensor is placed after the filter.

Installation of the SPCS-EF is more involved than a standard VFD, and will be more expensive to purchase and install.  The SPCS-EF system integrates better with the circulation, filtration, heating and chemical systems than a standard VFD.

Full-Drive Smart Pump Control System (SPCS-FD)

The greatest energy savings for pool circulation pumps will be provided by the full-drive Smart Pump Control System (SPCS-FD). It is a fully automated system, and communicates with every piece of equipment in the pump room necessary to have automation.  It also provides the greatest reduction of electrical costs.  The SPCS has all the features of the SPCS-EF, along with many more functions that make it the best choice for aquatic facilities.

The SPCS-FD communicates with the filter controller to know when the filter is backwashing.  There is no need for a pool operator to flip a switch on the motor control to insure correct flow rates for backwash, nor does the operator need to remember to switch back to “normal filtration” at the end of the backwash process.  The SPCS-FD has a VFD as one of its many components, along with a line reactor to insure only “clean” current is transmitted to the pump motor.  It also incorporates a main circuit breaker, a magnetic contactor, motor overload protection, over and under voltage protection  (sustained low voltage is the #1 destroyer of VFD’s), an emergency shut-off relay, a power block with easy-to-land connectors for interlocks.  All of these components are mounted in a NEMA 12 enclosure, with a cooling fan to protect against overheating.  The SPCS-FD has five separate modes, with an easy-to-use selector switch.  All of this results in a true automated system that integrates with the existing pool equipment in the pump room.

The SPCS-FD’s exceptional energy savings is enhanced by its “Plus” mode.  The SPCS-FD has a PLC communicating with an easy-to-program multi-day, multi-event timeclock.  This allows the SPCS to automatically switch from the “normal” flow rate to a reduced “Plus” flow rate whenever desired, thus taking advantage of down-time whenever it may occur.  The “Plus” mode flow rate is selected to ensure that there is enough water circulating to allow water chemistry and heater flow switches to work, but at a rate lower than “normal” flows.

Installation of the SPCS-FD requires a contractor familiar with pool automation, and capable of wiring the interlocks from the pool chemistry controller and filter backwash controller.  The SPCS-FD requires an investment greater than the other energy options, but the savings provided by the SPCS-FD very rapidly compensate for initial higher cost.

The SPCS-FD is chosen by aquatic engineers and designers for new and existing facilities because it is the only completely automated system available, incorporating a VFD with all the other components required to automatically respond to all the changing situations encountered in a highly-used aquatic facility. The SPCS-FD will provide the greatest savings on electrical costs, day-after-day, year-after-year.