Variable Frequency Drives (VFD)

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

VFDs should be specified by the Counsilman-Hunsaker project manager working on the project, in Division 13 documents, with coordination of pump requirements by Counsilman-Hunsaker as listed in the Design Considerations and Consultant Coordination documents.

When a VFD is needed, consider if the pump needs to be controlled beyond the install start-up and adjustment to the proper flow rate.  A feature pump may not need to have a VFD if all it needs to do is start and stop.   Typically, applications for the VFD would include recirculation pumps, and possible sprayground feature pumps.

The VFD should require additional coordination with the electrical engineer and possible the controls engineer to confirm the intended sequence of operation (how the pump is intended to run).The VFD requires a controlled environment. The unit needs operating conditions between 15 and 100 degrees (storage temperature -40 to 150 degrees) and show be housed in a watertight enclosure.  It should be mounted or secured to the building structure

While the VFD can function as a standalone unit (without controller). It is preferred to have the unit controlled from either a manual potentiometer or from 4-20ma water flow sensor.  This will allow the VFD to modulate the pump motor to maintain a constant flow rate.

The VFD must be rated for the same voltage (power supply) as the drive motor.

The VFDs are available for either analog or digital input signals.

The analog signal could be similar to the variable input from a flow sensor.  The thing to remember is the signal varies per the input.

A digital input would be an interlock signal (either on or off) from the chemical controller, filter control panel, remote start/stop switch, or an e-stop switch.  The point to remember is the digital input must be made in order for the drive to operate.

Siemens VFD have a BOP (basis operation panel) as an option the Advance Operator Panels should be typically provided when NO building automation system is available. This will allow the VFD to store control sequences internally to the unit and provide the operator a greater than of control to the unit.

If a VFD is used on a recirculation system that utilizes a Defender filter, a baseline limit should be set so that the filtration rate does not drop below 0.50 GPM/SF.  This will help ensure proper filtration and the media will attach to the elements.  During the precoat process, the VFD will likely need to be increased to the normal design point.

VFD is the primary basis of design for feature pumps.  A soft starter may be used as an option at the discretion of the project manager.

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