Water Treatment Equipment Resources

Variable Frequency Drive Noise Reduction and Mitigation
By Craig Scroggins and Eric Anderson – 8-2011

Switching from an on/off pump control to a variable frequency drive pump may result in flow measurement errors and pH measurement errors. This document details many solutions that H.E. Anderson has observed to help correct electrical noise problems.

A power line filter of the type and size indicated by the VFD manufacturer should be used to reduce the electrical noise being fed into the A.C. power wiring. Power line filters are usually listed as optional equipment. All VFD drive cables should be of a shielded type. All power and signal wires to and from H.E. Anderson equipment should be kept at least 12 inches away from VFD wiring. If the VFD wiring must cross any H.E. Anderson Product wiring make sure they cross at a 90º angle. H.E. Anderson J Plus controllers should be kept as far away VFD’s as possible. Use separate branch circuits for VFD’s and H.E. Anderson equipment. If flow measurement errors occur, it may be necessary to change the meter cable from a 2 conductor with shield to a 3 conductor with shield and connect the shield to earth ground. If pH measurement errors occur, it may be necessary to connect the water stream at the pH probe to earth ground. One option is to use a grounding rod close to the pH probe. Another option is to connect the grounding conductor to earth ground at the receptacle where the P-1 is plugged in. Often times the second option will correct pH measurement errors when the first option did not.

This section contains information regarding motor installation collected from various sources. Short motor leads, when used with an EMI/RFI filter or drive isolation transformer, will help reduce electrical noise generated by VFD’s (1). One way to control common-mode noise is to provide a known path to ground for noise captured at the motor’s frame. A low impedance path, such as a properly designed cable ground/shield system, can provide the noise with an easier way to get back to the drive than using the building ground grid, steel or equipment, etc. (2). VFD Cables with heavy thermoset insulation are recommended because of the proven electrical benefits and improved high temperature stability they offers. Shielding systems including copper tape, combination of foil and braid and continuous armouring types are most appropriate for VFD applications because of the low impedance path they provide for common-mode noise to return to the drive. (2). The high frequency current ripple in the motor cables may also cause interference with other cabling in the building. This is another reason to use a motor cable designed for VSDs that has a symmetrical three-phase structure and good shielding. Further, it is highly recommended to route the motor cables as far away from signal cables as possible. (2)

Conducted disturbances can propagate to other equipment via all conductive parts including cabling, earthing and the metal frame of an enclosure. Conductive emissions can be reduced in the following way:

• By RFI filtering for HF disturbances
• Using ferrite rings in power connection points
• Using an AC or DC choke (even meant against harmonics, it reduce HF disturbances as well.)
• Using an LCL filter in the case of regenerative drives
• Using a du/dt filter. (3)

To be able to effectively prevent disturbance through the air, all parts of the power drive system should form a Faraday cage against radiated emissions. The installation of a power drive system includes cabinets, auxiliary boxes, cabling, motors, etc. Some methods for ensuring the continuity of the Faraday cage are listed as follows:

Enclosure

• The enclosure must have an unpainted non-corroding surface finish at every point where other plates, doors, etc. make contact.
• Unpainted metal-to-metal contacts shall be used throughout, with conductive gaskets, where appropriate.
• Use unpainted installation plates, bonded to a common earth point, ensuring all separate metal items are firmly bonded to achieve a single path to earth.
• Use conductive gaskets in doors and covers. Separate the radiative i.e. “dirty” side from the “clean side” by metal covers and design.
• Holes in enclosure should be minimized. Cabling & wiring
• Use special HF cable entries for high frequency earthing of power cable shields.
• Use conductive gaskets for HF earthing of control cable shield.
• Use shielded power and control cables. See product specific manuals.
• Allow no breaks in the cable shields.
• Select low impedance shield connections on the MHz range.
• Route power and control cables separately.
• Use twisted pairs to avoid disturbances.
• Use ferrite rings for disturbances, if necessary.
• Select and route internal wires correctly.
• See product specific manuals

Installation

• Auxiliaries used with complete drive modules (CDMs) should be CE marked products conforming to both the EMC & Low Voltage Directives, NOT ONLY to the LV directive, unless they are intended for incorporation into an apparatus by another manufacturer or assembler.
• Selection and installation of accessories in accordance with manufacturer’s instructions.
• For wall-mounted units, strip the sheathing of a motor cable back far enough to expose the copper wire screen so that the screen can be twisted into a pigtail. Keep the short pigtail short and connect it to the ground.
• For cabinet –models, lead the cables into the inside of the enclosure. Apply 360° grounding of the cable shield at the entry into the cabinet. See product specific manuals.
• 60° earthing at motor end. See motor manuals.(3)

Sources:

1. Martino, F. J. (2001). AC Drives and EMI/RFI Mitigation. Power Quality and Drives LLC. Retrieved March 25, 2011 from http://www.powerqualityanddrives.com/emi_rfi /
2. Shuman, Brian. (2009). Building a Reliable VFD System. Belden. Retrieved March 25, 2011 from http://www.belden.com/pdfs/Techpprs/VFD_WP.pdf
3. (2008). EMC Compliant Installation and Configuration for a Power Drive System. (Technical guide No. 3). ABB drives. Retrieved March 25, 2011 from http://library.abb.com/global/scot/scot201.nsf/veritydisplay/a8dc0a0e66d66118c12575d6002fd22d/$File/Tg3_EMC_CompliantInstallation_61348280_Rev_D.pdf

Copyright 2011 H.E. Anderson Company. All rights reserved.

Several different strategies can be used to route water lines through greenhouses. Each has drawbacks and advantages. Please note that this article is not all inclusive and many details, such as valve locations and required plumbing accessories, may be omitted. Please see our installation guidelines for complete installation instructions. Also, while H.E. Anderson equipment can be used in recirculating hydroponic applications such as grow towers, Dutch buckets, NTF, etc., this article does not cover hydroponic applications.

Main Factors:
• Do multiple recipes of ingredients or solutions need to be sent to specific zones, houses or benches or will the whole facility operate on a single recipe at a time
• Will the injection system or systems be in a central location or distributed throughout the facility

Single Solution throughout Facility
If only one or at least very few solutions will be used at the facility then the injection system should be located downstream of where the water source enters the facility but upstream of where the pipes branch out to go to individual benches. Systems should always be installed into a bypass so the equipment can be isolated for service and water can continue to be supplied to zones if the injector malfunctions.

Figure 1 – Single Injector

Figure 2 – Single Injector with Clear Water Line

Figure 2 shows how a second water line can be run in parallel to the line containing treated water for added redundancy. This would give the option for each zone, valve or bench to be watered with treated water or clear water. If the injector requires maintenance then clear water can continue to be used to supply the zones.

Multiple Solutions throughout Facility
If multiple solutions are needed throughout the facility then several factors such as budget, flexibility, redundancy, and ability to run new pipes must be considered. The more investment up front yields a greater degree of flexibility and automation. The figures below show different options for providing multiple solutions throughout a facility. The two main different strategies are (1) locating all the injection systems in one central location or (2) distributing them throughout the facility.

Multiple Systems Distributed Throughout Facility
Having multiple systems spread out through a facility can be a good way to provide different solutions to different crops or zones. This is a great strategy because it allows for a high degree of customization. Each system can be a different size to accommodate each zone’s special needs such as high or low flow rates, many or few solutions, or even different injector technologies. This technique also offers a high degree of redundancy. If a recipe needs to be changed, a smaller amount of the previous recipe is left in the water lines because the zones are closer to the injectors. Drawbacks include sometimes taking up space inside greenhouses or rooms, not being able to use one system for multiple zones without overly complicated plumbing and delivering, storing and mixing chemicals at several different sites. Access to systems is also more difficult to regulate. Figure 3 below shows an example of multiple systems throughout a facility.

Figure 3 – Multiple Distributed Injectors

Multiple Systems Centrally Located
There are many advantages to centrally locating injection systems. It is easier to install more flexible piping, fertilizer and chemicals are all located in one room, there is only one location to perform injector maintenance, it is easier to control access to systems and multiple zones can be supplied with one system. Downsides include the length of pipe between the injector and the zone and a lack of redundancy if only one system is responsible for an entire facility. Injectors must be sized for the maximum number of zones or flow rate that it could possibly experience. Figure 4 shows how irrigation systems can be plumbed when all injection systems are located in one area.

Figure 4 – Multiple Central Injectors

Anderson Aqua System Centrally Located
The Anderson Aqua system offers several advantages over other systems. One single Anderson Aqua can be used to provide any recipe to any zone. EC and pH are automatically adjusted and recipes can be tied to irrigation valves to automatically switch recipes. Since one single system can be used to supply many different recipes a simpler irrigation piping system can be used. Figure 6 below shows a typical direct piping situation for use with an Anderson Aqua.

Aqua Plumbing

Figure 6 – Anderson Aqua Recommended Plumbing

Anderson Aqua System with Batch Tanks
Batch tanks are intermediate reservoirs that are placed between the injection system and the zones that are filled with water and nutrients for a specific zone or crop. Batch tanks may be necessary under some circumstances. The main reasons batch tanks may be used is when growers “micro dose” or “micro pulse”. This means watering crops in extremely short intervals throughout the day in order to maintain optimal moisture content in the growing media. Watering in extremely short intervals creates problems when mixing nutrients real-time because the system does not have time to react to the new recipe that is being blended and lines and blend tanks don’t have enough time to “turn over”. This means the irrigation system and water lines will be filled with a recipe that was intended to be sent to a different zone. Batch tanks solve this problem by storing pre-mixed recipes that can be dedicated to a zone or crop. Batch tanks also allow the recipe to be double checked before sending the recipe to the crop. See the image below for a visual representation of how batch systems are typically setup.

Figure 7 – Anderson Aqua Typical Batch Tank Configuration