AC fan-driven cleanroom fan filter units benefit from TRIAC and network control
10/01/2006
AC control options and networked solutions offer advantages and cost savings
By J. Fishbein, AirCare Automation Inc.
Cleanrooms utilizing fan filter units (FFUs) have experienced rapid growth. Enabling this growth has been the flexibility to provide controls to clean environments, from minienvironments as small as workbenches to ballroom-sized clean spaces. Higher purity work areas are also being incorporated within larger cleanrooms, adding further complexity to controls and systems implementation.
The evolution of low-cost AC fan controls has improved the performance and reduced the cost of AC fan-driven FFU systems. This article will identify key AC control options and trade-offs, as well as additional advantages and cost savings that are realized by incorporating “smart” networked solutions.
AC control options
When it comes to controlling cleanroom AC FFU fan speeds, there are a number of approaches available including: powering the motor directly off the AC line, autotransformer control, inverter drive control, “dimmer-switch type” 2- or 3-wire TRIAC control, and networked 2- and 3-wire TRIAC control.
The most basic control is powering directly off the AC line, where the control is based on the motor being fully on (high speed) or incorporating a multitap winding together with a switch to enable selection of a discrete high or low, or a high/medium or low, setting. Another simple approach is to use autotransformers to adjust the motor speeds.
In terms of more classical electronic control, inverter control and TRIAC control techniques are most commonly incorporated. Inverter control, or variable frequency drive (VFD), provides a true frequency control solution allowing “overdrive” and is usually sophisticated enough to be included in a networked solution. TRIAC control, which makes use of a phase control technique, can be implemented through either a manual dimmer-switch approach where the switch controls can be wall mounted, or through remote controller modules that can be mounted anywhere, but are usually located near the motor. Adjustment of the remote controller modules can be accomplished via a speedpot or can be operated in a closed-loop mode. Smart controller modules can also be integrated into a networked addressable system.
AC control approach attributes
Powering the motor directly from the AC line provides the lowest initial cost solution and allows for FFU operation at full speed. It may also allow for predetermined lower speed or speeds if the FFU contains tapped motor windings and a switch. With the controls on the FFU housing, however, adjustments to the FFU often require breaching the cleanroom ceiling. As a result, the FFU motors are often simply set to full speed, which unnecessarily increases energy cost and reduces filter life.
Autotransformer control is typically placed on the AC breaker line to vary the voltage to a bank of FFUs. However, although the use of an autotransformer eliminates the low frequency harmonics, it also adjusts all FFUs on the line to the same speed, which can prove expensive if used with too few FFUs.
The use of an inverter VFD control both eliminates the low frequency harmonics and reduces FFU power consumption. And, as will be shown later, this approach provides reduced power consumption compared to TRIAC control, at least at lower speeds. However, special consideration must be given to radio frequency interference (RFI) and power factor (PF), as VFD controls can create problems in these areas. Additionally, inverter drives require the use of more expensive inverter-grade motors to eliminate pitting of the motor bearings and motor insulation breakdown. Thus far, inverter drives have not experienced broad usage in cleanroom applications.
Dimmer-switch type 2- or 3-wire TRIAC control, similar to that used for lighting control, provides wall-mountable variable-speed control capability for FFUs. The 3-wire TRIAC control, as will be explained in more detail later, provides improved efficiency, as well as reduced harmonics and mechanical noise, or hum.
Networkable 2- and 3-wire TRIAC control units can incorporate a soft-start feature to limit high motor currents at start-up and, when operated in 3-wire mode, provide the same advantages as dimmer-switch control. Speed control can be achieved individually by connecting and varying a 10kΩ potentiometer, or the controllers can be operated individually in a closed-loop mode using a reference and a differential input to maintain variables such as pressure, temperature or airflow. It is advantageous to maintain a constant pressure in workstations, such as laminar-flow stations within cleanrooms, where doors can be opened during operation, resulting in pressure drop. Likewise, new developments aimed at maintaining constant airflow can have major ramifications in helping to maintain cleanroom integrity over changes in such variables as line voltage and the condition of the filter media.1
Network architecture
Smart controllers can also be networked using a single console to access each controller (see Fig. 1) The controllers are addressable FFU TRIAC drivers that control the FFU motors. The primary function of the console is to provide system control by communicating with all FFU controllers to set speed and monitor performance. Networking through a console also allows for a global setback (reduced speed) to further reduce energy consumption during nonwork times.
The network system console can also be used in closed-loop systems to provide the reference input with a digital readout, eliminating the need for a separate reference at each controller, and may incorporate other features such as shutdown or alarm in an emergency situation such as a fire or release of a toxic gas. A network solution can be implemented via a fully programmed console, a programmable logic controller (PLC) console, or a computer.
Technical advantages of TRIAC controls
The only two types of control that can be categorized as electronic controls are the inverter and TRIAC controls. Both control topologies can be integrated into network systems and the inverter drive can provide overdrive capability and additional efficiency improvements, at least at the lower FFU fan speeds (see Fig. 2). However, the higher cost of the inverter controls and the inverter-grade motors has thus far greatly limited their usage in AC cleanrooms.
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TRIAC type control, both wall-mountable, dimmer-switch TRIAC controls and the smarter networkable module type, provide more cost-effective AC control. Both can also provide 2- or 3-wire capability when connecting to the permanent split capacitor (PSC) motor commonly used in AC cleanrooms. The 2- and 3-wire wiring connections are shown in Figures 3 and 4, respectively.
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Implementing the 3-wire connection requires splicing the additional connection to the motor capacitor, thereby providing a separate drive to the auxiliary winding of the motor. For the 2-wire connection, both the main winding and the auxiliary winding are driven from the same source. Incorporating the 3-wire approach, whether with the dimmer switch or the networkable module approach, reduces power consumption, harmonics and noise. Figure 5 compares the harmonic content of a motor operating in the 2- and 3-wire modes. Of particular interest is the reduction in the third harmonic, which contains, by far, the highest amount of harmonic content.
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Over the usable speed range of about 60 to greater than 90 percent of full speed, the 3-wire approach reduces overheating, which is prevalent in many PSC motors, by reducing the motor power consumption by 10 to 20 percent typically (see Fig. 6). It is important to not run the motor at too low a speed because of the risk of stalling, at which point the motor will tend to draw more current and can be damaged by excess heat. Smart controllers contain a minimum speed setting to prevent operating the motor at too low a speed.
Networked TRIAC control advantages
Traditionally, network solutions for cleanrooms have been used in higher-end cleanrooms that use programmed, DC brushless motors to drive the FFU fans. Networking is accomplished either via computer control as part of a building management system or through the use of a programmed PLC unit. Also included are interface control boards that allow the user to vary fan motor speeds remotely, making changes by adjusting up or down their preprogrammed speed and torque curve. More recently, menu-driven, lower-cost, easy-to-install, stand-alone consoles with displays have been added to the tools to adjust the smart DC motors.
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These consoles also mate with some inverter drives as well as smart TRIAC controllers for use in network solutions in more cost-sensitive AC cleanrooms. TRIAC control of fan motor speed is accomplished via a phase control technique, which essentially is turning off the voltage to the motor for a portion of the AC cycle (in the U.S., there are 60 cycles per second). However, as shown in Figure 7, the phase angle varies nonlinearly over the cycle. This means that, when adjusting the dimmer-switch type TRIAC control, there can be a large amount of adjustment of the control with very little effect on FFU fan speed over part of the cycle, followed by large speed changes with smaller adjustments of the dimmer switch. This phenomenon complicates adjusting the FFU fan speed.
Figure 7 also illustrates that using a smart controller facilitates achieving linear speed control and, by incorporating a network solution, provides a visual display of the speed and ease of adjustment. It also allows for adjusting all fans from a single convenient location, as opposed to performing the adjustments at each FFU. Often, when smart control is not used, the FFU fans are simply set to maximum speed, which certainly is more costly in terms of energy consumption. Operating at full speed also reduces the FFU filter media life.
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To illustrate the energy cost savings that can be realized, two examples have been included in Tables 1 and 2. Both examples are for a 50 FFU cleanroom, comparing a 2-wire vs. a 3-wire system to a reference system being operated at full speed on a 24/7 basis.
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The assumptions have been taken from an actual application that was field retrofitted. The system in Table 1 included TRIAC controllers, but was not in a network solution and thus did not take advantage of the setback mode during nonwork time. If this system had been implemented using the 3-wire approach, the annual energy savings would have been over $3,200/yr, double the savings of the 2-wire approach.
The energy-savings results for the actual system, which is a networked, 3-wire control solution utilizing setback, are documented in Table 2. The network solution provided annual energy savings of over $5,500/yr, or about 70 percent more than if the non-network, 3-wire approach had been implemented. These annual energy savings are realized year after year for the life of the cleanroom.
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In addition to the energy cost savings, there are also installation cost savings that can be realized. For example, the 3-wire approach reduces the current draw at speeds less than full speed (see Fig. 8). At reduced speeds, the 2-wire approach results in elevated currents even though, as was shown in Fig. 6, the power consumption does decrease. The reduced currents with the 3-wire topology, along with reduced start-up currents (if the smart controllers contain a soft-start capability, which can limit peak start-up currents by up to about 50 percent), can result in being able to run more FFUs per circuit breaker, decreasing the number of circuit breakers required. There is an installation cost of some hundreds of dollars per breaker line installed. Thus, the cost savings will depend on the number of breaker lines that can be eliminated. Additionally, going to a network system will certainly reduce balancing costs by so many minutes per FFU. The reduction of circuit breaker lines installed and balance of time saved will vary.2
Conclusion
Despite all the advantages cited in this article, there remains one major obstacle to more broad-based implementation of network controls in AC cleanrooms: the ongoing short-term (and maybe shortsighted) requirement from the end customer for the lowest initial cost cleanroom. Cleanroom suppliers are often pushed for lowest cleanroom cost and are often competing for the job based upon that cost. Also, some larger companies utilize different personnel or departments for capital acquisitions, such as cleanrooms, and for managing operating costs. These, coupled with the fact that network control providers have been remiss in quantifying cost savings, have all been impediments to implementation. More recently, however, as this information is being distributed, some companies are even performing field retrofits to achieve the benefits, even though field retrofits are more expensive than including network control initially.
References
1. Abramowitz, H. “New Closed Loop Controls For Minienvironments,” Controlled Environments, February 2006.
2. Schwartz, J., et al. “Utilizing Lower-Cost ‘Smart’ FFU Systems,” Controlled Environments, November 2005.
Jeff Fishbein holds a BSEE and has over twenty years of experience in power electronics engineering, most of which was spent at Bell Laboratories and International Rectifier. He has held a number of positions in design and applications engineering, as well as in engineering management. For the past three years, he has been working with AirCare Automation (www.aircareautomation.com) as an applications engineering manager.