Don't Let The Lights Go Out On Your Dimming System

Your new fluorescent dimming system doesn't work the way you expected. You discover your ballasts are incompatible with your control system. No wonder you've been losing lamps left and right! Does this describe your situation? Just look around, and you can almost see "Install Dimmable Fluorescent Here" signs on every conference room, office, mall, and atrium. And that's just for starters. Everybody

Your new fluorescent dimming system doesn't work the way you expected. You discover your ballasts are incompatible with your control system. No wonder you've been losing lamps left and right! Does this describe your situation?

Just look around, and you can almost see "Install Dimmable Fluorescent Here" signs on every conference room, office, mall, and atrium. And that's just for starters. Everybody wants adjustable lighting. We want to create a mood one minute and read fine print the next. Many people want to augment usable daylight to save energy. This requires dimming, yet everyone wants the advantages of fluorescent. So the demand for dimmable fluorescent goes nowhere but up.

Meanwhile, we are seeing products with better cost-effectiveness, increased reliability, and increased availability. The new electronic dimming ballasts make daylight harvesting and energy management attractive options. Most installations use skylights or large window walls with a photosensor to dim lamps to desired levels. These applications can use the 10-0VDC electronic ballasts with photosensors and manual controls. Alternatively, they can use the 3-wire light level switching ballast where a photosensor signals relays to switch the ballast from full bright to full dim. So more specifiers and designers are taking the plunge into dimmable fluorescents.

When augmenting your artificial light with natural light, you reduce demand for artificial light. That's obvious, isn't it? From Fig. 1 (original article), you can see that reducing the light output (via dimming) gives a linear reduction in the input power required to power the light: to a certain point.

The 20% barrier. Traditionally, you dim a conference room via incandescent lamps and a wallbox control. Those with deeper pockets might opt for expensive control systems, which can dim down to 1%. Dimming to 20% is usually the best option. This level gives you enough light to take notes. Dimming to 20% during a presentation still allows you to view projected images comfortably. Going past the 20% threshold adds significant cost. But if that cost produces an energy savings, isn't it worth it? Good question. Let's find out.

When you think of fluorescents, you think of energy savings. When you think of dimming fluorescents, you think of even better energy savings. This is true to a point. A common misconception is the lower the light output, the lower the energy consumption. Notice the curve in Fig. 2 (on page 61, original article). You don't see a linear decrease in power to go with a decrease in light. In the 20% to 100% range, the curve is nearly linear. However, when you cross the 20% threshold, the curve loses any semblance to linearity. Every fraction of a lumen takes more power as you slide down the curve past the point of diminishing returns.

Types of electronic dimming ballasts. If you laid all the electronic dimming ballast designs one on top of the other, you could probably build a stack as high as the Sears Tower. Despite the variety, nearly all of these designs use one of four types of controls. They are 3-wire phase control, pulse width modulation (PWM), 10-0VDC, and 3-wire high-voltage switching.

Pulse width modulating is the least popular method of control because it costs more and isn't widely available. The advantage of PWM is the large number of ballasts you can wire on one control circuit.

The 3-wire phase control system allows you to dim fluorescent lamps down to 1%. This refers to the "architectural dimming" you may have heard someone talk about. The cost-effectiveness of this system is acceptable in many dimming applications. Unfortunately, it is prohibitive in many others. Furthermore, only a few companies manufacture the controls.

The 3-wire high-voltage switching (or "light level switching") is a low-cost alternative for dimming applications. The design calls for controlling the electronic ballast by switching two high-voltage (120V or 277V) input leads with wall switches or relays. This type of ballast gives the end user two or three levels of light output, with no expensive control panels or special control switches. In many applications, this is a great choice. Unfortunately, this dimming method lacks flexibility and loses its cost-effectiveness when you make changes.

The 10-0VDC system is cost-effective for most applications. The control system sends a low-voltage signal (10 VDC to 0VDC) to the ballast via two control input leads. The typical 10-0VDC electronic ballast can source or sink control current and gives you continuous dimming over a range of 100% to 20% light output. Some ballasts dim to 10% or lower, and you may need such ballasts for your application. Be aware, though, to get past the 20% threshold requires such trade-offs as lower ballast efficiency, higher input watts, and sub-par lamp performance. Plus, you really don't save any energy. So, in most applications you aren't going to want to dim below 20%. That does not mean you won't ever have a reason to do so. But your reason (using today's technology) will have nothing to do with energy savings or prolonging equipment life.

Proper dimming for lamp performance. The most critical trait of any dimming ballast is how it operates the lamp. Most systems coming into the market are for T8 lamps. With the Energy Policy Act of 1992 (EPACT) eliminating the F40T12/CW lamp, the T8 lamp becomes the logical choice for dimming applications.

Don't use the 34W, T12 energy-saving lamp for dimming applications. Unlike incandescent lamps, which have extended life through dimming, fluorescent lamps do not. It's easy to reduce fluorescent lamp life with dimming unless you know what you're doing. You maximize lamp life by controlling minimum lamp current, lamp starting, cathode voltage, and lamp current crest factor (LCCF) during all phases of the dimming process. See "What Is A Lamp Current Crest Factor," below, for details about LCCF.

Dimming ballasts lower the lamp current from approximately 200mA to 30mA. Some ballasts go as low as 10mA. The lamp current directly relates to the light output of the lamp; 190mA is 100% output and 30mA is 20% output. As the lamp ages or the environment changes (colder due to air conditioning), lamp instability problems can occur. With the lamp current too low, the lamp may begin swirling or striating and, if this continues, will quickly degrade lamp life.

Lamp starting is another performance factor. Dimming ballasts should be rapid start; that is, they should have voltage (3.0 to 4.5) at each cathode at starting and during operation. This is critical to maximum lamp performance during the dimming process.

Unlike a standard rapid start ballast, which has a constant cathode voltage, the dimming ballast should raise the cathode voltage as it lowers lamp current, typically from 3.0V to 5.0V (Fig. 3, on page 62 of original article). This keeps the cathode temperature constant as the lamp cools.

Another important feature is the ballast's ability to start the lamp at full current, independent of the controller setting, and then dim to the preset level. This assures correct lamp starting.

Other performance issues. Always take a close look at total harmonic distortion (THD), power factor (PF), ballast factor (BF), and energy management. THD should be lower than 20% through the entire dimming range. However, don't get lulled into the idea that less THD is better. You can tolerate a certain amount without ill effect.

The PF should be above 90% ("high power factor"), but cast a wary glance at 98% PF ballasts. Going beyond 95% PF requires trade-offs that can degrade your system.

When designing the installation alongside standard ballasts, be sure the BF is the same as the standard ballasts you use. This assures equal light levels between fixtures at full light output. Otherwise, you're going to have a system that just doesn't look right. The BF should be above 90% through the dimming range.

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