Efficiency is only the beginning

Efficiency is only the beginning

Piper, James

IN LESS THAN 20 YEARS, electronic ballasts have revolutionized the way fluorescent lighting systems are designed and operated. The electronic ballast has become the standard for both new construction and renovation projects. As sales have gone up, prices have come down, helping boost sales even higher. The result has been further ballast improvements and innovations.

Energy efficiency has been the key to the acceptance of electronic ballasts as a replacement for older electromagnetic ballasts. An electromagnetic ballast operates on 60 Hz alternating current: The arc that energizes the light-producing phosphors in a fluorescent lamp switches on and off 120 times per second. Because the phosphors generate light only when they are activated by the lamp’s arc, and because they rapidly dissipate energy from the arc, there are distinct periods each cycle when the lamp’s light output falls off.

Electronic ballasts operate at a much higher frequency, typically between 25,000 and 45,000 Hz. The times when the arc is off are short enough that the phosphors produce light for a longer period of time each second. As a result, the total light output of the lamp is greater.

Rather than produce more light, electronic ballast manufacturers reduce the power that the ballast-lamp system requires to produce the same quantity of light produced by an electromagnetic ballast.

Another advantage of electronic ballasts is that they have approximately onethird the internal energy losses of electromagnetic ballasts. This lower loss rate means improved efficiency and reduced heating loads.

Although electronic ballasts have been marketed for years, their current widespread use is the direct result of the passage 10 years ago of federal legislation that established minimum ballast efficiency standards. The legislation banned the manufacture of any ballast that failed to meet those standards.

While a few electromagnetic ballasts could meet the new standard, it became clear that the future largely belonged to the electronic ballast with its higher efficiency and lower losses.

Full-Range Dimming

In the 10 years since the legislation was enacted, electronic ballasts have continued to evolve. A good example is fullrange dimming. Dimming electronic ballasts are well-suited for use in applications where sunlight can supplement the fluorescent lighting system, multiple levels of lighting are required for different operations, and the lighting system is connected to a central energy management system.

Although some electromagnetic ballasts allowed a limited range of fluorescent lamp dimming, it took the development of the electronic ballast to make it practical and affordable. Even then, early generation dimming electronic ballasts could only be dimmed to between 15 and 45 percent of full light output – adequate for some applications but not for others. The new generation of electronic ballasts provides continuous dimming capabilities from 100 percent to as little as one-half of one percent of full light output, with little or no perceptible flicker.

What’s more, today’s dimming electronic ballasts are designed to accept a signal from a number of different sources, including manual controls, photocells and energy management systems. One application for the latter is to help control electrical demand, while photocells can be used in daylighting applications. With continuous dimming abilities, changes can be made automatically and smoothly to prevent disrupting operations.

Dimming ballasts have also become more energy efficient and generate less heat. Although the overall efficiency of the lighting system decreases as it is dimmed, the decrease in operating efficiency is more than compensated for by the reduction in system energy use. Additionally, today’s dimming electronic ballasts maintain a high power factor over the entire dimming range, typically varying between 0.95 and 0.99.

Thanks to these innovations, dimming electronic ballasts are rapidly becoming an important element in designing energy efficient lighting systems in both new construction and renovations.

Step-Dimming

Before low-cost, dimmable electronic ballasts became widely available, the most commonly used method for controlling the light output from fluorescent lamps was step-dimming. Unlike the dimmable ballasts whose output can be varied continuously, step-dimming ballasts produce only a limited number of light levels, typically 100, 75, 50, and 25 percent.

There is a new approach to step dimming that makes use of the existing wiring to transmit the dimming control signal from the controller to the connected ballasts. Use of the existing wire reduces the installation costs in retrofit projects. This new generation of step-dimmable ballast also accepts signals from a wide range of sources, including switches, photocells, occupancy sensors, and central energy management systems.

Step-dimming electronic ballasts are ideal for use in lighting retrofit applications that do not require full-range dimming. They can also be used with a cental energy management system to reduce lighting loads during periods of high electrical demand.

Compact Fluorescent Dimming

Compact fluorescent lamps have been in widespread use for more than 15 years. In spite of their energy efficiency and long rated life, compact fluorescents had a serious limitation: They could not be dimmed. The reason is that compact fluorescents were marketed with electromagnetic ballasts. The new generation of electronic ballasts for compact fluorescent lamps overcomes this limitation by providing a wide rage of dimming capabilities, allowing the compact fluorescent lights to be used in many dimming applications that currently use incandescent lamps controlled by a dimmer.

Electronic ballasts have also helped to improve the efficiency of these lamps, making them even more attractive as a replacement for the standard incandescent bulb.

Soft-Start Technology

One of the most recent developments in electronic ballast design is the development of a soft-start ballast. Each time that a fluorescent lamp is started, it is exposed to stresses that work to reduce lamp life. The more starts a lamp experiences, the shorter its life. Soft-start ballast technology reduces these stresses by ramping up the voltage and current to the lamp over a short period of time instead of suddenly applying full voltage and current. This soft start doubles the number of starts that a lamp can experience over its rated life, extending the time between relamping intervals. That means reduced lamp costs and – more significantly – labor savings.

Soft-start technology is particularly beneficial in applications where lamps are started frequently, such as in areas controlled by occupancy sensors.

Better Performance

In addition to the new features and capabilities, electronic ballasts also offer improved performance in four key areas: reliability, low total harmonic distortion, high power factor and reduced in-rush current.

Reliability. When electronic ballasts were first introduced, maintenance managers expressed concerns over their longterm reliability. Electronic devices, in their experience, were not very reliable and could be readily damaged by the transients commonly found in facility electrical systems. Their concerns were in part justified. While many ballast designs proved to be effective and reliable, some early designs were subject to failures.

The reliability of electronic ballasts is no longer a concern. Improvements in manufacturing and design have reduced the number of failures to an average of less than one percent after five years of operation. By comparison, magnetic ballasts have a five-year failure rate of 0.5 percent. Although the failure rate for electronic ballasts is still higher, it is well within an acceptable level for building equipment with an expected life of ten to fifteen years. In addition, the energy savings more than offsets any increased replacement costs.

Low Total Harmonic Distortion.

A concern with early electronic ballasts was how they modified the magnitude and wave shape of the power being supplied to them. This modification, known as harmonic distortion, is measured by the degree to which the current deviates from a perfect sinusoidal wave form. The measure is known as total harmonic distortion (THD). THD can cause motors, transformers and neutral conductors to overheat, fuses to blow, and computer equipment to malfunction, and can interfere with communication equipment. The greater the harmonic distortion, the greater the risk of problems.

As a result, many utilities required that electronic ballasts have a THD of less than 20 percent of the fundamental in order for them to qualify for rebates. While many earlier generation electronic ballasts met this requirement, some did not. Today, nearly every ballast manufactured meets the 20 percent THD requirement and most are well under it.

High Power Factor. Inductive loads, such as motors, transformers, and fluorescent ballasts, change the relationship between the current and the voltage in an electrical circuit, shifting them so that they are no longer in phase with each other. The greater the shift, the lower the system’s power factor. Low power factor causes capacity problems for both utility companies and their customers. To avoid these problems, utilities recommend that customers take steps to correct their facility’s power factor to be 0.90 or greater. Many utilities impose penalties when power factor falls below 0.85.

Low power factor was a concern with some early models of electronic ballasts. As a result, some utilities required electronic ballasts to have a power factor of at least 0.90 to qualify for a rebate. As a result, electronic ballasts now typically have power factor ratings between 0.95 and 0.90.

Limited In-rush Current All electronic power supplies, including those in electronic ballasts, cause a current spike when they are first turned on. This spike, known as in-rush current, is momentary but may reach as high as 100 times the steady-state current draw of the electronic device. Because it is so short in duration, the spike does not release a significant amount of energy and does not cause heating or other problems for the electronic device.

While the spike does not affect the operation of the electronic device, it can cause disruption in the power system, particularly if a large number of devices, such as electronic ballasts, are switched on at the same time. Their combined effect is often sufficient to cause the intermittent tripping of circuit breakers for no apparent reason. The spike can also cause damage to other equipment and components. For example, the high inrush current can burn and destroy the contacts in the switch used to operate the lighting system.

To limit the impact of high in-rush current, some manufacturers are offering electronic ballasts with a built-in circuit designed to limit the in-rush current to one-fourth to one-tenth the in-rush current generated by a standard electronic ballast. At least one manufacturer is offering a ballast that delays the timing of the in-rush current until after the switch or operating control contacts have closed, to help protect them from damage due to arcing and burning.

Ballasts that limit or delay in-rush currents are well suited for applications having large lighting systems controlled by a single switch or lighting control. Similarly, the low in-rush ballasts can minimize contact burning and arcing in applications where a group of fluorescent fixtures are switched on and off several times per hour, such as in areas where the lighting in controlled by occupancy sensors, photocells, or by a central energy management system.

Once considered an expensive and risky alternative to the standard magnetic ballast, electronic ballasts have become the standard by reducing lighting system energy requirements and by improving the flexibility and capabilities of the systems in which they operate. As the market continues to grow, look for ballast manufacturers to continue the evolutionary process, further enhancing the performance and capabilities of their ballasts.

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James Piper is a consultant and writer with 25 years of experience in the facilities field.

E-mail questions to edward.sullivan@trades.com.

Copyright Trade Press Publishing Company Nov 1998

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