Plant Lighting: Pulse Start Technology Improves Metal Halide Performance
Important changes are occurring in metal halide lighting. New lamp-ballast systems have become available under the generic name pulse start. They offer significant improvements over previously available metal halide products.
By Sri Rahm
Important changes are occurring in metal halide lighting. New lamp-ballast systems have become available under the generic name pulse start. They offer significant improvements over previously available metal halide products. Nowhere is the impact as high as on indoor lighting in the commercial and industrial arena, because the wattages, 50-400 W, are exactly those needed for indoor spaces.
This article describes pulse start technology and its benefits, then examines the somewhat confusing jumble of lamp-ballast combinations called pulse start systems. Not all benefits are available with every combination, and some are more cost-effective than others.
Metal halide lamps are high intensity discharge (HID) devices that produce intense white light. Originally produced in the 1960s for stadium lighting and other high wattage applications, they are available today in sizes ranging from 32-2000 W and constitute the fastest growing segment of the lighting market. They are ideal for efficiently lighting large areas such as plant floors, high-bay areas, warehouses, and parking lots. Metal halide lamps offer excellent efficacies (lumens/watt), white light with excellent color rendering and sparkle, compact source size, and long life.
The light is produced by an electric arc confined within a quartz arc tube, together with certain metallic compounds. Metal halide lamps require a ballast to operate, as do other gas discharge lamps such as mercury vapor, fluorescent, high pressure sodium, and neon.
Most ballasts used to operate metal halide lamps were designed 2 to 3 decades ago, and the standard lamps have changed little over the last 25 yr. Designers have been constrained to only minor improvements because of the need for new lamps to operate on existing ballasts. At the same time, ballast engineers have been limited because their new designs needed to be compatible with existing lamps.
In the last few years the industry has turned to more of a systems approach. Lamps and ballasts have been conceived and designed together to take advantage of interactive features to provide more value to the end user.
This trend started with low wattage lamps that were introduced in the 1980s and early 1990s, but now all of the major lamp manufacturers offer higher wattage lamps (150-400 W) that incorporate this new pulse start technology. These systems offer attractive options for engineers designing lighting layouts for new spaces, but they are also worth considering as retrofits for existing fixtures.
To understand the story it is useful to construct it from the viewpoint of those who design the lamps and ballasts. Examination of design features and related benefits helps distinguish between the many combinations of lamp and ballast that have appeared in the marketplace, all calling themselves pulse start systems.
The pulse start story
Historically, metal halide lamp engineers had to perform a balancing act in specifying the pressure of argon gas used in the arc tubes. If the pressure is too low, tungsten coming off the electrodes at each start builds up into a light-absorbing film on the arc tube wall. At higher fill pressures, the buildup is reduced (leading to better long-term performance) but the lamps are harder to start.
As a result, lamp engineers specify as high a pressure as they can get away with while still making sure that the lamps start.
Reduced lamp degradation due to starts
Pulse start designers get around the starting problem by designing the ballast to produce a 3000-4000 V ignition pulse to initiate the electric discharge. Older ballasts do not produce such a pulse; instead they require a third electrode in the arc tube called a starter electrode. With the new ballasts, the electronic pulse starts arc tubes even if the argon pressure is 2 to 3 times higher than traditional arc tubes. This higher pressure results in less electrode deterioration with each start.
The new ballasts with the added ignition pulse are slightly more expensive, but these few dollars are well spent. At $0.10/kWh, a 400-W lamp costs $800 in electric power over the 20,000-hr life of the lamp. It's often overlooked that these operating costs are much higher than the initial costs of lamps and ballasts.
For the lamp just mentioned, even a 10% improvement in mean lumens translates to an $80 added value coming from saving in power costs over the life of the lamp. These savings can be realized by using 10% less fixtures or by using the same number of fixtures with lamps of 10% lower wattage (still providing the same lumens). With the best systems, it is possible to realize up to 20% improvements in mean lumens/watt.
A system designated "pulse start" must, as a minimum, meet two requirements: ballast with an ignitor pulse output, and arc tube with higher than traditional fill pressure.
Ballasts can have smoother waveforms
In the past, ballast engineers attacked the difficulties of starting even traditional metal halide lamps by designing ballasts with exaggerated peak voltages. These were sometimes referred to as peaked lead or constant wattage autotransformer (CWA) ballasts. The high peaks provided extra voltage to start the arc and allowed lamp manufacturers to use slightly higher fill pressures than in early lamps. (This did not eliminate the need for a starter electrode.)
Traditional metal halide ballasts in use today all have this peak; it is part of the ballast design and cannot be turned off. It also shows up in the current waveform during steady state operation, with two undesirable results: electrodes are damaged over the long term, causing lumen degradation; and the ballast contributes to power line harmonic problems.
With an ignitor pulse to start the lamp -- a pulse that shuts off once the lamp has started -- ballast engineers no longer have to create the exaggerated peaked waveform. The lower current crest factor then allows a smoother, more benign current waveform for the operation of the lamp and reduces lumen degradation.
Formed body arc tubes increase output
For years lamp engineers had the ability to design a highly efficient formed body arc tube that provided a 20% improvement in light output over traditional pinched body designs. However, this arc tube design had no starter electrode and would not work with existing ballasts. It needed a special ignitor pulse start system which only the ballast manufacturer could provide. Since such ballasts were not available in the marketplace, this improved lamp design stayed on engineers' desks.
Now that pulse start ballasts are generally available, several lamp manufacturers are providing these special high-output formed body arc tubes. In addition to more light output, these lamps also show improved color uniformity, quicker warmup, and faster restrike.
Ballast system options
Ballast selection requires particular attention. Any ballast with an electronic high-voltage pulse to start a metal halide lamp qualifies as a pulse start ballast. Some pulse start ballasts on the market still have the old, dirty waveform. These cheap pulse start ballasts merely have an ignitor pulse circuit added to a standard ballast (M-57, M-58, or M-59). Improvement over old-style systems is minimal, and even this is lost if standard lamps are used.
With pulse ignition, reactor or lag ballasts can be used (see "Ballast system comparison" table). They are much more gentle on the lamp electrodes and maximize lumen maintenance. Additionally, some reactor systems are almost twice as efficient as other ballasts. Reactor ballasts are usually designed to operate on 277 V without transformers that contribute to losses.
Another ballast parameter is regulation. This feature refers to how much lamp performance is affected by line voltage fluctuations. Although very efficient, reactor and lag ballasts offer poor regulation. If voltage fluctuations are a problem, regulated lag ballasts offer very good regulation, even though they are not as efficient as reactors.
In continuous burn applications (where the lamps are turned off only once a week), a pulse start system with a regulated lag ballast can extend the life of a 400-W lamp to 30,000 hr.
Many different combinations of ballast, ignitor, and arc tube can be used. A key performance indicator to look for is mean lumens over life, usually stated as a percent of initial lumens. (Note that performance improvements are compounded when both the initial lumens and the lumen maintenance are raised.)
New lamps are designed to the same light center lengths as the old, so the same fixtures and reflectors can be used. Changing to pulse start lamps requires that the old ballasts be replaced or modified. The best option is to take out the old ballasts and wire in new ones. Lag or reactor ballasts are most efficient and also provide the cleanest current waveform.
A simple cost-benefit analysis will usually come out way on the side of replacing the ballasts. Retrofitting requires a choice between two attractive options.
- Use the same wattage lamps as previously. Lamps with formed bodies yield 10% to 20% higher lumens than standard lamps.
- Use lower wattage lamps that provide the same light output as the previous ones, with corresponding energy savings.
It is also possible to merely add an ignitor pulse circuit to the old ballast and technically get a pulse start system. This approach gains some of the pulse start benefits, but retains the disadvantages of the older ballast.
There are real benefits available from metal halide lighting using pulse start systems. Many combinations of ballasts and lamps that qualify as pulse start systems are on the market. Based on user needs and ease of retrofit, it is possible to convert to systems which provide pulse start benefits.
It is important that specifiers ask these questions.
1. Does the lamp have higher initial lumens?
2. Does it have a higher argon fill pressure and therefore better maintained-percent lumens?
3. Does the ballast have a low current crest factor (gentler waveform)?
4. What are the ballast losses in this system? Is a low-loss ballast available?
5. If voltage fluctuations are a problem, how good is the regulation?
The answers to these questions help in evaluating systems being offered as "pulse start" and to avoid systems labeled "pulse start" just for some superficial benefits.
-- Edited by Gary Weidner, Senior Editor, 847-390-2689, email@example.com
The designation pulse start applies to many lamp-ballast combinations.
Products can differ considerably.
Ask the right questions when buying or specifying.
Pulse start benefits
- Higher initial lumens
- Less lumen degradation
- Improved color uniformity
- Faster warmup and restrike
- Improved ballast efficiency
- Energy saving possibilities
- Better ballast waveshape
Note: Any given product will not have every benefit.
The author is willing to answer technical questions concerning this article. Mr. Rahm is available at 440-248-0600, ext 2297; e-mail: sri_rahm@ adlt.com.