Fluorescent Ballasts
All gas discharge lamps, including fluorescent lamps, require a ballast to operate. The ballast provides a high initial voltage to initiate the discharge, then rapidly limits the lamp current to safely sustain the discharge. Lamp manufacturers specify lamp electrical input characteristics (lamp current, starting voltage, current crest factor, etc.) required to achieve rated lamp life and lumen output specifications. Similarly, the American National Standards Institute (ANSI) publishes recommended lamp input specifications for all ANSI type lamps. Ballasts are designed to optimally operate a unique lamp type; however, some ballasts will adequately operate more than one type of lamp. In these cases, optimum lamp performance is generally not achieved under all conditions. Less than optimum conditions may affect the lamp’s starting characteristics, light output, and operating life.
Circuit Type and Operating Mode
Fluorescent ballasts are manufactured for three primary types of fluorescent lamps: preheat, rapid start, and instant start.
Preheat Operation Lamp electrodes are heated prior to initiating the discharge. A ‘starter switch’ closes, permitting a current to flow through each electrode. The starter switch rapidly cools down, opening the switch, and triggering the supply voltage across the arc tube, initiating the discharge. No auxiliary power is applied across the electrodes during operation.
Rapid Start Operation Lamp electrodes are heated prior to and during operation. The ballast transformers has two special secondary windings to provide the proper low voltage to the electrodes.
Instant Start Operation Lamp electrodes are not heated prior to operation. Ballasts for instant start lamps are designed to provide a relatively high starting voltage (with respect to preheat and rapid start lamps) to initiate the discharge across the unheated electrodes.
Rapid start is the most popular mode of operation for 4-foot 40 watt lamps and high output 8-foot lamps. The advantages of rapid start operation include smooth starting, long life, and dimming capabilities. Lamps of less than 30 watts are generally operated in the preheat mode. Lamps operated in this mode are more efficient than the rapid start mode as separate power is not required to continuously heat the electrodes. However, these lamps tend to flicker during starting and have a shorter lamp life. Eight-foot ‘slimline’ lamps are operated in instant start mode. Instant start operation is more efficient than rapid start, but as in preheat operation, lamp life is shorter. The 4-foot 32 watt F32T8 lamp is a rapid start lamp commonly operated in instant start mode with electronic high-frequency ballasts. In this mode of operation lamp efficacy is improved with some penalty in lamp life.
Energy Efficiency
Fluorescent lamps are reasonably efficient at converting input power to light. Nevertheless, much of the power supplied into a fluorescent lamp-ballast system produces waste heat energy.
There are three primary means of to improving the efficiency of a fluorescent lamp-ballast system:
- Reduce the ballast losses
- Operate the lamp(s) at a high frequency
- Reduce losses attributable to the lamp electrodes
Newer, more energy-efficient ballasts, both magnetic and electronic, exploit one or more of these techniques to improve lamp-ballast system efficacy, measured in lumens per watt. The losses in magnetic ballasts have been reduced by substituting copper conductors for aluminum and by using higher grade magnetic components. Ballast losses may also be reduced by using a single ballast to drive three or four lamps, instead of only one or two. Careful circuit design increases efficiency of electronic ballasts. In addition, electronic ballasts, which convert the 60 Hz supply frequency to high frequency, operate fluorescent lamps more efficiently than is possible at 60 Hz. Finally, in rapid start circuits, some magnetic ballasts improve efficacy by removing power to the lamp electrodes after starting.
Ballast Factor
One of the most important ballast parameters for the lighting designer/engineer is the ballast factor. The ballast factor is needed to determine the light output for a particular lamp-ballast system. Ballast factor is a measure of the actual lumen output for a specific lamp-ballast system relative to the rated lumen output measured with a reference ballast under ANSI test conditions (open air at 25 degrees C [77 degrees F]). An ANSI ballast for standard 40-watt F40T12 lamps requires a ballast factor of 0.95; the same ballast has a ballast factor of 0.87 for 34-watt energy saving F40T12 lamps. However, many ballasts are available with either high (conforming to the ANSI specifications) or low ballast factors (70 to 75%). It is important to note that the ballast factor value is not simply a characteristic of the ballast, but of the lamp-ballast system. Ballasts that can operate more than one type of lamp (e.g., the 40-watt F40 ballast can operate either 40-watt F40T12, 34-watt F40T12, or 40-watt F40T10 lamps) will generally have a different ballast factor for each combination (e.g., 95%, <95%, and >95%, respectively).
Ballast factor is not a measure of energy efficiency. Although a lower ballast factor reduces lamp lumen output, it also consumes proportionally less input power. As such, careful selection of a lamp-ballast system with a specific ballast factor allows designers to better minimize energy use by “tuning” the lighting levels in the space. For example, in new construction, high ballast factors are generally best, since fewer luminaires will be required to meet the light level requirements. In retrofit applications or in areas with less critical visual tasks, such as aisles and hallways, lower ballast factor ballasts may be more appropriate.
To avoid a drastic reduction in lamp life low ballast factor ballasts (<70%) should operate lamps in rapid start mode only. This is particularly relevant for 32-watt F32T8 lamps operated at high frequency.
Finding the ballast factor for lamp-ballast combinations may not be easy, as few ballast manufacturers provide this information in their catalogs. However, if the input power for a particular lamp-ballast system is known (usually found in catalogs) an estimate of the ballast factor is possible.
Flicker
Electromagnetic ballasts are designed to condition the 60 Hz input voltage to the electrical requirements of the lamps. A magnetic ballast alters the voltage, but not the frequency. Thus, the lamp voltage crosses zero 120 times each second, resulting in 120 Hz light output oscillations. This results in about 30% flicker for standard halophosphor lamps, operated at 60 Hz. The flicker is generally not noticeable but there is evidence that flicker of this magnitude can cause adverse effects, such as eyestrain and headache.
Most electronic ballasts, on the other hand, use high-frequency operation, which reduces lamp flicker to an essentially imperceptible level. The flicker percentage of a particular ballast is usually specified by the manufacturer. For a given ballast, the percent flicker will be a function of lamp type and phosphor composition.
Audible Noise
One characteristic of iron-cored electromagnetic ballasts operating at 60 Hz, is the generation of audible noise. Noise can be increased by high temperatures, and it is amplified by certain luminaire designs. The best ballasts use high quality materials and workmanship to reduce noise. Noise is rated A, B, C, or D in decreasing order of preference. An “A” rated ballast will hum softly; a “D” rated ballast will make a loud buzz. The number of ballasts, their sound rating, and the nature of ambient noise in the room determine whether or not a system will create an audible disturbance.
Virtually all energy-efficient magnetic ballasts for F40T12 and F32T8 lamps are “A” rated, with a few exceptions, such as low temperature ballasts. Still, the hum of magnetic ballasts may be perceptible in a particularly quiet environment such as a library. Well-designed electronic high-frequency ballasts, on the other hand, should emit no perceptible hum. All electronic ballasts are “A” rated for sound.
Dimming
Unlike incandescent lamps, fluorescent lamps cannot be properly dimmed with a simple wallbox device such as those used for incandescent lamps. For a fluorescent lamp to be dimmed over a full range without a reduction in lamp life, its electrode heater voltages must be maintained while the lamp arc current is reduced. As such, lamps operated in rapid start mode are the only fluorescent lamps suitable for wide-range dimming applications. The power required to keep electrode voltage constant over all dimming conditions means that dimming ballasts will be less efficient when operating lamps at dimmed levels.
Dimming ballasts are available in both magnetic and electronic versions, but there are distinct advantages to using electronic dimming ballasts. To dim lamps, magnetic dimming ballasts require control gear containing expensive high power switching devices that condition the input power delivered to the ballasts. This is economically viable only when controlling large numbers of ballasts on the same branch circuit. In addition, luminaires must be controlled in large zones that are determined by the layout of the electrical distribution system. Since the distribution system is fixed early in the design process, control systems using magnetic dimming ballasts are inflexible and are unable to accommodate changes in usage patterns.
Dimming of electronically-ballasted lamps, on the other hand, is accomplished within the ballast itself. Electronic ballasts alter the output power to the lamps by a low-voltage signal into the output circuit. High power switching devices to condition the input power is not required. This allows control of one or more ballasts independent of the electrical distribution system. With dimming electronic ballast systems, a low voltage control network can be used to group ballasts together into arbitrarily-sized control zones. This control network may be added during a building renovation or even, in some circumstances, during a lighting retrofit. Low voltage wiring does not have to be run in conduit, which helps keep installation costs down. In addition, it is less costly to modify the size and extent of lighting zones by reconfiguring low voltage wiring when usage patterns change. Low voltage wiring is also compatible with photocells, occupant sensors, and energy management system (EMS) inputs.
Dimming range differs greatly among ballasts. With most electronic dimming ballasts, light levels can vary between full output and a minimum of about 10% of full output. However, electronic, full-range dimming ballasts are also available that operate lamps down to 1% of full lumen output. Magnetic dimming ballasts also offer many dimming options, including full-range dimming.
Adapted from the Advanced Lighting Guidelines: 1993 (Second Edition), originally published by the California Energy Commission.