When considering energy-efficient lighting systems, facility executives can select from a wide range of options, including fluorescent lamps, compact fluorescent lamps, electronic ballasts, efficient fixture designs and automatic lighting controls. While each of these components is important to the overall operating efficiency of the system, none is more important than the ballast selected. Selecting the proper ballast can make the difference between a lighting system that is simply more efficient than the one it replaced, or one that is highly energy efficient.
All lighting system upgrades today involve replacement of the ballasts. In most cases, the upgrade involves moving from the older magnetic ballast to one of the newer electronic ballasts. Electronic ballasts offer improved operating efficiency over magnetic ballasts, reducing ballast energy requirements by an average of 35 percent. But not all electronic ballasts are created equal. Manufacturers have developed a number of different ballast designs with different characteristics for use in different applications. There are five major ways in which ballast selection will impact the energy performance of the lighting system. To achieve optimal performance from the new lighting system, ballasts must be carefully selected to match the needs of the space.
- Electronic Ballast Efficiency
Simply stated, electronic ballasts are significantly more energy efficient than their magnetic counterparts. As a result, the manufacture of magnetic ballasts will most likely be curtailed for general purpose lighting applications.The improvement in energy efficiency achieved by electronic ballasts comes as a result of several factors, including lower internal losses and higher operating frequencies. Magnetic ballasts suffer from core losses that are inherent in the design and operation of the ballast. Electronic ballasts operate without the need for windings and magnetic fields, thus eliminating magnetic losses completely.
Electronic ballasts also operate at a higher frequency than magnetic ballasts. Magnetic ballasts operate at line frequency, 60 Hz. This means that the arc that excites the phosphors on the inside surface of the fluorescent tube is triggered 60 times each second. Electronic ballasts operate at frequencies ranging from 20 to 60 kHz. At these higher operating frequencies, the arc excites the phosphors for a longer period of time, resulting in the generation of 10 to 15 percent more light for the same energy use.
Operating at a higher frequency also eliminates one of the most common complaints associated with fluorescent lamps: flicker. With the arc being triggered at 60 times per second with magnetic ballasts, some people can detect a flicker in the light output of the lamps. Electronic ballasts, with their higher operating frequency, completely eliminate this flicker. While the lamps are still switching on and off with the arc, the higher operating frequency makes it impossible for the eye to detect.
- Ballast Factor
One of the most frequently overlooked yet important items when selecting lighting system components is the ballast factor. The ballast factor is a measurement that rates the relative light output of a ballast with respect to the light output of the same lamp operated by a reference ballast. The higher the ballast factor, the higher the light output of the lamps. The lower the ballast factor, the lower the light output of the lamp and the lower the energy use of the system. The ballast factor is dependent on both the ballast selected and the lamp installed.The ballast factor of the units selected for a particular installation will significantly impact the total light output and energy use of the lighting system. While magnetic ballasts were available in a narrow range of ballast factors, between 0.925 and 0.975, electronic ballasts are available in a wide range of ballast factors — from as low as 0.73 to as high as 1.2. This allows facility executives to select ballasts that provide a wide range of light output.
Consider a lighting system upgrade where fixtures with magnetic ballasts and T-12 lamps are replaced on a one-for-one basis. If the lighting levels in the existing system were adequate, simply replacing the fixtures with ones using electronic ballasts with a ballast factor of 1.0 and T-8 lamps would reduce energy use, but it would result in an increase in lighting levels. Selecting a ballast with a ballast factor of 0.88 would reduce lighting system energy use even further while maintaining the original lighting levels within the space. A higher ballast factor would increase overall lighting levels, while a lower ballast factor would reduce lighting levels and energy use.
It should be noted that ballast factor can impact lamp life. High ballast factors generally shorten lamp life and accelerate lamp lumen depreciation. In system designs, these factors must be evaluated against the cost of having to alter the layout of the lighting fixtures.
By selecting a ballast with the proper ballast factor rating for the lamps that will be used allows facility executives to control both light output and lighting system energy use.
- Dimming
With magnetic ballasts, dimming was an option, but it was not cost effective in most applications. Dimming controllers for magnetic ballasts were bulky, expensive and not very effective. Most systems could not reduce light output much below 50 percent of full brightness. Dimming tended to increase the problems associated with lamp flicker. Many systems required lamps to be started at full brightness, then dimmed to the desired level. As a result, most applications that required dimming yet wanted the efficiency of fluorescent lamp operation installed two separate lighting systems — one fluorescent and one incandescent.Dimming electronic ballasts have eliminated the problems associated with trying to dim magnetic ballasts. Controllers are small and inexpensive. Light output can be controlled to as low as 5 or 10 percent of full brightness. Flicker at low light levels is eliminated due to the high operating frequency of the ballasts. Costing approximately 35 to 50 percent more than conventional electronic ballasts, this new generation of electronic ballasts has made fluorescent lamp dimming practical, efficient and affordable.
Most dimming electronic ballasts are controlled by a 0-10 volt DC signal from the controller. Some models make use of AC line-phase controller signals, eliminating the need for special control wiring.
Dimming electronic ballasts can be used in any application where dimming is desirable, such as in meeting rooms. While use of dimming ballasts in these applications will save energy by eliminating the need for a separate incandescent system, the real potential for energy savings comes from applications that can make use of natural light.
In spaces with even moderate areas of exterior glass, natural light is available for use in lighting the space. To be used effectively though, there must be a way to automatically sense the total light level in the space and adjust the light output of the fluorescent fixtures as the amount of natural light varies. Dimming electronic ballasts give facility executives that capability.
The typical dimming system uses a sensor to measure the overall lighting level in the space. The sensor, connected to the dimming controller, regulates the light output of the fluorescent fixtures to keep the total light levels constant. A single controller can be connected to all dimming ballasts, or multiple controllers can be connected to individual banks of ballasts based on their distance from the glass. The result will be a decrease in lighting energy requirements of 35 to 75 percent for the typical office application.
- Rapid-Start vs. Instant-Start
Electronic ballasts, like the older magnetic ballasts, are available in two basic types: rapid-start and instant-start. Selection of the ballast type impacts overall energy use because of differences in the operation of the ballasts.All fluorescent lamps need a means of starting the lamps as well as regulating their operation once started. Rapid-start lamps have two circuits. One circuit controls normal operation of the lamp, and the second provides a low voltage to the lamp’s electrodes. This low voltage heats the electrodes to between 70 and 100 degrees Celsius in a few seconds, allowing the lamp to light. Once lit, most rapid-start ballasts continue to supply the voltage to the electrodes, resulting in an energy use of two to three watts per 40 watt lamp.
Instant-start ballasts do not heat the lamp’s electrodes to start them. Instead, they temporarily apply a high voltage to the lamp that strikes the arc. Once the lamp is lit, the high voltage is turned off. As a result, energy requirements are reduced. This improvement in energy performance comes with a penalty. The use of high voltage to start the arc accelerates erosion of the emissive coating on the lamp’s cathode, decreasing lamp life. Compared to rapid-start ballasts, instant-start ballasts have a lamp life that is approximately 25 percent shorter. The more frequently the lamp is started, the shorter the lamp life.
The decision to use a rapid-start or instant-start ballast is usually made based on the application. Applications that are switched more frequently, such as those in areas controlled by occupancy sensors, are better suited for rapid-start ballasts. Applications where lights burn for longer times per start can benefit from the energy savings produced by instant-start ballasts, without significantly reducing relamping intervals.
A third type of electronic ballast is becoming more common: the programmed-start ballast. Programmed-start ballasts offer the energy efficiency of instant-start ballasts without sacrificing lamp life.
Programmed-start ballasts apply a low voltage to the lamp’s electrodes for starting, like instant-start ballasts. Unlike instant-start ballasts, programmed-start ballasts turn off that voltage once the lamp is lit. By turning off the voltage, these ballasts achieve the energy efficiency of instant-start ballasts, while providing the lamp life of rapid-start ballasts. While programmed-start ballasts are relatively new, they are projected to replace most rapid-start units.
- Power Factor
Power factor is the ratio of active power in a circuit to apparent power. In purely resistive loads, the power factor in the circuit is 1.0. But as electrical loads are added to the circuit that are inductive, such as those from induction motors and ballasts, there is a phase shift in the circuit between the current and the voltage, and the power factor decreases. In most commercial and institutional facilities, the overall power factor is typically between 0.85 and 0.97. In industrial applications, particularly those having a large number of induction motors, it can fall to 0.75 or lower.Power factor impacts the current-carrying capacity of a circuit and the cost of electricity. Depending on the rate structure in effect for the facility, low power factor can carry a significant penalty, costing facilities thousands of dollars each month.
Power factor is important when considering electronic ballasts because they can cause reductions in overall power factor by distorting the current wave shape. These distortions are measured by the unit’s total harmonic distortion (THD). The higher the THD, the greater the ballast’s impact on power factor. Correcting this distortion is difficult and expensive. Although the load drawn by a single electronic ballast is very low, given the number of ballasts that may be installed in a particular facility, their overall impact on system power factor can be significant and costly.
Electronic ballasts that limit THD and therefore the impact on power factor tend to cost more than those with a higher THD, but their additional first costs can be quickly recovered, particularly in applications with a high-cost penalty for low power factor and a high lighting load. High THD generated by electronic ballasts and other sources can also cause problems within other electrical equipment in the facilities, such as the overheating of induction motors and interference in office equipment.
For most applications, limiting electronic ballast THD to values of less than 20 percent will avoid problems with other equipment and low power factor. Electronic ballasts are available with THD ratings of below 10 percent for specialized applications.
