Up until a few years ago, magnetic ballasts were the only option for HID light sources, and they are still the type that is most often used. The most common type of magnetic ballast sold today is the constant-wattage autotransformer, which effectively regulates lamp power to minimize flicker and unexpected shutoffs if the line voltage varies. Composed mainly of magnetic coils, these ballasts often include a capacitor in the circuitry to improve power factor. Magnetic ballasts generally have internal losses of at least 10 percent of the lamp wattage—and the percentage is even higher for lower-wattage lamps. Manufacturers produce magnetic ballasts capable of working with lamps of up to 1,500 watts (W).
Electronic ballasts are a relatively new offering, and they are now available for lamps of up to 1,000 W. They use switching electronics and small high-frequency inductors, rather than large line-frequency ones, to control current and voltage to the lamp. Though costly, electronic ballasts offer a host of benefits over their magnetic counterparts, including higher efficiency, better dimming capabilities, better light quality, and shorter warm-up times.
Efficiency. Electronic ballasts for HID lamps consume about 60 percent less power than their magnetic equivalents—a typical electronic ballast for a 400-W lamp uses 15 to 30 W, compared with 50 to 60 W for a magnetic unit.
Light output. Metal halide lamps are notorious for the color variability of their light output, but lamps operated by electronic metal halide ballasts provide more stable output than lamps operated by magnetic ballasts. That’s because the electronic ballasts reduce the variability of the voltage supplied to the lamp.
The light output of lamps driven by electronic ballasts also degrades more slowly over time, resulting in greater light output at the mean and end of the lamp’s life (see Figure 1). This in turn enables systems with electronic ballasts to use fewer fixtures, or lower-wattage lamps, to provide the same output as systems with magnetic ballasts.
Figure 1: Electronic ballasts improve lumen maintenance
Lamp output degrades more slowly with electronic ballasts than with magnetic ballasts. Output declines more rapidly on probe-start metal halide lamps operating on magnetic ballasts.
Dimming capability. Electronic ballasts can be made continuously dimmable, down to about 50 percent of full output power. This characteristic makes them more amenable to daylight harvesting than magnetic ballasts, which, at best, offer step-dimming down to one or two lower levels. Step-dimming to 50 percent of full power is common in warehouses and other irregularly occupied building spaces. Both electronic and magnetic HID ballasts lose efficiency at a similar rate as they are dimmed, so that when power has been reduced to 50 percent of full power, the lamps put out only about 35 percent of maximum light output. However, dimming any HID lamp, even with an electronic ballast, may lead to significant color-shifting, seriously limiting this capability in areas where color is of concern.
Warm-up and restrike times. Metal halide lamps take several minutes to warm up to full output and several minutes to cool down and restart once they go out (either intentionally or due to a power failure). With electronic ballasts, warm-up times are significantly shortened for both quartz and ceramic lamps, but restrike times (the time it takes for a lamp to cool down and restart after a momentary outage) are only shortened for quartz lamps (see Table 1). None of the times is shortened to the point where metal halide lamps can be used with on/off controls.
Table 1: Warm-up and restrike times for metal halide lamps
Electronic ballasts have shortened warm-up and restrike times for pulse-start metal halide lamps. The ranges of values account for differences in ballast starting techniques, fixture heat dissipation, lamp type (coated or clear, open or enclosed rating), lamp age, and whether or not there is a hot restrike capability.
Noise. Magnetic ballasts produce noise because the metal laminations within them vibrate as the magnetic field changes at the line frequency of 60 times per second. Electronic ballast products are available that operate at both high and low frequencies, but operation is silent because the ballasts don’t have laminations to vibrate. This quality is important in areas such as libraries, concert halls, and retail shops.
Light flicker. Some electronic ballasts for metal halide lamps operate at much higher frequencies than the 60 hertz (Hz) of magnetic ballasts. This high-frequency operation eliminates the flicker that can accompany line-frequency operation and can cause headaches or otherwise affect a room’s occupants. High-frequency electronic ballasts also eliminate the stroboscopic effect. Although other types of lamp flicker are just annoying, the stroboscopic effect can be dangerous. When circular saws, drill presses, and other machinery operate at a certain speed, the stroboscopic effect from magnetic ballasts can cause the machines to appear as if they are not operating. Other electronic ballasts operate at lower frequencies (75 Hz for one product), but they have a square wave design that also eliminates flicker.
Longer lamp life. Ballast manufacturers report that pulse-start metal halide lamp life may be increased by about 25 percent through the use of electronic ballasts. This effect is plausible, because electronic ballasts provide more-precise control of current and therefore place less stress on electrodes when a lamp is started. But as of yet, no independent confirmations of increased lamp life have been made, and it may take some time for manufacturers to verify longer lamp life. Meanwhile, when trying to estimate the life of a lamp operating with a particular electronic ballast, use data from a lamp manufacturer rather than from a ballast manufacturer.
Cost. Incremental costs for electronic ballasts vary widely and could be anywhere from $60 to $150 more than a comparable magnetic pulse-start metal halide ballast, with about $100 being the most common differential. However, electronic ballasts have reasonable simple-payback periods over magnetic ballasts based on efficiency and lumen depreciation alone (see Table 2).
Table 2: Cost comparison—electronic versus magnetic ballasts
Systems with electronic ballasts currently cost significantly more than those with magnetic ballasts. However, energy savings can lead to a reasonable payback, depending on the application.