September 1996 Volume 2 Issue 16
Ballasts: Picking the Best
All gas discharge lamps — including fluorescent and high-intensity discharge (HID) types — must work together with an electrical device called a ballast. The ballast performs certain functions for the lamp. These differ somewhat from situation to situation, but include limiting the lamp current; providing a high voltage pulse to ionize the gas in the lamp; and in the case of electronic ballasts, converting the 120 voltage pulses per second of 60 Hz power into tens of thousands of pulses per second. This article discusses some possible lamp/ballast pairings, and tells how to compare their efficacy.
FLUORESCENT LIGHTING BALLASTS
With fluorescent lighting, there are three ballast options:
The iron-core electromagnetic ballast
This type of ballast has been used in fluorescent fixtures for the past 50 years. It consists of a magnetic core of laminated transformer steel around which is wound a coil of insulated copper or aluminum wire. The assembly is then impregnated with one or more insulating and sound absorbing substances, and is mounted in an elongated sheet steel housing (see illustration). The principle functions of this type of ballast are to initiate the gas discharge in the lamp and then, once the arc has been established, limit the arc current.
In 1995, Natural Resources Canada (NRCan) introduced regulations establishing minimum efficiency standards for all ballasts imported into Canada or sold inter-provincially. In effect, these regulations ensure that grossly inefficient ballasts will not be sold, but they do not require that all ballasts have high efficiency. For example, in a fixture equipped with two 4-foot 34 watt fluorescent lamps and an iron-core ballast which meets the NRCan standard, the ballast loss may be as great as 17% of the fixture input power. Some energy-efficient electromagnetic ballasts have lower loss than this.
Left: a typical fluorescent ballast. Right: an HID ballast with lamp socket attached.
The low-frequency electronic ballast
Low-frequency electronic ballasts are energy-efficient electromagnetic ballasts with added electronic circuitry which cuts off current to the lamp filaments once the gas discharge has been established. Using this type of ballast with the same lamps as the previous example, the ballast loss would typically be only 5% of the input power.
The high-frequency electronic ballast
Instead of delivering 120 current pulses per second to the fluorescent lamps as electromagnetic and low-frequency electronic ballasts do, high-frequency electronic ballasts deliver tens of thousands of pulses per second. This boosts lamp efficiency. A lamp fed with 32 watts of 25,000 Hz power produces more light than the same lamp fed with 32 watts of 60 Hz power. High-frequency
electronic ballasts have losses, just as other ballasts do, but in some cases the boost in light output due to high-frequency excitation is greater than the drop in light output due to power losses within the ballast. When this happens, the ballast appears to have a negative power loss (a power gain) which in strict energy terms is impossible. Where different lamp excitation frequencies are involved, the best approach is to compare the efficacy (lumens out per watt in) of lamp/ballast combinations. With this approach (described in the next section) there are no confusing “negative losses.”
While more expensive than electromagnetic ballasts, one HF electronic ballast can serve as many as four lamps. And when properly matched with high performance T-8 lamps, the combination gives both top-quality lighting and minimum operating cost.
EVALUATING LAMP/BALLAST PERFORMANCE
Each fluorescent lamp type is rated to produce a specified lumen output when fed with its rated wattage (60 Hz excitation) through a standard test ballast. Commercially available ballasts are often designed to work with several types of lamps, and for each lamp type the ballast will have a Ballast Factor rating. The ballast factor tells you how much light you will actually get from a particular type of lamp when used with that ballast:
Actual Lumens Out = Ballast Factor x Rated Lumens Out
Another figure found in the ballast catalogs is Input Watts or ANSI Input Watts. This is the amount of power delivered by the power line to the lamp/ballast combination.
The efficacy (lumens per watt) of each combination can be calculated as follows:
Actual Lumens Out
Efficacy (lm/W) = ——————————
Ballast Factor x Rated lumens out
For example, the 4-foot 34-watt T-12 F40T12/ES lamp has a rated output of 2775 lumens when used with its standard test ballast. If we use two of these lamps per fixture with one electromagnetic ballast to operate both lamps, what will the efficacy be? The proposed ballast has a Ballast Factor of 0.88, and an Input Watts rating of 72 watts. Two lamps under standard conditions produce 2 x 2775 or 5550 lumens. Thus:
0.88 x 5550
Efficacy (lm/W) = ———————— = 68 lumens/watt
This figure can then be compared with the efficacy of other lamp/ballast combinations. Two 4-foot 32-watt T-8 lamps and a typical high-frequency electronic ballast gives the following result:
0.95 x 6000
Efficacy (lm/W) = ———————— = 92 lumens/watt
The operating cost of this particular T-8 system would be 14% lower than the T-12 system (62 ÷ 72 = 0.86), and its light output would be 17% greater ([0.95 x 6000] ÷ [0.88 x 5550]= 1.17).
Almost all ballasts for HID lamps are of the electromagnetic type, and are designed to deliver rated lamp wattage to the lamp. Because ballast manufacturers specify Watts Input to the lamp/ballast combination, it is easy to calculate ballast loss:
Ballast Loss (watts) = Watts Input – Rated Lamp Watts
When comparing different HID ballasts for possible use with a given lamp type, the ballast producing the most light per electricity dollar will be the ballast having the lowest loss.
If you would like further information about lighting efficiency and lighting options, call Mike Proud or Ron Estabrooks at 368-5010 (toll free)