HID Lighting De-Mystified (a little)
By Erik Biksa
Learning about the electrical principles of HID lighting from the hydro store can at times be challenging. This is not to say that your friendly and familiar retail staff are not qualified to provide you with the level of service you demand, but sometimes the scope of the subject may exceed their scope of expertise. For example, you might not expect your car salesman to run through the principles of the combustion engine with you, but you would expect that they know the horsepower rating on a given make or model.
From my experiences and discussions, I have come to the realization that very few of us know how the HID ballasts we use really work. It is the purpose of this article to provide a general understanding of the principles and workings of the more commonly used HID lighting transformers/ballasts and components. Hopefully this will help to dispel and bring to light a lot of the myths and rumours surrounding HID lighting. Much of the information in this article is available on the web, and by no means am I an expert in the subject. Each manufacturer has their own “angle” in terms of the type of ballast they produce, so be aware of the way information is presented to you and who’s letterhead it may be printed on. It is not likely that you will see many false claims, but some pertinent information may be omitted to make one product seem more favourable than the next.
For HID lighting systems, ballasts serve two main purposes; to ensure that the correct voltage is available and sent to strike the arc (start) at the HID lamp, and to provide and maintain the correct current to the lamp once the lamp has started (arc is established).
The way in which a particular ballast system addresses the two requirements as above, determines what type of ballast it may be classified as.
Essentially each lamp type, wattage, and brand has a unique start-up voltage and operating current. HID lamps also have a negative resistance characteristic. This basically means that they have very specific electrical requirements for them to start and to continue to produce light, and if not carefully regulated the lamps will draw excessive power to the point of “self-destruct”. Conversely, as power supplies fluctuate, voltage may dip below the required lamp operating voltage, extinguishing the arc. This is why a ballast system is required, as it helps to generate and maintain these requirements. For some, this may help explain why specific wattages of lamps must be matched with the correct wattage of ballast.
Below are working descriptions of three basic ballasts types (although several variations have been developed):
Reactor Ballasts ®
These ballasts can be used when the input voltage to the lamp fixture meets or exceeds the starting voltage of the lamp. Reactor ballasts may be best suited to mercury vapour lamps as many of these lamps are designed to start between 240 to 277 volts. So if you have a 240 or 277 volt power supply, the reactor core will simply act as a resistor/regulator ensuring that voltage supplied to the lamp does not exceed the lamp requirements. The short coming of this system is that you can only run one lighting system per circuit in this example and a 25% fluctuation in line voltage may cause the lamp to extinguish. A capacitor may be wired into the power supply before it reaches the reactor core to reduce the current draw and increase the power factor (PF) ratings, but will not alter the lamp wattage regulation.
High Reactance Autotransformer (HX)
HX ballasts are used when the input voltage (i.e. 120 V or 240 V) is not able to meet the starting voltage requirements of the lamp. As with the standard reactor ballast, the core limits the current to the lamp, but may “step-up” the input voltage to the level required for the lamp to operate. For example many standard 1000W HPS lamps have a minimum open circuit requirement of 450+ volts, but the power supply coming in may not exceed 240 volts or even 120 volts. Therefore the voltage must be stepped-up via core winding(s), hence the “autotransformer”. This type of lighting system could be very effective, but requires careful matching of lamp to ballast type to power supply. A capacitor(s) may also be used as with the reactor ballast to create a higher power factor rating (PF) and help to regulate the input voltage.
Constant Wattage Auto Transformer (CWA)
This is the most common type of ballast found in our industry. The primary difference in this ballast versus R and HX ballasts is that it uses a capacitor(s) in series with the lamp. Note that a pair of capacitors may be wired in parallel together, but in series with the ballast. The capacitors wired in series with the ballast improves the regulation characteristics to the lamp. Essentially, when the power supply fluctuates the lamp will have a better chance of staying lit than with other types. For example it may require up to a 40% change in line voltage for the lamp to extinguish versus the 25% more commonly associated with the R and HX ballast types.
In most of the ballast used within the indoor gardening industry, there are three critical points at which voltage must be measured to ensure that lamps may run at optimal.
- Power input to ballast - If you are using a multi-tap ballast and decide to use the 240 volt tap, make sure that you are supplying the ballast with 240 volts. You would be really surprised at how many people out there are using 240 volt set-ups while only supplying the ballast with 208 volt. A lot of industrial space in Western Canada is supplied with 208 volt, not 240 volt. Also keep in mind that voltage drops with distance traveled. With CWA type ballasts people have been able to get away with this, as they are not as sensitive to line fluctuation (i.e voltage drops). CWA ballasts can fool the grower into thinking things are OK. With these systems, just because the lamp is lit, does not mean it is running anywhere near optimal. For example you are consuming 1050 watts, but really only getting 900 watts of light (but all you see is the lamp is lit). With HX type ballasts, the lamp will not stay lit if voltage is not near optimal; an easy way to determine your efficiency!
- Ballast open circuit voltage - In HX and CWA type ballasts this is the point where the electricity has been “transformed” at the ballast winding(s), most commonly power is “stepped-up”. For example, the line voltage coming into the ballast is 240 volt, but leaving the windings it may be higher than 480 volt plus on a CWA 1000 watt HPS system. If this power falls short in an HX system, the lamp will not be able to stay lit. In a CWA system the result may be that the lamp has the ability to stay lit, but is performing below optimal efficiency (low lumen per watt conversion). An increase in this “power out” in an HX system would likely damage the lamp or lead to lamp failure. In a CWA system it may lead to the failure of the capacitor. Either way, the core may be subject to burn out if it does not operate within the correct voltages.
- Nominal lamp volts - This is the amount of power being sent to the lamp via the lamp cord. In CWA ballasts, this power has been “regulated” by the capacitor(s) wired in series with the ballast. For example, in a 1000 watt HPS lamp this value is typically 250 volts or in a 600 watt system it is closer to 105 volts. If this voltage rises too greatly, the lamp may burn brighter at the expense of pre-mature lamp failure or worse. If a lamp has a crest factor rating of 1.8 it means that the normal open voltage can be 1.8x as high as the required voltage for operating the lamp (ratio of peak to RMS current). A pulsing/strobbing lamp is a good indicator that the nominal lamp voltage is being exceeded.
The role of a capacitor is to provide a set level of resistance. For example, the ballast open circuit may be 480 volts plus, but by the time it discharges through the lamp cord in a CWA system it has been reduced to 250 volts. The capacitor has reduced and should maintain this voltage level. However, during start up, the capacitor should have low resistance, allowing the open circuit voltage of 480 volts plus to engage the pulse ignitor to send a spike of up to 5000 volts to strike the arc. Near instantaneously, as the arc is established, the capacitor gains resistance and helps reduce the nominal lamp voltage to within the acceptable range allowing the lamp to operate within the normal pre-set range.
For some further food for thought Chart A illustrates what is typical of most HPS lamps in North America: (horticultural lamps may have higher input requirements)
Chart B is a table that is typical of Metal Halide HID lamps in North America: (Horticultural Lamps may have higher input requirements)
There have been a lot of innovations in horticultural ballasts and particularly lamps. Growers have long recognized the benefits of increased lamp wavelengths in the blue spectrum during early vegetative growth and enhanced red spectrums for flowering. One of the newer technologies is the pulse start metal halide lamp and ballast. Basically, MH lamps may now use an external ignitor, eliminating the bi-polar switch in the arc tube (formerly a “built-in” ignitor). This creates a lamp with a higher lumen per watt ratio than traditional metal halides with the increased lamp life and decreased warm-up time associated with HPS lighting systems. Digital or electronic ballasts (EB) are the avant-garde in horticultural lighting today, and basically make this article obsolete. Fortunately for the time invested in this writing, it will probably be a few years yet before they replace current popular technology.