| Electrode voltage drop :
Is a fairly constant voltage. Higher lamp arc voltage means that the electrode voltage drop is less significant part of the overall voltage of the lamp. As the current is the same in the series circuit (electrode, arc, electrode) the voltage drop is directly proportional to the losses towards there
In large lamps the limiting factor on arc voltage is staying within voltage range that does not complicate too much the gear :
- If the choice is between a choke vs complex ballasts (HX, CWA) the choke will win, both in cost and efficiency, so it makes sense to match the lamp to the max voltage with which it can work with a choke
(On a side note, i dont understand why in the US lanterns for 277 and 480V still use complicated and inefficient ballasts, even in places with good and stable line voltage)
- If a complex ballast is made anyway, some upper limit is chosen based on the lamp discharge and ignition voltages not exceeding the limits of normal lighting components, insulating materials according to some safety standards etc
The law of diminishing returns applies : The higher you go, the additional gain is less significant, so at some point it makes no sense to continue pushing the voltage up
In small lamps the limiting factors don't come from the arc voltage (with the exception of 55V HPS). It is low because some other design factors took precedence first
Required losses to keep different parts of the lamp at the correct temperature :
HID lamps must be hot in order for the metals to be evaporated and at the right pressure. The coldest spot in the arc tube will be a place where metals and halides can get out of the discharge and do no useful work
In some lamps (HPS) this is an essential part of the design, but still the coldest spot temperature must be hot to a certain level
In some lamps the design goal is to avoid the coldest spot altogether
In high power lamps, the lamp power and power density are high enough so that several particular places where heat escapes from the lamp (e.g. conduction through electrode wires, radiation from the arctube surface behind the electrodes) are not significant compared to it
In low power lamps, those heat losses are significant and must be reduced - By better arctube shape design, heat reflective coatings, etc. But once those are improved as much as possible, there is still heat to be generated to make up for what is still lost, so some loss is essential
Light output drop during lamp life :
Electrodes sputter and cover the arc tube walls. In a short arctube, this reduces the light output more significantly
This may not be reflected in initial Lm/W but instead in the lamp actual performance and life ratings
Life ratings - Both because the light output drop in itself is a life rating, and because other mechanisms of lamp failure can be designed to fail at around the same time that the light drops below the wanted levels : There might be performance improvement during the lamp's useful life - like thinner electrodes that lose less heat, and for cost cutting
While it varies for each lamp type, for some 250W is around the break even point, between things that pull down the performance which come from small things in the lamp vs. from the general limitations of materials used
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