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Understanding power factor.

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Mike:
The low wattage street lights here have NPF ballasts and the older low wattage MV fixtures did too. You'd think the electric company would install HPF street lights to reduce the burden on the grid but the NPF ones are the cheapest so that's why they use them... :-\

lights*plus:
Ha. Cheap, cheap, cheap, cheeeeap! Is what the chick said to the hen having her head chopped!

Medved:

--- Quote from: Mike on February 07, 2014, 10:00:40 PM ---The low wattage street lights here have NPF ballasts and the older low wattage MV fixtures did too. You'd think the electric company would install HPF street lights to reduce the burden on the grid but the NPF ones are the cheapest so that's why they use them... :-\

--- End quote ---

If the existing wiring have a margin, they don't have to bother with HPF as well.
Other reason for not using PFC capacitors inside the streetlights is with multiphase installation with fixtures connected in "Y", when the Neutral get loose, the PF compensated fixtures may form low impedance resonators with the other ballasts, creating a lot of consequence damages. Therefore with such systems the capacitors are common for larger group of fixtures and connected to "D".
Other reason to not use capacitors inside the fixtures are the group dimming systems, where the required compensation capacitance changes with the dimming level, so it is usually moved to the dimmer box.

Medved:

--- Quote from: lights*plus on February 07, 2014, 07:58:30 PM ---
--- Quote from: Medved on February 07, 2014, 03:51:14 PM ---To really utilize the advantages of the HPF fixtures at home (higher efficiency,...), the power factor correction would have to be incorporated deeper in the ballast design than just a correcting capacitor parallel to the mains terminals.
--- End quote ---

But I thought that a working ballast (like a cheap one found in wallpacks & floodlights) gets less hot with the suggested capacitor, thereby increasing its life (make a cheap ballast into a better one). Don't be afraid to ellaborate on your deeper in ballast comment.


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No, because what the ballast always see on it's input is just plain 120VAC, regardless if you connect the capacitor there or not. So with the same lamp, the primary current will be the same as well.
Where the current become lower, are the wires from the CB box down to your fixture.

Something else is not using the specified capacitor on a (mainly multi-tap) HPF ballast. There the capacitor is usually connected across a winding with the highest voltage. That mean, the complete primary is there to handle the reactive power, so the related current becomes lower. The 120V section then handle just the real power. But when you disconnect the capacitor, the short 120V section will have to handle the complete apparent power, so becomes overloaded. So even the ballast may seem to be working, the missing capacitor causes it's overheating.
But these ballasts I have categorized as the "power factor correction incorporated deeper in the ballast design" and these are always installed with their specified capacitor from the factory (a cheepeese fixture maker use cheaper 120V-only NPF ballast).
Because of this trick, the multi-tap ballasts become actually cheaper as HPF than they would be as NPF, because as NPF, the 120V section would have to be made with thicker wire, what would mean more complicated manufacture (swapping wire gauges), larger ballast body and generally more material to pay, so an additional capacitor with the HPF format is just cheaper solution...

lights*plus:
Ok good stuff. That's why some really good HPF ballasts have such thin windings..so easy to damage if the wrong tap voltage is used (dogh)!

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