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Capacitor size for HPS HID LPF ballast


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Capacitor size/ HID Ballast factor





I have a 100W HPS HID outdoor light with a low power factor ballast. Could you tell me what size capacitor to add that would improve the power factor to approximatly .90-.95?


Currently this single 100W system is consuming 2.23Amps at 120V input.....OR.....268W.


100W lamp/268W Consumed= .37 "The data sheet on the ballast states the power factor is .50"


"Now I know why it was so low priced."


Any help would be appreciated greatly as this light stays on all night.


Thank you.

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Hello mrcibotti


From the figures that you quote, the lamp is currently drawing 268VA (not watts) and if the genuine lamp power is 100W, then there would be a pf of 0.37 as you suggest. I would expect hwever, that there is also some poswer dissipated by the ballast, and this may be another 10 or more watts, so the pf may be a little higher.

Let us assume however that your figures are correct.

The current dawn is made up of resistive current and reactive current. Working on the theoretical, the resistive component is equal to P/V = 100/120 = 0.83A


The reactive component is square root of ((2.23 x 2.23) - (0.83 x 0.83)) = 2.07 Amps.


For a powerfactor of better than 0.95, you want to have a reactive component that is less than 0.83 x tan (cos-1(0.95)) = 0.27A

This is a current reduction of 2.07 - 0.27 = 1.8 amps, or a VAR rating of 1.8 x 120 = 216VAR.

The capacitor that you need to add is 216VAR for a final power factor of 0.95.


This assumes the the ballast disipation is zero and that the current is sinusoidal and the lamp is drawing 100W.


Best regards,

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Hello again mrcibotti


On further thought on this, is the low power factor really a problem. Is this a domestic installation, in which case you pay for KW only and the poor power factor is not relavent, or is this an industrial installation where you are paying a power factor penalty?


Remeber that a power power factor does not make your meter read higher.


Best regards,

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