| There is no single way how a HP Fluorescent circuit can be implemented, and each circuit will behave differently. Some possible behaviors may include :
The lamp current going down as the capacitor value is going down, while some other current draw in the circuit (the filament heating, the ballast primary winding losses, etc) remains unchanged. So while the phase angle of the current component affected by the capacitor may be going more and more like 90deg (towards capacitive), at the same time its effect is becoming less significant for the complete ballast PF which is seen from the line outside, so the PF of that one stays high
The lamp current not changing, but ballast primary current is rising and along with it the ballast's resistive losses, which pull the PF towards unity. Within some range of capacitor value (as it is failing), it is possible that the change in PF (towards inductive) is masked by the increasing "resistive" component of the current
The capacitor is parallel to the line input, and not actually part of the ballast circuit (even if it is hidden inside the ballast enclosure). The change in PF (towards inductive) will be clearly visible
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