In the magnetic ballast type fluorescent lamps (old ones), what is the need of a capacitor in the starter circuit and what determines its ratings?
If my understanding is correct, it's a bi-metallic strip opening and closing producing an inductive kick, so it should work fine without the capacitor too.
The capacitor in old Fluorescent Starters is for EMI suppression. This is typically a fairly-small value - anywhere between 1n to 100n, depending upon who made your particular starter.
The capacitor may also reduce contact erosion on the starter contacts - I honestly don't know. But I do know that in olden days when everyone had an AM radio sitting on the kitchen counter, you could immediately tell if someone turned on a Fluorescent lamp that didn't have that capacitor inside the starter.
The capacitor is (in most common fluorescent lamp circuits) is for power factor correction. Since there is a coil in the ballast, the capacitor is used to bring the power factor back towards unity. Probably not such a big deal when you consider individual lamps in homes, but when you start looking at hundreds or thousands (aggregate of homes or a typical business), keeping a unity power factor is important. For business power, at least in my area, there is a steep penalty on power costs if the power factor is not close to unity.
