When it comes to Fluorescent stuff I can change a ballast in under 5 minutes and explain the workings and life of a fluorescent tube flawlessly.  But when it comes to the HID lighting I'm still learning the stuff.  Some of the older HID fixtures I've replaced bulbs in and ballasts in, specifically HPS, I've noticed something.  When I have to do a ballast replacement the new ballast kits for the fixture include an ignitor.  The old setup did not have an ignitor in it, just the ballast.  What is the purpose of the ignitor, and why would the older systems not have them?

Thank you

HID and fluorescent ballasts really aren't all that different, and do all the same jobs, in many of the same ways.  A key difference, which can cause some confusion, is packaging.  In North America, fluorescent ballasts are usually packaged in potted "cans" with all of their components (except starters on preheat) contained inside.  HID ballasts, on the other hand, often have all of their components separate.  The parts you have will be the ballast core/coil, an ignitor (for pulse start systems) and a capacitor (for high power factor systems).

HPS lights (and pulse-start metal halide) require a very high voltage (up to 4000 volts) to start, but than run on a very low voltage (around 55 volts for low wattage HPS lamps).  This is done using an ignitor.  There are three styles of ignitors, external 2 wire, external 3 wire and internal.  It sounds like your old HPS ballasts had internal ignitors, where the ignitor is packaged within the ballast core/coil, either underneath the core wrappings (Advance), or in a box attached to the core (Universal).  Your new ballasts sound like they have external ignitors.  2-wire style produce the high voltage pulse internally, and are wired in series between the lamp and ballast (as well as neutral).  3-wire style ignitors generate a pulse, than use the windings of the ballast to build it to the necessary voltage to start the lamp.  Those wire to the connected between the lamp and ballast, the neutral connection and a tap in the ballast.  For best performance, those are matched to the ballast.

And important to note is, then some HID lamp families do need ignitors, some don't.
So if you were replacing e.g. the existing MV or probe start MH (by the HPS or pulse start MH), the existing lamps did not require the ignitor, so the ignitor was not there at all.

HPS lamps can have some ways of starting :

Ignitor inside the lamp - as can be found in many 70W Euro HPSes. Usually a glow starter (identical to that of fluorescent) + thermal cutout to prevent it from attempting to restart a hot lamp. This lamp in 240V works with plain series inductor (so is very often used as replacement for 80W mercury, allthough the ballast and the lamp dont match but are close enough to work well)

Starting aid inside the lamp requiring somewhat high starting voltage - the lamp is filled ith mixture that has lower starting voltage than normal xenon HPS, and can also have stuff like starting probe on the exterior of the arc tube. This lamp needs either a ballast with high enough OCV (perhaps there are HX'es capable of starting it directly - i dont know) or some basic ignitor that makes a starting pulse from the ballast by momentarily shorting the ballast's output

Standard HPS - the lamp requires external pulse of 4-5 KV to start



Here we have the 70W HPS with internal ignitor and the standard HPSes that need 4-5 KV

We have only 3 wire ignitors (for HPS / MH) of 2 types :

Semiparallel which make few high power pulses/sec (seen as flashes in a cycling lamp) by discharging a capacitor into part of the ballasts coil, getting high voltage acrss the entire coil

Superimposed which make 50-100 low power pulses/sec (seen as dim glow) by internal pulse transformer which is in series with the ballast / lamp

Ash: HPS lamps with an internal glow starter have the usual xenon buffer gas, as the regular HPS lamps.
HPS retrofits for MV ballast with a neon-argon penning mixture, self-starts directly from the mains with the aid of a starting coil, with no ignitor. But here the thermal disconnector is to disconnect the starting coil when the lamp is operates at full output, in order to pervent sodium loss from the arctube. So the "Delay" during cycling or turning off/on, for the thermal disconnector to cool down until its closes and connects the starting coil back, than the lamp strikes.