I have discovered that both 1 and 2 lamp LPF F20T12 trigger start ballasts can be dimmed using a CFL/LED dimmer on the highest dimming range possible. Is this safe? Is is safe to dim the same ballast types on 3 speed fan controls?

it will sputter the lamps as your diming the cathode heating too.

it will sputter the lamps as your diming the cathode heating too.

I didn't notice any blue glow in the cathodes.

I didn't notice any blue glow in the cathodes.

Blue glow comes only when it is killing them at the highest gear...

For fluorescent dimming you need a circuit that provides more heating power as the main arc current gets reduced. So in a cheap way it means separate filament transformer...

And what 3-speed fan controller you mean? Because these use to be just 1p3t switches, it is the fan design itself, what usually makes the three speeds (each speed has its tap on the motor winding, the speed is selected whenthat tap is powered)...

Blue glow comes only when it is killing them at the highest gear...

For fluorescent dimming you need a circuit that provides more heating power as the main arc current gets reduced. So in a cheap way it means separate filament transformer...

And what 3-speed fan controller you mean? Because these use to be just 1p3t switches, it is the fan design itself, what usually makes the three speeds (each speed has its tap on the motor winding, the speed is selected whenthat tap is powered)...

It is a wall switch with only one output for the fan. The fan is supposed to be set on high speed for the wall control to work correctly.

Meved  he duscribing a motor speed controller
same Idea use on sewing nachines

fluorescent05
thing is yes this ballast have a cathode heater built im
with a ballast to run your fluorescent lamps

thing is your also dimming the cathodes with the ballast
causing them to no lionger to properly heat them  runing the fluorescent lamp in cold cathode mode.  your better
off using preheat gear with two independant 3-4 volt transformer. to keep cathode heated and put ballast in series with fan control. 

there is such ballast Designed to Flash fluorescent lamps.  cause it keep cathode heated then you haveva flasher citcuit to turn ballast on and off.

Meved  he duscribing a motor speed controller
same Idea use on sewing nachines

fluorescent05
thing is yes this ballast have a cathode heater built im
with a ballast to run your fluorescent lamps

thing is your also dimming the cathodes with the ballast
causing them to no lionger to properly heat them  runing the fluorescent lamp in cold cathode mode.  your better
off using preheat gear with two independant 3-4 volt transformer. to keep cathode heated and put ballast in series with fan control. 

there is such ballast Designed to Flash fluorescent lamps.  cause it keep cathode heated then you haveva flasher citcuit to turn ballast on and off.

If you were to open the ballast up, would you see seperate transformers for high voltage across the tubes and low voltage to heat the cathodes? Also, if the cathodes are shut off on Advance PowrKut ballasts (only when the lamps have started), why is it harmful to dim or shut off the cathodes when dimming the tubes?

the way powerkut ballast works is once  tubes started and warmed up at full power it can Maintain the Emission on the cathodes. from that point it can no longer have to be heated.  it is very closely to preheat
ballast with electronic starter inside the ballast can.
 now dimming it with fan controller you are reducing the
lamp current with no cathode heating. Can put lamp out of hot thermonic emission running them in cold cathode
cathode. once this happens it sputters the lamp off quickly do underdriving the fluorescent lanps.  This applys to all  Discharge lamps.  now if you ever taken
apart a normal advance rapid start ballast it will help you
understand the internals bit better.  cause you have one ballast core cause it wrounded up with a auto transformer and a choke section.  along with 2 independant winding on same core for heating the lamp
on 2 lamp ballast is same idea with more turns of wire
in the auto transformer section with 3 independant cathode heating but one of them is use bit thicker wire
to supply both cathodes on yellow wire. while passing lamp current though this  to next lamp in series  to compleat the circuit.  this one of reasons it is hard to dim it.

now diming ballast supplys the contant voltage to cathode once it stays heated  you can do can dim them and wont harm the lamp.  if you ever starred a rapid start ballast on a damp cold day you can see cathode glow at both ends ends of flourescent tubes.

if you ever look at UK quick start gear. i think i have Thorn Invincable on my gallery.   it uses a transformer with two low voltage windings to heat the cathodes and using a standard preheat ballast on 240 volt OCV.

the only way to properly dim your tube is use the Ocv from ballast to drive your lamp with one wire at both ends using your speed control and useing two seperate transformer.  no center tap transformer wont work as you will short the ballast out though transformer and not lighting your lamp. as golden rule electricity like take shortest path.

Never tried it with a LPF F20 RS ballast,
But I have dimmed a 2xF40 LPF with no problems. Doesn't seem to affect lamp life either.
(obviously if you dim too much, it won't stay lit, and that may damage lamps/ballast ).
(Also works far better if you include a small incandescent load on the dimmer)

Meved  he duscribing a motor speed controller
same Idea use on sewing nachines

These I know about that correspond to your description (triac dimmer like, just modified range) are smooth regulated with a pot, not with 2 or 3 fixed speed levels.
If the fan system had fixed 2 or 3 speed levels, all I've seen were actually 2-3 speed motor and a switch.

Using the triac dimmer with fixed 2 or 3 speeds makes not much sense, because such switch cost more than a pot it replaces in the circuit and offers less marketing value (few fixed speeds instead of smooth variable control). Plus the levels with triac dimmers tend to vary a lot, what would make the few speed levels very inconsistent. With a pot system this is masked just because the user can set anything from the minimum to maximum.