What would happen if a self ballasted MV lamp such like a Philips ML 250W 235-245V would be connected to let say 300Hz via a frequency converter?
Are there any issues comming up with this set up?
How does the light output behaves? Less flickering?
How does the electrical properties changes?

Higher frequency don't affect efficiency on HID lamps, but only on fluorescent and LPS lamps.
I think the lamp would simply have less flickering.
Operating an HID lamp on HF like 20,000Hz, may cause the arc to resonate and overheat the arctube. I don't know if with 300Hz this is enough to cause resonances.

The frequency is of no problem at all (of course if it does not go to higher kHz range where the arctube resonances may occur).
The problem may be the voltage, namely the resulting filament loading and peak voltage for ignition.

At higher frequency the arc restriking after zero crossing would be better, so possibly longer duty cycle and the lamp burning at a bit higher power as result. I doubt it would be significant difference tho

Ok thanks! So if I provide some higher frequencies to the lamp the voltage drop ratio between the arc tube and filament changes? However the resistance of the filament stays always the same but the arc tube?

The resistace yes, but at low frequency it takes rather long time after a current zero cross to restore the plasma in the arctube. This delay cuts out a part of the sinewave (something like a regular triac dimmer does). The overall lamp design count on that, so the filament voltage rating is tunet for that to fit. So if the different frequency cause the amount of the missing sinewave to change, the resulting rms voltage/current across the filament changes too.
But first here we are talking about really very small differences, so I have quite some doubts if that is really important at all.
Second it may be not that straight forwar of higher frequency -> faster reignition -> higher power. Higher frequency->faster reignition link is quite straight forward and without any questions. But the thing is, although the reignition may be faster due to the higher frequency, but for the same timing difference the supply "sinewave" progresses faster too, so the same delay significantly reduces the power. So goes against the faster reignbition. What is the net result? who knows...

But anyway, as I expect this to be insignificant anyway (mainly compare to other factors like the voltage accuracy, exact waveform shape,...).
So I would really make more focus on the voltage and shape - these contribute to the filament loading  with way greater extend.

Just a question: Why you are asking that? Or in other words where the higher frequency supply comes from? Because the real sinewave is not that common shape with common generators, mainly because the exact shape uses to depend on the actual generator design and loading...

Thank's Medved for the detailed information!

I was just asking because I have several Frequency converter and very recently I got about 100 NOS MVSB lamps. Some of them I like to use in our workshop but with less flickering if possible so I thought to use such a converter for this application. I'm making a test in autumn when I'm back from my vacations!

What I still don't know is how does the sine shape looks like on the output of these converters. I know that this might important too in regarding of the reigniting of the arc after every zero cross.

The resistace yes, but at low frequency it takes rather long time after a current zero cross to restore the plasma in the arctube. This delay cuts out a part of the sinewave (something like a regular triac dimmer does). The overall lamp design count on that, so the filament voltage rating is tunet for that to fit. So if the different frequency cause the amount of the missing sinewave to change, the resulting rms voltage/current across the filament changes too.
But first here we are talking about really very small differences, so I have quite some doubts if that is really important at all.
Second it may be not that straight forwar of higher frequency -> faster reignition -> higher power. Higher frequency->faster reignition link is quite straight forward and without any questions. But the thing is, although the reignition may be faster due to the higher frequency, but for the same timing difference the supply "sinewave" progresses faster too, so the same delay significantly reduces the power. So goes against the faster reignbition. What is the net result? who knows...

But anyway, as I expect this to be insignificant anyway (mainly compare to other factors like the voltage accuracy, exact waveform shape,...).
So I would really make more focus on the voltage and shape - these contribute to the filament loading  with way greater extend.

Just a question: Why you are asking that? Or in other words where the higher frequency supply comes from? Because the real sinewave is not that common shape with common generators, mainly because the exact shape uses to depend on the actual generator design and loading...

What I still don't know is how does the sine shape looks like on the output of these converters. I know that this might important too in regarding of the reigniting of the arc after every zero cross.

It is by far not just about the zero cross, but mainly what happens to the voltage shape (so then the rms value), when the arc voltage of a constant voltage clamp characteristic is subtracted from the supply voltage.
With a 100V arc the remaining rms voltage across the filament is normally (with true sinewave supply) about 140V ( =sqrt(arccos(100V/(230V*sqrt(2))/(Pi/2))*((230V*sqrt(2))-100V)/sqrt(2) so in other words a rms value from what remains of a normal 230Vrms sinewave waveform exceeding the 100V of the arc).

E.g. the "modified sinewave" using mostly +325/0/-325/0 pattern keeps across the filament sqrt(1/2)*(325-100)=160Vrms, which is about 10% filament overdrive. And that is quite a lot.

If the inverter generates a +325/-325V rectangle (common for the low power, very cheap inverters intended to supply just the mobile chargers and similar devices), the filament voltage would be 225V, so about 50% overdrive.
With a rectangle of +230V/-230V (to maintain the rms) the filament would be a bit underdriven, but the 230V may not really suffice for a good and reliable ignition.