Hi guys....just bought a variac or variable transformer those with the turning knob on top. It is rated at 500VA which I figure to be 500watts? So it should keep a 125w MV happy but it seems that it's not getting enough power from the variac. I checked the output voltage I increased it from mains 230v to output 240v. I used the same bulb at the outside wall outlet and it's brighter and lights faster than with the varic.... Anyone knows Y this is so? Please help..:/.

IMJ

500VA mean product of MaxVolts x AllowedCurrent, so in your case 500VA with max setting 240V mean a bit more then 2A maximum current. 125W MV need 1.15A, what would be the input current of an uncompensated ballast (so ~250VA; Compensated would be about 0.6A), so the 500VA rating should be OK.

Check the output voltage when loaded with the lamp circuity, if it does not correspond to variac setting. If not, look for loose contact in the variac.

The "work well in the wall outlet outside" mean you tried it with the same ballast, or you tried only the lamp? If only the lamp, then check the ballast...

Hi thanks for replying. Yes it's the same bulb and same ballast same wires at the outside wall outlet. The output voltage of the variac I measured at all stages of warm up, all show 240v. Could it be the resistance of the variac coil in series causing this loss of power? Thanks again.

IMJ

Should not be.There is clearly something wrong, only to find what...
Do you own a scope?
If yes, connect some low voltage transformer directly parallel to the 125W fixture input and make snapshot of the secondary voltage waveform (the LV transformer would scale down voltage and provide insulation, as most scopes are grounded, so you cannot measure directly on mains potentials).

No I do not own a scope but I suspect the waveform is the problem. The MV flickers more than without the variac even at highest setting of 260v. Anyways I know how our flat distribution is wired maybe you can see if it's the problem? Our flat has 12 floors....the 3 phase Red, Yellow, Blue are split among 4 floors like Red phase is given to 4 floors, BLUE phase is another 4 floors and Yellow another 4 floors. Maybe it is this way of distribution that causes the waveform to distort from RMS since I think it has unbalanced phases. I don't think it's the variac cause I tested it with 12v halogen and it can give 11.9v and lights it very bright. I apologize for not being specific but it could be anything right...:/.

IMJ

Has the MV fixture PF compensation capacitor? It might help to provide current for higher harmonics, if they are present...
And did you tried different ballast?

If both lead to an improvement, i would suspect the ballast not to be in very good shape...

Yes I used a 12mF cap for the 125w MV...no change and it's the same with all HIDs I used.

Maybe it's not actually rated for 500VA?

I doubt it though because if it were so the fuse would blow. The fuse is rated at 3A and I already blew it when I tested a 12v 50w halogen.