Vintage Variac Restoration!

[Sigurthr]

Ok this doesn't make sense to me... sorry if this sounds stupid, but a volt-amp is literally the output voltage multiplied by the amperage... right?

So if the windings in my case can handle 8A at 240V giving a VA rating of 1920VA... Doesn't this mean at 336V the current handling capability now diminishes to ~5.7A? Otherwise you are violating the 1920VA rating.... :thinking:

The variac only dissipates energy lost in the form of copper losses, which is determined by current through the winding, not voltage. This is the entire reason your mains operate at 240v instead of 120v, in order to get say 1kW of power out of 240v it takes half the current as on 120v, so the conductors have half the losses for the same diameter wire, or they can use half the amount of conductor and maintain equivalent losses.

You can pass 250kV @ 10mA through a 30ga piece of wire with no problem, that is 2.5kW (or kVA) of power. If you take that same amount of power and re-arrange it to say 100V @ 25A it is still 2500VA, but that 30ga will go up in a flash of smoke instantaneously. A conductor's power carrying capability is determined by its diameter which determined how much current it can carry, not the voltage. The wire doesn't care if it is 1V or 1MV.



What happens when you push too much voltage through a variac? You might get flashovers between windings if the insulation is insufficient (I've seen 120V variacs handle 400V with no problem), but that is it. What happens when you draw too much current from a variac? The windings overheat. Yes, this means that if you turn the variac down to say 20Vac output and then short the output drawing 20A it will kill the variac, even though it is only outputting 400VA and it is rated for 1920VA.


Edit:
My interpretation of his goal is to use the variac to power a load that will draw more than 8A of current at voltages above 240V. If this is correct then you either have to limit run times to keep winding temperature under control or you have to increase the impedance of the load so that it no longer draws more than the 8A the variac can safely handle for extended periods of time.

 

[bobhaha]

The variac only dissipates energy lost in the form of copper losses, which is determined by current through the winding, not voltage. This is the entire reason your mains operate at 240v instead of 120v, in order to get say 1kW of power out of 240v it takes half the current as on 120v, so the conductors have half the losses for the same diameter wire, or they can use half the amount of conductor and maintain equivalent losses.

You can pass 250kV @ 10mA through a 30ga piece of wire with no problem, that is 2.5kW (or kVA) of power. If you take that same amount of power and re-arrange it to say 100V @ 25A it is still 2500VA, but that 30ga will go up in a flash of smoke instantaneously. A conductor's power carrying capability is determined by its diameter which determined how much current it can carry, not the voltage. The wire doesn't care if it is 1V or 1MV.



What happens when you push too much voltage through a variac? You might get flashovers between windings if the insulation is insufficient (I've seen 120V variacs handle 400V with no problem), but that is it. What happens when you draw too much current from a variac? The windings overheat. Yes, this means that if you turn the variac down to say 20Vac output and then short the output drawing 20A it will kill the variac, even though it is only outputting 400VA and it is rated for 1920VA.

Ohhh ok I see how it correlates now. This makes perfect sense! Thank you +1 mate! :beer:

Edit:
My interpretation of his goal is to use the variac to power a load that will draw more than 8A of current at voltages above 240V. If this is correct then you either have to limit run times to keep winding temperature under control or you have to increase the impedance of the load so that it no longer draws more than the 8A the variac can safely handle for extended periods of time.

Yup... hit it right on the head!

 

[Cyparagon]

So, you're confusing current with voltage. You want to over-current it, not over-volt it.

Basically the two limiting factors for the current are the core size and the winding size. The first referring to the saturation of the core and the second referring to the resistance of the windings. Unfortunately you can't do anything about either. Using a ballast just limits the current - it does not permit you to run at higher currents than you regularly would. The resistance seems to have more of an effect than the core saturating. So as a rule of thumb, as long as it doesn't get too hot, it's fine. They tend to be okay up to twice their current ratings as long as it's only for short durations. I'd recommend putting a 15A breaker on the low side and an ammeter. Ballasts are only to provide an active current limiting. That isn't needed if you monitor the current and don't try to drive shorts.

 

[bobhaha]

So, you're confusing current with voltage. You want to over-current it, not over-volt it.

Basically the two limiting factors for the current are the core size and the winding size. The first referring to the saturation of the core and the second referring to the resistance of the windings. Unfortunately you can't do anything about either. Using a ballast just limits the current - it does not permit you to run at higher currents than you regularly would. The resistance seems to have more of an effect than the core saturating. So as a rule of thumb, as long as it doesn't get too hot, it's fine. They tend to be okay up to twice their current ratings as long as it's only for short durations. I'd recommend putting a 15A breaker on the low side and an ammeter. Ballasts are only to provide an active current limiting. That isn't needed if you monitor the current and don't try to drive shorts.

I wanted to over-volt it... but I was scared of over-currenting it at elevated voltages. But as Sigurthr pointed out, either way 8A is 8A... the windings do not care if its 10V or 10,000V.

Also yeah I read that they are fine to work up to 200% overload for a while... I'll probably put in a 16A breaker since that's all I have lying around. If It gets to the 16A point I'm probably doing something wrong and I should stop that right quick...