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Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
I was wondering if any of you guys know what the maximum voltage a variac can withstand. I have a motor that is capable of taking up to 155VAC, and my variac goes to 140VAC. I want it to MAX at 150VAC.
I was thinking of perhaps adding some windings down below the "first winding" past the range of the wiper. I would not be able to use these windings for anything, they would even "raise" the zero winding to a few volts, but I never need that winding anyway. I guess I would just wind about 10 turns down below the lowest winding, and tie them into the line.
Any thoughts or suggestions? I don't want to move the tap because then the core will likely saturate.
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
I wouldn't recommend modifying the variac. Instead you can add a "buck/boost" transformer to increase the range of your output voltage. A buck/boost transformer is simply a transformer with a low voltage high current secondary winding that can be connected in series with the supply voltage. When connected in-phase, it will add to (boost), and if out of phase it will buck (reduce) the supplied voltage. By driving the primary of the B/B transformer from the output of your variac and connecting its secondary in series (in-phase) with the variac output, you can effectively "spread out" the output range to 0-150 or 0-155 VAC. A 10-15 volt filament transformer with a current rating that at least matches the variac capability should do the trick.
Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
Thanks Bert. After reading your post, an image of the inside of a variac flashed through my mind, and I remembered that the core of those things is not very accessible for modifying, let alone adding windings anywhere.
I understand what you describe, initially it sounds more complicated than it really is. But if I'm going to use an external transformer like that, I might as well re-wind the secondary of a medium sized MOT to supply 155VAC, and then power that with an unmodified variac set for 0-120VAC.
Another question comes to mind now, because I calculated that 155VAC based on the internal resistance of this universal motor. I realize I better ask some of you guys about the power output of this motor. I think it is appropriate to start a new thread since it really is a different question.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, the boost transformer arrangement is much more economical on iron and copper. Say you want to supply 10A at 155VAC. Then you need a 1.55kVA transformer with your method. But a boost transformer (used with a variac cranked to 140V) only needs to be 150VA, since it's supplying 10A at 15V.
Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
Thanks for clearing that last bit up Steve. In my case, I am trying to reach 600W, so I would need a 6kVA transformer.
After I repeated my calculation, I made an error somewhere in my math (typical of me) and the value I am reaching for is 146.9VAC, so I don't think it's worth the modifications just for 6 volts. I am going to stick with the variac as is, since I measure on it's output 141 Volts. It's close enough.
Registered Member #594
Joined: Tue Mar 20 2007, 04:02PM
Location: Joplin Mo USA
Posts: 6
If you are off by only 6V an old led acid battery charger might be a good place to find a transformer they are pretty tough most of the time and can be found used or trashed quite often so they are cheap as well.
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Variac - reconfigure to get ~150VAC?
Tesla^3