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Come-on, is my 10A variac REALLY only good for 10A?

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GeordieBoy

Mon Sept 21 2009, 09:20PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Why not simply bypass the variac once you are up to "running voltage"?

-Richie,

Bird-Dawg

Tue Sept 22 2009, 12:11AM

Registered Member #2360 Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14

Two of my variacs are 5amp rated but they have 6.3amp slow blow fuses in them and i have used them for a year and they work just fine. Though my 10amp i let run a little to long with to much current and had to order a new brush set from the manufactuer.

Josh Johnson

Tue Sept 22 2009, 01:25AM

Registered Member #793 Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35

Thanks to everyone for all the great replies! Nice to see some real data/graphs on variac ratings.
Sounds like I'll have no problem running the variac a bit harder for my few minute runs. I think I'll go for a 12.5 slow blow fuse.

I think the fundamental problem is with your OCD. If it's limiting streamer length already, then the current should not increase much on ground strikes, since the OCD should limit it there too.

The OCD has worked well in past at lower current settings, even with ground strikes. I think the grounding target was at just the right distance. I bet if I would move the grounding target farther away, and bring the variac to >90% the OCD would kick in and/or fuse would blow without any ground strikes. I think I was right on the edge.

Why not simply bypass the variac once you are up to running voltage

I guess I like the "instant" control the variac provides. If something looks fishy I can quickly turn down the power level.

This question about removing the variac got me thinking about its effect of power factor.
Does the variac provide some PF correction by way of its inductance? Would it's removal cause a decrease in PF? Has anyone measured this?

Which brings up another possible solution: An increase in PF should allow me to run the same power levels without blowing the fuse. I believe a rectified capacitive load gets one about a 0.6 PF (without the variac). There is probably some room from improvement there.

Thanks again,
Josh

Frosty90

Tue Sept 22 2009, 08:27AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

I have pushed a small variac (well) beyond its rating (about 1000w through a 300w variac), and stopped when it got too hot, but for some bizzare reason, after a certain point it began to make a ticking noise....

But it still works (after I replaced the brush), and seems to be fine as long as it doesnt get too hot.

GeordieBoy

Tue Sept 22 2009, 12:42PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> I guess I like the "instant" control the variac provides. If something looks fishy I can quickly turn down the power level.

You could always rig up a limit switch at the end of the variac's travel and use this to switch the variac out of the circuit only when the dial is at 100%. As soon as you move the knob away from 100% the variac would click back in, and you have control again!

As for the ticking noise from a heated variac, it is probably just due to different rates of thermal expansion for the different materials the variac is made from.

-Richie,

Proud Mary

Tue Sept 22 2009, 08:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GeordieBoy wrote ...

As for the ticking noise from a heated variac, it is probably just due to different rates of thermal expansion for the different materials the variac is made from.
-Richie,

Hi Richie, unfortunately we can't hear this sound for ourselves, but I would suspect (a) magnetostriction or (b) (worse!) a thermocouple switching in and out.

I'm must say, I'd always take the case off and find out pronto, rather than trying to guess what is going on inside.

Harold Godwinson

Frosty90

Wed Sept 23 2009, 02:30AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi Richie, unfortunately we can't hear this sound for ourselves, but I would suspect (a) magnetostriction or (b) (worse!) a thermocouple switching in and out.

Well it certainly didn't have a thermostat switch , and it only 'ticked' exactly at each pulse of current (interuped SSTC), so I would say magnetorestriction.

Cheers,
Jesse

klugesmith

Wed Sept 23 2009, 07:52PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Well, transformers do hum due to magnetostriction. But -that- noise source would not increase with load current, in fact it should go down slightly along with the amplitude of magnetic flux in the core. The extra amp-turns in secondary cancel the magnetizing effect of extra amp-turns in primary. As Dr. Slack would be happy to explain at length, AC flux depends only on frequency and applied voltage. The latter is smaller under heavy load, because of I*R drop in the winding.

I suspect the audible ticking is from microscopic motion of the windings or brush mechanism with each pulse of current.
Strictly (no pun intended) speaking, that isn't magnetostriction.

[edit] Found a nice tutorial about transformer noise abatement here:

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