Posted Sun Jul 29, 2007, 6:00pm Subject: Is my Variac dead?
I recently picked up a used Variac, one of the red 20A models from China that Sweet Maria's used to sell. I don't have much experience with these things and it seems to be acting strange.
Here's the scenario - I plug it in the Variac with no devices attached, turn the big knob down to zero volts, then flip the switch to On... so far so good. Next, I slowly turn the knob up until the output voltage reads 110.
Here's the problem... as soon as I touch the knob again, the needle on the output meter starts going crazy. It'll drop to zero, then peg all the way up to 150, and back to zero again. Seems like anything attached to it when this happens could easily be fried.
Has anyone else experienced this? Is this thing serviceable at all? I'm guessing it's not...
Posted Sun Jul 29, 2007, 8:03pm Subject: Re: Is my Variac dead?
It sounds like one of the wire windings in the variac is shorting out against another one. That can happen if there's a problem with the insulating shellac that coats the windings.There's also a carbon sweeper that comes off of the center variable shaft on a horizontal arm that sweeps across the top face of the windings when you dial in a voltage.
Posted Sun Jul 29, 2007, 11:15pm Subject: Re: Is my Variac dead?
I think Craig is right. I would look at and hope for the fixable stuff first. The carbon brush may be worn. If you can get an exact replacement that would be prefered. The tension spring may be weak. I think Circuit Specialists (http://webtronics.stores.yahoo.net/ensavat1.html) sells that variable transformer. They may be able to get the brush and the spring. You can also file to fit some other brush. Look to a local motor rebuilder for help there. If the transformer varnish has burned, the transformer is probably not worth rewinding.
Posted Mon Jul 30, 2007, 8:22am Subject: Re: Is my Variac dead?
Thwack,
For what it is worth. I have the same model purchased from SM around 15 months ago. I have not used more than a total of 30 times and have not used it any for the last year. I am willing to part with it. If interested email me at (davidm@cablelynx.com)
Posted Mon Jul 30, 2007, 4:26pm Subject: Re: Is my Variac dead?
Thanks for the replies guys!
Craig and Phil, I took the shell off of the Variac and it appears that the varnish covering the windings is OK. Should the carbon sweeper be in contact with the top of the windings? I noticed that it is on the lower settings, but when it is turned around into the area where the problem occurs it seems to lift off just a bit. Attached is a picture with it in the position where the problem occurs.
Also, the center shaft seems to be kind of loose if I wiggle it, and if I push it down it has some play in that direction too.
Posted Mon Jul 30, 2007, 4:56pm Subject: Re: Is my Variac dead?
Thwack Said:
Thanks for the replies guys!
Craig and Phil, I took the shell off of the Variac and it appears that the varnish covering the windings is OK. Should the carbon sweeper be in contact with the top of the windings?
Nice pic, I was actually Googling one for you., but didn't find a good one. Yes, that's how the voltage is dialed in & acts like a "T" tap to select varying voltages across the windings.
I noticed that it is on the lower settings, but when it is turned around into the area where the problem occurs it seems to lift off just a bit. Attached is a picture with it in the position where the problem occurs.
Also, the center shaft seems to be kind of loose if I wiggle it, and if I push it down it has some play in that direction too.
It should be in firm spring contact/tension with the top of the flattened out windings. Any lifting isn't good & will produce arcing & the intermittent voltage "jump" you experienced. Yes, the cheap ones are not exactly built with the tightest/greatest of tolerances..
Posted Mon Jul 30, 2007, 5:00pm Subject: Re: Is my Variac dead?
If you can find an appliance / vacuum cleaner repair shop, they should be able to come up with a brush that's a close enough fit for you to file down. Barring that , Mcmaster-Carr can sell you a chunk of raw brush stock that can be readily filed /sanded to fit. Look for a shop with an old proprietor!
Posted Mon Jul 30, 2007, 6:52pm Subject: Re: Is my Variac dead?
Success! I loosened the 2 set screws that hold the horizontal arm onto the center shaft. There were 2 copper spring washers under it that needed to be repositioned, then I reattached the arm while applying a little bit of tension. Now there is nice contact between the carbon brush and the windings all the way around.
I turned it on and cranked it up to 120v. Before it would only make it to 110 then crap out.
So, now that it appears I have a working unit, what would you recommend I set it to for my first Variac-controlled roast? I wanted to try a Variac mainly to deliver steady voltage to the iRoast2, not necessarily to throttle it up or down during the roast. I'm thinking to just set it to 110v and go from there.
Posted Mon Jul 30, 2007, 7:27pm Subject: Re: Is my Variac dead?
Thwack Said:
Success! I loosened the 2 set screws that hold the horizontal arm onto the center shaft. There were 2 copper spring washers under it that needed to be repositioned, then I reattached the arm while applying a little bit of tension. Now there is nice contact between the carbon brush and the windings all the way around.
Posted Tue Jul 31, 2007, 12:30pm Subject: Re: Is my Variac dead?
A variable transformer is useful to control the voltage to your roaster if you are trying to acheive a particular temperature vs. time profile, or if the voltage from the line is not what you want. It will not provide more voltage stability on its own. It will provide voltage stability wit you as part of the feedback loop watching the meter and making corrections with the knob.
To establish a good starting voltage I would recomend looking at your line voltage and using that value if you are happy with your previous roasts. If your roasts were too fast start with about 5 to 10 volts less that line voltage.
After you get comfortable with the transformer / roaster combination, I would recomend getting a thermometer probe and expermenting with controlling the roast profile.
Phil
So, now that it appears I have a working unit, what would you recommend I set it to for my first Variac-controlled roast? I wanted to try a Variac mainly to deliver steady voltage to the iRoast2, not necessarily to throttle it up or down during the roast. I'm thinking to just set it to 110v and go from there.
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