Variac Humming - Is It Safe to Use? - Page 1
Forum section: electronics / beginners

author: K1JOS
author_rank: Regular Contributor
timestamp: February 26, 2014, 08:13:46 pm
content: Hello,I bought a very nice 15A Superior Variac (0-140) and it has worked well for several years in light intermittent use. It has had an intermittent soft hum for brief occasions unrelated to presence of any load or Variac voltage setting. I have monitored AC mains line voltage and appears totally unrelated to any fluctuations or swings. This hum now has gotten much louder and continuous. Is there any safety issue that would caution me to continuing to use the Variac or is it just a noise issue from old laminations?thanks in advancejerry

author: johansen
author_rank: Super Contributor
timestamp: February 26, 2014, 08:20:37 pm
content: is it the carbon brush wearing out? sharpen the point down a bit.

author: K1JOS
author_rank: Regular Contributor
timestamp: February 26, 2014, 09:39:18 pm
content: What could I use as some guide to determine if the carbon brush is worn down? Changing the variac knob (brush also I assume) setting does not lead to any change in hum, with change in voltage. Is there some way of testing for the source of the hum?jerry

author: xygor
author_rank: Regular Contributor
timestamp: February 26, 2014, 09:53:13 pm
content: They usually hum when they don't know the words.Do you recognize the tune?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2014, 10:16:43 pm
content: Do you have any means of measuring power? (Current is one thing, but there'll be plenty of reactive current too...)My 10A variac draws 10W at idle, a reasonable figure for core loss... I forget how much current, like an ampere or something.Tim

author: Psi
author_rank: Super Contributor
timestamp: February 26, 2014, 10:33:58 pm
content: It's normal for a transformer to hum louder as it gets old as the core sections start to delaminate, usually its not a safety problem.I would check the no-load current though, as suggested, to ensure it's not a shorted turn or something more serious.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 26, 2014, 11:23:37 pm
content: Check for any rubber isolation washers/bumpers/etc. that are deteriorating. Also look for loose mounting hardware.Ed

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: February 27, 2014, 12:34:58 am
content: Take the cover off , see if it still hums , if it does , then listen and poke around trying to find some thing that is lose , some type of fastener etc.If you find that its vibrating at the brush and all the fasteners at the brush area are tight .Then remove the brush holder , maybe with some magnification to see if there is signs of arcing between the carbon brush & the brass holder , I have repaired some were the glue holding the brush to the holder after some load melted the glue and it lost contact - no current to run load , in your case it could be lose and transferring current but making intermittent contact creating the hum .For the last 15 amp variac , I priced the brush & holder and they wanted $90 or about $30 if they but new brush in my old holder .I thought that was too much for what was there and I did not want to be with out the use of it for sending mine in and wait to get back .It can be very tricky if it is still stuck like mine , the glue was still holding but no continuity between holder & brush .I was able to get the brush out , without breaking the carbon brush and then clean the old glue off - what remained was a ruff surface on the glued side [ without the glue ] so I filed the notch in the holder until I had a bright surface where the holder had held the brush , then floated a lot of solder and kept it liquid and positioned the carbon brush inplace and let cool , then put all back together and tested with some equipment including a motor .And all is good .Humming in AC equipment , most of the time there is something lose , like a fastener somewhere .

author: Smokey
author_rank: Super Contributor
timestamp: February 27, 2014, 01:41:17 am
content: Humming is probably ok.Arcing sounds are probably not ok.I had a cheap Chinese Variac (the bright red ones... you know what I'm talking about..) and after a while the brush started to wear out and arc a little as you changed the voltage. It charred the coils a bit. Check where the brush makes contact and see if there are any burn marks. The bright side is that it cleaned up and still works once I replaced the brush.