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Variac

Discussion in 'Troubleshooting and Repair' started by Hopper, Oct 31, 2013.

  1. Hopper

    Hopper

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    0
    Oct 31, 2013
    Good afternoon,

    I inherited this old Variac from my father and after several years have decided to use it. I wanted it to test various voltage fan motors to see if they run OK.

    When I first switched the thing on nothing happened :(
    So I stripped it down and found that the "swipe arm" was not making contact with the coil and the top of the coil, the contact point was very dirty. Before I started any work, I tested the coil and all tappings for continuity - everything fine.
    I spent the next hour cleaning everything up and adjusting the "swipe arm" so that it contacted the coil all the way round - I then measured the resistance right across the coil whilst moving the "swipe arm" - all good, the resistance increased/decreased as expected :).

    I then connected the power as shown below and put my volt meter on the outlet points.

    Wow...everything worked as it should :D for a few seconds........ then the coil overheated and started to smoke!:confused: The coil overheated exactly in line with the "swipe arm", a neat brown line from the bottom of the coil to the "swipe arm" contact!
    I tried the arm in several different positions but every time the same happens!!

    The picture of the "swipe arm" is not mine, it is one found on the net but is almost identical
    What am I missing?
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,395
    1,919
    Nov 17, 2011
    You did use AC voltage, did you?
    Oh, and I think live in should be terminal 4, not 5. Although that probably doesn't matter much.
     
  3. Hopper

    Hopper

    8
    0
    Oct 31, 2013
    Hi Harald,

    Yes, AC voltage was applied.

    I believe that you can connect to various tappings on the coil depending on what "overpower" you are looking for. i.e.. on a 230v ac power supply you get 260v if you use point 5 for example.

    I have read a bit on the net about these Variacs but can not pretend to understand exactly how they work - they appear to be very simple!

    I was wondering if at the contact point on the "swipe arm" there was perhaps a carbon rod missing or worn, would that cause the overheating?
    I say this because some Variacs are described as having a carbon rod contactor - Mine is a flat disk ..not sure what it is made of!
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,395
    1,919
    Nov 17, 2011
    I think you're right there.

    A variac is a transformer with only one winding. The wiper makes contact to different "taps" on the coil to control the amplitude of the output voltage. Therefore the material of the wiper is as a first approximation irrelevant for the operation of the variac.

    You could start by removing the wiper completely. The variac then is nothing but a big coil to the primary voltage. If you apply primary power, nothing should happen. After some time of power-on the coil could be a bit warm from the losses inherent in any coil (copper resistance, magnetic losses). But don't touch the coil with power on!
    You can check the basic operation of the variac by measuring the voltages at the different fixed taps while the wiper is removed.
    Make this test in a dark room and watch out for any signs of arcing (sparks, light effects) - there should be none.

    Now put back the wiper - don't forget to unplug the variac from mains before - but apply no load. The variac should still operate without visible signs (particularly no smoke or brown stripes). Measure the output voltage at the tap. Does it follow the wiper's movement?

    If at any step you observe problems, turn off the variac immediately and disconnect it from mains. Check for any open circuits (broken wires) or short circuits. Clean the whole variac thoroughly and let completely dry before reassembling.

    If these ideas don't help, I'm sorry to say but I'm at a loss.
     
  5. Hopper

    Hopper

    8
    0
    Oct 31, 2013
    Hi Harald,
    thanks for your ideas - I will give it a try tomorrow.

    I'll let you know how I get on.
     
  6. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    A metal disc will short out some turns of the transformer.

    Variac brushes are made of carbon so there is some resistance between turns. This is increased by using directionally made carbon or by using two brushes glued together so the output current will use the two sides in parallel and the sides will be in series between turns.

    A Variac is made very accurately and should never be overloaded or the gold track will be distorted.

    I was lucky to find a disused diddy Variac without a brush. I used some 6B pencil lead which worked without too much excess heat.
     
  7. Hopper

    Hopper

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    Oct 31, 2013
    Hi Duke,
    so you think if my brush is missing or worn then that may be the cause of all the localised heat?
    What about using an old washing machine motor carbon brush? It already has a wire attached and I could file it to shape to fit my current "sweep arm"?
    What damage (if any) will have been caused by the overheating during my testing?
    There are 4 places around the coil where I have sent the wire a little brown - I presume from the varnish on the coil burning!? I was quick enough each time to switch the power off and have checked the continuity - I'm afraid I have no idea what the Ohms readings should be, only that they do change as the "sweep" arm is moved.
     
  8. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    Any resistance readings should be very low.

    As Harald has said, try energising the winding with the arm disconnected. Nothing should happen.

    You could use a second hand motor brush cut to size. I would make it only wide enough to be able to travel smoothly from one turn to the next. A motor brush from an electric motor, running on a copper commutator may be much harder than the proper brush running on a soft gold track.

    You may be able to find a proper brush replacement.

    You have obviously done some damage but if the turns are not shorted or distorted, then it should work. Do NOT overload which can be easily done with unknown loads.
     
  9. Hopper

    Hopper

    8
    0
    Oct 31, 2013
    Thanks Duke,

    I'll have a go over the weekend.
     
  10. Hopper

    Hopper

    8
    0
    Oct 31, 2013
    Had a go this weekend - everything went great.

    First I removed the "sweep arm" and powered the unit - No overheating at all.
    I then examined the arm to find the remains of a small (2.3mm) carbon rod in the centre of what I first thought was the contact plate. The picture posted at the beginning of this thread is what made me think there was something missing!

    The carbon rod was a push fit so I made a new rod from a piece cut from an old washing machine motor brush. I turned it circular and to the correct diameter on my lathe (you could use a drill if you don't have a lathe). Refitted it and "hey presto"!

    Everything now works perfectly - I still don't fully understand why the introduction of a tiny carbon brush stops the coil overheating but ....... it does!
     
  11. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    There is about 1V between the turns on the Variac and so if two turns are connected together, a very large current will flow. Going across more than one turn will make the matter much worse.
    The small brush is only wide enough to be able to step from one turn to the next and has a significant resistance to limit the current. I think special directional carbon is used for the proper brushes.
     
  12. Hopper

    Hopper

    8
    0
    Oct 31, 2013
    Hi Duke,

    That makes sense, using the whole surface of the metal mounting plate would have made the contact point around 8 or 10mm - no wonder it got hot!
     
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