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Variac wiring

  1. Jun 14, 2009 #1
    hi

    can you tell me what should be the proper wiring for this mains variac?
    input 240V 50Hz

    Once connected, it keeps blowing up the fuse.
    I think it is because of the inrush current, so i want to double check with you, that i have it wired correctly.

    Thanx
     

    Attached Files:

  2. jcsd
  3. Jun 14, 2009 #2
    I've worked with variacs before but I've never seen one like this. For testing purposes you can connect a 220 volt light bulb or two 120 volt light bulbs in series with the primary. This will prevent the fuse from blowing and you can experiment with different connections. If the variac is shorted the bulbs will come on with full brightness even with no load on the secondary.
     
  4. Jun 15, 2009 #3

    berkeman

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    Staff: Mentor

    The first picture shows *very* clearly that this variac expects a 120V input, and can produce up to a 240V output. The diagram at the right of the first picture shows how to hook it up. Do not put in 240V.
     
  5. Jun 15, 2009 #4
    I think that the Variac can be wired for either 125 or 220 volt input, on the terminals in center picture.. The label very clearly shows this. The common for either is the tab with the "0". Using 240 volt input on thr 220 tab might push the iron into saturation. Use the light bulb in series as per above post by Turtlemeister. For the output, I think "S" in the third picture is the slider, and "0" is the common, which is hard-wired to the input common. The label implies that the slider may go to 250 volts with either 125 or 220 (not 240) volt input. The input is rated at 12 A for either 125 or 220 volt input. There is a lot of inductance here, and I get surges when I turn my 1 KVA Variac on.
     
  6. Jun 15, 2009 #5

    dlgoff

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    I don't think S is the slider. Look at the first picture. It seems to me that "0" is the common, "A" is for 125 volts, "S" is for 220 volts, "E" is the earth ground. Don't know where the slider is however; from the pictures that is.

    Edit: After a second look, I'm not sure about "S" now. Why wouldn't it be shown in parenthesis like the other terminals? Is that an arrow I see?
     
    Last edited: Jun 15, 2009
  7. Jun 15, 2009 #6

    dlgoff

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    Now I think it is like Berkeman says. Wired for 125 volt with the slider "S" going up to 220 volts. It is an auto-transformer.
     
  8. Jun 15, 2009 #7

    berkeman

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    I think you might be right, Bob. I've never used a variac like that, putting in a voltage near the top of the autotransformer, but I suppose it may work.

    To the OP -- you can use a resistance meter with the variac not connected to any input power source, to measure the resistance of the various windings, and verify that the slider S is doing what is expected.
     
  9. Jun 15, 2009 #8
    I think my Variac had the option of either having a 0 to 120 v or a 0 to 140 v slider range, depending on how I hooked up the AC. I think there were 3 input tabs; comm, 120 v and 140 v. I built it into a box in 1958, and I don't want to take it apart now (it is still working)..
     
  10. Jun 15, 2009 #9
    It is not uncommon for variacs to have multiple input voltages. The Superior Electric model 236B for example has 120/240 inputs. As Bob has pointed out, putting 240 on the 220 input may be to much and could be what's causing the excessive inrush current. Try connecting 120v to the 125 input. That should give you the same voltage output range but at reduced current on higher voltages.
     
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