Transformer used for producing Electric Arc

Discussion in 'General Electronics Chat' started by SVS, Jul 10, 2014.

  1. SVS

    Thread Starter Member

    Aug 16, 2012
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    Hi everyone,

    Could someone explain me the working principle of the HV transformers which are used in generating electric arc or Plasma ?? may be with some equivalent circuit and simple explanation?

    How does Leakage inductance help in these kinda transformers ?

    Looking forward for your support.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    When you say HV electric arc, the traditional one is SMAW or Shielded Metal Arc Welding or 'Stick' welding with a typical current of ~200amps, for this the open circuit voltage is around 100vac, this high voltage is required to initiate the arc by striking the work piece, once the arc is struck however the current has to be reduced otherwise the current would be excessive at this voltage, in order to do this, it was traditionally done using a magnetic controlled coupling between primary and secondary, IOW the operator could control the final current by means of an external crank that inserted or withdrew a coupling core.
    At this point the arc current is maintained at a manageable level by collapse of secondary current due to the loose coupling procedure.
    Plasma is usually produced by HV (~200+ vdc) some method is used to initiate ionization of the air/oxygen/inert gas shield at the torch, the plasma arc then transfers from the torch to the +ve referenced work piece which results in immediate and the extremely high temperature melting point.
    Constant Current is maintained throughout.
    MIG or Metal Inert gas is used as a shield for small gauge wire feed welding using a low voltage ~35v DC constant current source.
    Max.
     
  3. SVS

    Thread Starter Member

    Aug 16, 2012
    89
    4
    What about High Voltage and Low current atmospheric Plasma where high voltage is used and current flow is in milliamps range ...
    Please check my attachment..
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am only familiar Plasma cutting systems where a large current is obviously required.
    In the scenario you show, what is the end usage and what does the plasma current flow to?
    I imagine though that the exact same principle is at work?
    At some point the negatively charged plasma source has to return to the positive side of the source supply?
    Max.
     
    Last edited: Jul 10, 2014
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    plasma cutters and tig welders use "high frequency stabilazation" which puts a high frequency high voltage on the electrode to actually jump a spark. he spark is ionized gas, witch is conductive and allows the high current lower voltage arc or plasma to conduct. this is used instead of dragging the electrode across the surface to strike an arc. the high frequency is generated using a high voltage transformer (6kv or so) to jump a spark across a tuned circuit ( think tesla coil) inductively coupled to the welding leads.
     
  6. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Common high voltage transformers use arcs to ignite some sort of fuel, such as found in oil burner furnaces. A transformer (often with a voltage between 6 and 20 thousand volts) creates an arc. The heat then ignites the oil and the furnace starts up. Another example is an automotive ignition transformer, which generates around 20,000 volts to make a spark in the spark plug, which then ignites the gas/air mixture in the cylinders.

    High voltage transformers are also often used to excite noble gases in lighting, the most common of which being neon.

    The operation is very simple--a transformer (with a winding of, say, 1000 turns on the primary and 100,000 turns on the secondary) could take a 100v input and, due to the turns ratio, output 100 * (100,000/1000) = 10,000v on the secondary.

    Obviously this is in an ideal case, and wire resistance, leakage inductance, core type, etc all have an effect on the output voltage and current.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This is the type of typical spark gap ignitor used on most hi end industrial Plasma cutters, the transformer is mains fed (50/60hz) the combination of cap and coil primary form a relaxation osc.
    and it is used for the 'ignition' to occur between the torch nozzle and the gas shield, that is the Torus and GND connection indicated on the attached DWG. then once the plasma 'ignition' occurs, the arc transfers from nozzle to table or work and the ignitor is turned off.
    Max.
    [​IMG]
     
  8. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    The above is the schematic diagram for a spark gap Tesla coil.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes, I just borroweds the picture.;)
    The component layout is the same.

    Official description:-.

    The arc igniter in a plasma cutting system is a High Voltage High Frequency (HVHF) Pulse Generator typically realized using a Tesla coil resonant circuit.
    The ignition voltage pulse is a 5-15kV pk., 2-3 MHz sinusoid lasting for not more than 5-10 cycles.
    It is used to ionize the gas inside the torch thereby producing plasma
    The Tesla coil resonant circuit is the preferred method since it is composed of rugged components like the Tesla coil transformer, spark gaps and capacitors.
    However, the conventional Tesla coil transformer is expensive since it requires a special custom bobbin and the windings are manually wound.
    This also leads to variations in unit to unit performance. In view of this, a planar technology based Tesla coil transformer as a cost effective alternative is proposed. With this approach, there is no need for a custom bobbin or manual winding. This simplifies the build, reduces cost and ensures consistency in unit performance. The design procedure consists of obtaining characteristics of the current transformer and designing the planar version to match those characteristics.

    But I must say I have never seen a planar coil used yet?
    Max.
     
    DerStrom8 likes this.
  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Perhaps I should clarify, I should have said "ALSO the circuit for a Tesla coil". I wasn't trying to correct you :D:p
     
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