Transformer spec. "No-load voltage (app. x factor)"?

Discussion in 'General Electronics Chat' started by DickCappels, Dec 8, 2014.

  1. DickCappels

    Thread Starter Moderator

    Aug 21, 2008
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    One of the specifications for a transformer that I am evaluating is the no-load voltage. The spec reads:

    No-load voltage (app. x factor) 1.8

    Its an overload-proof PCB-mounted isolation transformer for 50/60 Hz operation.

    Does anybody here know what "(app. factor)" means?

    Thank you in advance for any help.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am guessing it is no load voltage = 1.8 x full load at rated Kva?
    Max.
     
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  3. RichardO

    Well-Known Member

    May 4, 2013
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    The "1.8" factor sounds high. A PCB mounted transformer will normally have a no-load voltage about 25% higher than the voltage with the specified load. By "app" they may be including the external rectifiers, filters and regulators.
     
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  4. DickCappels

    Thread Starter Moderator

    Aug 21, 2008
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    Its probably that or something close. This transformer has very poor regulation.
     
  5. DickCappels

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    Confirmed. the ratio between voltages when driving a load of 1% of VA rating to that when driving a 100% VA rating load is 1.77. Pretty close to 1.8. :)

    Thank you both for your help thinking about this.
     
  6. RichardO

    Well-Known Member

    May 4, 2013
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    Interesting. I have measured a number of small transformers and get surprisingly close to the 1.25:1 ratio I mentioned.

    I am curious what is different about your transformer. Can you give the part number for the transformer you have?
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I'm guessing a very skimpy core.
     
  8. DickCappels

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    Aug 21, 2008
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    It is a Block AVB 0.35/2/15, a 0.35 VA unconditionally short circuit proof isolation transformer.

    The datasheet says "Coil shell in 2-chamber technology"

    The windings are high impedance.

    Pri #1 6.79 k, 7.61 H
    
Pri #2 8.09 k, 7.63 H
    Sec #1 642 Ohms, 800 mH
    
Sec #2 541 Ohms, 795 mH

    Connected with the primaries in series (for 230 VAC) and the secondaries shorted, the leakage inductance on the primary is 9.1H, which is 2.8 k at 50 Hz. Under this condition the total primary impedance is 15.5k, which I limits the current to 10 to 20 milliamps at 240 VAC.
     
  9. RichardO

    Well-Known Member

    May 4, 2013
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    Studiot may be unto something. I think the "unconditionally short circuit proof isolation transformer" description is telling. I suspect that the inductance or resistance of the windings is intentionally made high to limit the current during a short circuit. This will make the transformer less efficient as indicated by the higher than normal no load/full load ratio.

    Thanks for giving me something to think about... :)
     
  10. studiot

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    I agree, the high impedance, also accounts for the poor regulation.
     
  11. ramancini8

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    Jul 18, 2012
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    The high resistance winding transformer is common in applications where the energy must be limited. The application drives the selection of the transformer, and the transformer performance has to be accepted. P. S. transformers are not regulators.
     
  12. studiot

    AAC Fanatic!

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    ?
     
  13. ISB123

    Well-Known Member

    May 21, 2014
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    Transf. Regulation=Voltage difference in percentage between load and no-load.
     
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