AC transformer 500:5A

Discussion in 'General Electronics Chat' started by superway, Dec 10, 2013.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    Hello,

    I am looking for an AC current transfomer 500:5A on omega.com website.
    There are 2 versions of this product, 500:5A with Burden(VA) 15, and 500:5A with Burden 7.5.
    What is the differnce between these Burden value (VA)?

    Thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,365
    Believe it or not, 7.5va :eek:
    Max.
     
  3. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    I will install 100 Ohm, 25Watt accross to secondary of CT and then monitor AC current on Scope. So is Scope a VA load? What O'scope VA rating? can I use 7.5VA or 15VA CT?
    Thanks
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
    2,365
    VA, Volt x Amp is the power rating of the device, and is directly proportional to any ratio change, as in a transformer secondary etc.
    You need to state the voltages you are working at.
    If you placed a 100Ω across a supply to draw a maximum of 15va
    Or e.g. a voltage of 38.75v with the 100Ω load will be .3875 amps or 15va..
    Max.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    The burden VA determines the maximum load that can be connected on the current transformer output as determined by VA = I^{2}*R. Thus for 7.5VA and a 500:5A transformer the maximum load (burden) resistance is R = VA/I^{2} or R = 7.5/25 = 0.3Ω (thus you can't use the 100Ω resistor you selected). This gives a maximum output voltage of 5A * 0.3Ω = 1.5Vrms. You can do a similar calculation for a 15VA maximum burden.

    Note that this resistance includes any lead resistance at the transformer output.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Current transformers do not care about the primary voltage, only the current. The only voltage of concern is the secondary voltage across the burden resistor as determined by the VA burden rating.
     
  7. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    Thanks.
    For this CT rating 500:5A, 5A is maximum on secondary rating as your calculation and required 0.3 Ohm.

    But for my product application, a maximum 40 Amp is running through CT primary and the secondary will be produced 0.40A ( 500:5A).

    So VA = I^2 x R = 0.40^2 x 100 Ohm = 16 VA, is it correct?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    3,220
    No. It's the maximum output voltage that's critical since core saturation is determined by the voltage and frequency, not current. For a 15VA transformer, maximum output voltage would be 15VA/5A = 3V. Thus, for a maximum primary current of 40A, the maximum output burden resistance would be 3V/.4A = 7.5Ω
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    FWIW-

    Never have current flowing thru the primary side (ie-the "hole") of a CT with an open secondary ckt. This is like having a burden resistor of ∞Ω. Sparks will fly!
     
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