Transformer Tester Project (high VA vs low VA)

Discussion in 'The Projects Forum' started by Darksabre, Feb 27, 2012.

  1. Darksabre

    Thread Starter New Member

    Feb 27, 2012
    4
    0
    Hi guys,

    Just joined up to ask you all a question, and hopefully contribute some knowledge in the future.

    I am working on a project that will allow an operator to connect a multi-tap transformer to some test leads and it will step through each primary voltage, and read the secondary voltage. A PLC will be monitoring the voltage to let them know it is good, and to step through each voltage test.

    That's the easy part...there is one tiny, annoying detail that I'm trying to work through. The transformers being tested have dual secondary outputs: one continuous duty 120VAC @ 25VA and one 50% duty 120VAC @ 300VA. It needs to be able to tell if the operator has those two connections switched in the transformer assembly.

    How in the world can I tell the difference between the high and low secondaries?

    I've thought of maybe applying a load on on the transformer secondary that is right at the max VA and see if the voltage drops? But I'm worried about damaging the transformer...and we can't have a tester damaging every transformer it tests :(

    Any ideas at all?

    Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
    16,295
    6,807
    The 25 VA winding will be of smaller wire, thus will have a votlage drop with a 25VA load. Check for that voltage drop and if it isn't there, you must be on the 300va winding.
     
  3. Darksabre

    Thread Starter New Member

    Feb 27, 2012
    4
    0
    Sounds like a plan. That is the direction I was headed, but I wasn't sure. I will try to find a load as close to 25VA as possible.

    I'm just primarily worried about damaging the transformers if the load is too high.

    Thanks for the reply!
     
  4. Darksabre

    Thread Starter New Member

    Feb 27, 2012
    4
    0

    I've got a pretty large load on the transformer 25VA secondary, but the voltage is not dropping. :(

    I have a 390Ω 50W resistor on there, but the voltage is still steady at 121VAC. At this point its pulling way more than the calculated max current of .208A (25VA = 120V * 0.208A). I would have thought it would hit max current, and therefore drop the voltage, but that doesnt seem to be the case for transformers.

    How can I detect if I'm on the 25VA or the 300VA line? Resistence?
     
  5. Darksabre

    Thread Starter New Member

    Feb 27, 2012
    4
    0
    Resistance...of course! the resistance is about 10 times lower on the high VA vs the low VA windings.

    Now how to measure resistance automatically with no voltmeter and a PLC..?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    You measure the transformer output voltage with and without the load. From that you can calculate the winding resistance based upon the load current and the voltage difference.

    Transformers do not have a wall where the voltage suddenly drops. The VA rating is determined by the maximum operating temperature of the windings so the transformer will always deliver current based upon the load and winding resistance until if fails from overheating.
     
  7. #12

    Expert

    Nov 30, 2010
    16,295
    6,807
    The 25VA winding can supply about .2 amps. .2 times the resistance of the winding will cause a voltage drop when the load is connected. This is an absolute. If you can't measure it, you aren't measuring fine enough.

    I don't know what a PLC is. I don't know its capabilities. Perhaps you need another circuit to enhance the effects so the PLC can detect them.
     
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