Current transformer

Discussion in 'General Electronics Chat' started by kubeek, Apr 7, 2008.

  1. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Hi

    I just read the AAC article about current transformers, and I have question. How high can the voltage on secondary side get, when the ampermeter is disconnected?
    And will it be a peak, or continous high voltage?
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    In a current transformer, a shorted or low-Ohm secondary produces flux canceling most of the primary flux. An open secondary removes the opposing flux to the primary flux. Viola! You now have a step-up voltage transformer with an obscene turns ratio. Voltage on an open CT secondary can be several hundred to several thousand volts.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
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    So 0.10 Volts per Amp can be zappy?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    "Volts per Amp?" Is this a wide-band current transformer? If so, what is the size of your burden resistor? Working backward from that will give turns ratio.
     
  5. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Why doesn´t this happen with a regular transformer? The current through the primary should be highest as it is connected without any resistor in series.

    But I guess I am wrong cause it doesn´t happen..
     
  6. subtech

    Senior Member

    Nov 21, 2006
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    Hello kubeek
    Current transformers (like most everything else in our electrical world) have specifications and "nameplate" ratings. Current transformers are usually specified by designers/engineers with a particular application in mind.
    The accuracy required of the CT and "load" (i.e. "burden") that the CT's are to be connected to, will dictate the type and class of CT.
    I'm not familiar with CT ratings outside of the States, but if you will google "IEC Current Transformers" I think you can find an explanation of how CT's will be classified.

    To answer your original post generically, loaded CT's that have their secondaries open circuited will produce an infinite voltage (kilovolts). In practice however, this voltage will be in the hundreds usually due to saturation of the ct core material. I'm sure you already know this but, PLEASE keep mind that many people have been injured or killed outright in accidents that involved CT's. If you plan on experimenting with CT's, educate yourself FIRST, then play safely.

    Hope this helps.
     
  7. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    I still don´t understand how does a CT differ from potential transformer in mechanical construction.
    I think that CT has fewer turns, but that´s all I can guess.
    Will a 1:1 transformer whith one turn on each side be a CT or PT?
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Try to think of it this way.

    There is really no such animal as a current transformer or a potential transformer.
    Classification schemes are dreamed up by teachers but are only of value if the devices concerned can actually all be divided into the classes given.

    Actually there are many types of transformer, differential transformers, impedance transformers, current transformers, voltage transformers, power transformers, pulse transformers, radio frequency transformers and so on.

    Really the name depends upon what use you wish to put the transformer to. Some uses are so common that they have given their names to transformer types.

    But of course all transformers pass current, produce voltage, have impedance etc etc, its just which one is most important.

    Not all transformers have a core though. Some are simply one or more turns wound onto a conductor carrying a large current for the purpose of sensing the current passing through. These are often called current transformers in instrumentation work.

    Similarly when a transformer is used to supply power, the voltage is usually an important criterion so it is often called a voltage transformer or a power transformer.

    Potential transformer is a bit uninformative and my science dictionary suggests this term should not be used, but that

    Voltage transformer=power transformer=potential transformer.

    Final comment.

    Saying PT transformer or CT transformer is about as sensible as saying

    AC current or AC voltage.

    I actually grew up using CT transformer to mean centre tapped transformer.

    Life would be much less confusing if people didn't keep trying to redefine already well defined words. If they need more terms let them invent their own.
     
  9. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Ok, so much about names and definitions. :)

    Could you or anybody else elaborate more on the function of the current transformer?
    Thanks
     
  10. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    anyone? aaa
     
  11. recca02

    Senior Member

    Apr 2, 2007
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    current transformers are used where the current is too high to be measured without being stepped down.
    the ratio of turns is very low on primary side..They step down currents of KA ratings to 1-5 or even .1 A(IIRC)
    for construction details Google Current transformers.
    Another application of CT is to be used in conjunction with relays to form a tripping circuit of protection of transmission buses or instruments/Machines from overcurrent/undervoltage/and a lot other abnormal and dangerous situations and faults(like line to line,line to ground). They step down current to a level desired to be used with the relay so that the protection is achieved.
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    current transformers rely on induced voltaged from the wire they are adjacent to or wrapped around. Their function is to provide you with a sample of the signal on the wire. Some have nameplates that give the specifications typically in millivolts per amp. I've used some that provided .1 volt per amp. These allow you to measure higher currents without placing yourself in danger of contacting the energized circuit.

    I've wrapped a few turns of wire around another wire, making a "current transformer", to detect the presence of a signal. I never did the math to "calibrate" those gimmicks, as I wasn't looking for specifics at the time.
     
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