measuring current waveform

Discussion in 'The Projects Forum' started by abhishek3330, Mar 19, 2013.

  1. abhishek3330

    Thread Starter New Member

    Mar 19, 2013
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    I have to differentiate between inrush current and fault current of power transformer, for that i have to check the current waveform of both.
    The only measuring instrument i have is CRO so please tell me how to observe current waveform on CRO, starting from how to connect probe.
     
  2. ramancini8

    Member

    Jul 18, 2012
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    Inrush current will always be accompanied by rising input current. Fault current will usually not be accompanied by rising input current. Use a dpdt switch for power on; half the switch for tx power, and half the switch to trigger the scope. When the input power is on detect the rise in current to see the fault current. This is easily accomplished with a recorder.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    If the transformer is line powered, you need an isolated current sensor such as a Hall effect device or a current transformer to safely observe the current. Current transformers are the most commonly used device for this. Never try to measure line current directly with an oscilloscope.

    Alternately you could use a power line isolation transformer to power the transformer under test. That will reduce the hazard of directly measuring the current with a shunt resistor. But you still have to be careful of the high voltages present.
     
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  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Since the scope return is (typically, do check first!) the AC neutral you could put a burden resistor in line with the scope and see the current.

    If you need any more detail then that you don't have the experience to accomplish a dangerous measurement, so that is all I will say. Sorry.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    Sorry but that is a dangerous thing to do and I most emphatically state that it should not be done, even by an experience person (I'm surprised you recommended that :eek:) The ground and neutral can be inadvertently switched and you can get much spritzen sparks with dangerous consequences. Don"t do it.
     
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  6. abhishek3330

    Thread Starter New Member

    Mar 19, 2013
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    thank you all

    @CRUTSCHOW
    can i connect CRO probes to current transformer secondary.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    See if you can borrow/buy/"demo" a current probe for your scope.

    They are contact free and simply clamp around the insulated wire. Price isn't cheap, however. If you have a "big name" scope, such as Tek, Agilient, LeCroy, contact the dealer to see if you can "demo" a current probe if you only need it for one use. This may work out well if you have an off brand scope as well. You then get one for 1 week to a month or more in the hopes of you liking it a lot and buying one. :)

    You can connect to the secondary without a problem, it is isolated from the primary, and hence, ground/neutral of your power system. Inrush is typically on the primary side, though. A 500W 1:1 power isolation transformer can be had for about $125 (far cheaper than a current probe). If you work with line powered devices and a scope often, the usefulness and safety of an isolation transformer can't be understated. A Variac is also nice, but they often aren't isolating, so you'll need both if wanting to tweak voltages to simulate droops and peaks.
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    If one intends to use a standard toroidal core current transformer to measure line currents [such as one would use with a current indicating panel meter] there is usually a caveat that strongly recommends that the user terminate the CT secondary in a burden resistor of suitable value. This reduces the likelihood of the user receiving a shock across the unterminated CT secondary terminals due to the high induced voltage likely to occur with primary current flowing.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Yes. A current transformer secondary is isolated from the primary. That's why they are used.
     
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