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  1. #1
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    Circuit to capture the motor inrush current

    Hi,

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    I am trying to capture the inrush current waveform of a pmdc motor in an oscilloscope. When the actual inrush current value is 75A, we are able to capture only 44A with set up shown below. The artificial network used is a RLC filter circuit. Please suggest any other alternative arrangement for capturing the same.

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  2. #2
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    Re: Circuit to capture the motor inrush current

    Hi,

    Your picture does not show the exact "capture circuit".
    Thus we can't verify whether it is suitable or not. Also we don't know how you read out the peak value. A DVM or an analog voltmeter is too slow, I assume.

    It shows, that is is useful for signals in the nanoseconds up to microseconds range.
    But your motor current peak should be in the milliseconds range. This is off by a factor of 1000.

    Klaus
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  3. #3
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    Re: Circuit to capture the motor inrush current

    Inrush current can be captured with a current probe and a digital oscilloscope.

    It's not clear what you exactly want to achieve and how the schematic is related to motor inrush current.
    Obviously, the inrush current depends on the supply voltage and impedance. You can measure inrush current either with a specific application circuit or under certain nominal conditions, e.g. specified by the motor vendor. How do you set the conditions and why?

    When the actual inrush current value is 75A, we are able to capture only 44A with set up shown below.
    How do you know the actual inrush current?



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  4. #4
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    Re: Circuit to capture the motor inrush current

    A schematic showing boxes identified only with numbers or letters, is not an useful schematic.
    The cable distances 200+/-50, what are the units? Millimeters? Feet? Kilometers?

    A wideband current probe (wideband meaning: not designed for powerline frequencies only) and any half-decent modern DSO will be more than enough.

    Also as Klaus mentioned, because of the motor's inductance, the inrush current will be in the milliseconds range.
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