digital motor protection

Discussion in 'The Projects Forum' started by shoka, Mar 12, 2009.

  1. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    hello,
    i'm trying to make a digital motor protection.
    i'll measure the 3 phase voltage and current of a motor ,also i'lmm measure the frequency and P.F.
    MY problem is how to interface the voltage to the ADC of the PIC microcontroller.
    i use a transformer 220/15V 0.1 Amp, then i make a voltage divider and a bridge to rectificate the voltage .
    i then put a buffer and a diffrential op amp which doesn't work for AC .
    plz help me
    a file attached with the circuit
     
    • DMP.pdf
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It looks like a lot of elaboration. more is not necessarily better. The transformers' outputs could be rectified with the negative end of the bridge to circuit ground, just as you have it. You want to use the divider after the bridge, though - not between the transformer and bridge.

    Apply the positive output to your divider with the other end to circuit ground. Use the wiper to pick off the voltage range you wish, and apply the pot's wiper to a single op amp. The one extra op you have has its input tied to ground, so it is not going to show a common-mode output (UiA, U2A, U3A).

    The differential amp is operating single-ended as is, so it gives you no added signal processing. All you need is the pulsing DC output from the bridge and divider.

    The divider is oddly made, for that matter. Why not just use one pot? Make the resistance high enough not to load the output of the transformer, like 50 K.

    If you have some reason for the elaborated circuitry, please post up a narration of how and why it works. Perhaps I have missed something.
     
  3. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    u mean connect the transformer to the bridge and then voltage divider then the buffer.
    u also say that i don't need the differential amplifier.
    i understand what u have written ?
    thanks
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I think it will work correctly that way.
     
  5. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    thanks,
    now i want to measure the current , it's about 5A come from a C.T .
    i put a 0.1 ohm resistor in series, a voltage drop of 0.5 volt appears(0.1*5).
    a differential op amp is uses ... will it load a current ??
    a transformer of ferrite core is used with ratio 1/1.
    and then a bridge and a buffer.

    will it work??
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Have you looked into the ring/toroid current sensors?

    An optocoupler after the rectifier, with a large dropping resistor, would allow you to measure the voltage without contact/circuit break as well.

    The above would also allow the unit to be a non permanent installation.
     
  7. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    plz i want to know how to measure the AC current of 1-5 Amp?
     
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