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How to record voltage spikes?

Discussion in 'Electronic Basics' started by [email protected], Jun 2, 2011.

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  1. Guest

    We use an inverter to make three phase power from single phase in the
    shop. The inverter has been working quite well, all the CNC machines
    working properly, until recently. Then one of the machines kept
    shutting down because of over voltage to the VFD that drives the CNC
    spindle. It turns out that three of the four filter caps on the output
    of the three phase inverter power supply failed. These caps were rated
    60 mfd +or - 8%. The failed caps measured 17 mfd and were really hot.
    The company who makes the inverter sent us new caps and all seems to
    be well again. Almost. I used a Tektronix 465B scope to look at the
    power coming out of the inverter before and after the caps were
    replaced. The noise on the waveform was reduced by over 75% after the
    new caps were installed. It still looks good. But the machine that was
    getting overvoltage alarms on the VFD spindle drive is getting them
    again. Not as near as often, maybe once or twice a day instead of ten
    or twelve in just a few hours. When I put the scope on the inverter I
    did see voltage spikes but they were fast and I'm not sure if they
    were just artifacts. But if I can somehow document or record these
    spikes then maybe I can find out if they are from the inverter or the
    single phase coming in. Is there some cheap way to do this with a
    computer? Maybe using a the sound card or something?
  2. Jamie

    Jamie Guest

    We get those kinds of errors if one leg of the 3 phase hits ground
    some where.

    I am trying to follow your description and I think you're indicating
    that your spindle drive is a 3 phase VFD ? and you are operating this
    from a shop inverter? In any case, you may want test voltages of the
    three phase output to ground, or check for something that is hitting
    ground off that line.. Most inverters that drive motors will get a
    over bus voltage error because it's picking up a short through something
    and is multiplying the voltage on its internal bus... Also, if you have
    a DB (dynamic braking) Resistor on the drive, check to make sure it's
    still operating because that can cause problems due to the drives bus
    voltage going up
    from back spin on the motor or some unexpected pulse taking place on the

  3. Bob Masta

    Bob Masta Guest

    My Daqarta software can do this, but you will of course need
    to provide isolation if you are going to connect AC mains to
    a sound card. Most sound cards are good for only 5 Vpp or
    less, and you'll want to leave some headroom so you can see
    spikes above the normal level. You might use a simple
    voltage divider after a transformer... maybe a wall wart can
    be hacked for the transformer, if you don't have one handy.

    Note that Daqarta is limited to looking at 2 input channels
    at a time, since it uses "standard" Windows sound cards that
    are stereo only. (Exotic cards with 4 or more inputs
    require special drivers that Daqarta doesn't support.)

    If you need to monitor all 3 phases, you can use 3
    transformers for isolation, then sum the outputs of 2 of
    them together. You'll get a misleading voltage waveform due
    to the phase difference, but presumably any spikes above
    that waveform will still be detectable.

    The next issue is the duration of the spikes. Standard
    sound cards use a 48000 Hz sample rate by default, so each
    sample is 1/48000 = 20.83 usec wide. You'll be limited to
    catching spikes that are wider than this, probably at least
    twice as wide. Some cards can sample at 96000 Hz, but you
    will need to go into Windows Control Panel to enable that...
    by default, they just "fake" the higher rates (the
    assumption is that you are using the card for "entertainment

    Daqarta can show you the "live" waveforms in real time, but
    you will also want to simultaneously record them with the
    DDisk (Direct-to-Disk) option. For this particular use,
    check out the Decimate - Envelope option in the X-Axis
    control dialog (thin unmarked button under the X-Axis
    toolbar button). Toggle the Trigger toolbar button off and
    this will show a slow-scrolling trace (speed set by Decimate

    The Envelope mode means that the trace will show the max and
    min values at each time-point shown. Even if you have a
    high Decimate factor such that many raw samples are combined
    for each time point, you still see the max and min of any
    instantaneous sample peaks.

    If you are recording with DDisk, the DDisk control dialog
    allows you to specify Decimate Lock, which means the
    recording is of the same Decimate Envelope trace you are
    viewing, at a very slow effective data rate (but still
    preserving max.min info) to save file space in case you want
    to record for many hours continuously. Otherwise, leave
    Decimate Lock off and the raw full-speed waveforms will be
    recorded. Then when you go back to analyze them later, you
    can toggle Decimate off and see the original raw peaks.

    I'll be glad to answer any questions. (What will Daqarta
    cost? Nothing, if you can do the job in the
    30-session/30-day trial period.)

    Best regards,

    Bob Masta

    DAQARTA v6.01
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Sound Level Meter
    Frequency Counter, FREE Signal Generator
    Pitch Track, Pitch-to-MIDI
    Science with your sound card!
  4. shrtrnd


    Jan 15, 2010
    Dranetez makes a wide variety of voltage spike recorders.
    Everywhere I've worked, that's what we used to monitor power spikes.
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