How to record voltage spikes?

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? Thanks, Eric
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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 line.


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Record voltage by blowing fuses that are connected to 2 resistors in a voltage divider.

v1 =3D v2 ri/(r1+r2)

.25 amp fuse =3D v1/r1

v2 =3D 200v

v1 =3D 20v

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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 only").

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 factor).

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

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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!

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Bob Masta

Look for an AC Line Monitor by Dranetz.

These devices actively monitor voltage, and they do have 3 phase cards which will monitor all three phases. Look on eBay... they've been around for years.

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