How to record voltage spikes?

[[email protected]]

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

 

[Jamie]

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

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.

Jamie

 

[Bob Masta]

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

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

 

[shrtrnd]

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