Andy wrote:
On Nov 12, 1:09 am, "Phil Allison" wrote:
"notme"



http://assets.fluke.com/appnotes/1629920_.pdf
Very good description of the In-rush feature. I wish the guy at Fluke
would
have recommended it.
** The link was very hard to find, Fluke's site alluded to its existence but
was not clear on where it was.

Google helped out ....

Thanks for your observations, Phil. You've been very helpful.
** DMMs baffle the masses, it seems.

Mainly cos the name is so misleading.

.... Phil

I recently did some tests on the inrush current on a wound rotor
motor. I used two fluke meters, both with inrush capacity. And I
also built my own circuit using CT, a few resistors, a couple op amps,
and a data acquisition card. The data acquisition card was set to
sample at 1000Hz. I ran the tests by starting the data acquisition,
and then starting the motor. The samples were taken for 1 second.
With the data card I was able to get very good graphs of the
asymmetric starting current. However, the max amplitude of the
starting current measured by the data acquisition card was remarkably
different from that measured by the Fluke. The fluke does not
necessarily see the max waveform. The fluke takes a bunch of samples
in the first few cycles, and then spits out the max of what it
measured. I think the flukes are fine, but it should be noted that
they can be off by quite a bit. In my tests, the difference between
the fluke and the data circuit ranged from a few percent to almost
100%.

Now you've introduced another nuance to the discussion.

Reading the app note it seems that Fluke designed there 33 meters to
read the symetrical currents for motor starting.

But as you mentioned, large inductive loads often have a DC offset
component to their starting current. This comes as an artifact of
closing the starter when the sine wave is not at zero-crossing
(inevitable in a three-phase motor).

You can see this in oscillograph traces, or hi frequency samping such as
your set up. But it's hard to get repeatability unless you have a
zero-crossing motor starter. Each time the motor/transformer is
energized, it's likely to be at a different point on the sine wave.

(some large motor/transformer protection schemes avoid false-tripping on
this in-rush by using various techniques such has harmonic-restraint, or
simple time delays)

I wonder if Fluke deliberately filter out the DC offset just so they
don't have to explain why the reading changes each time you start the
motor :-)

daestrom