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From: daestrom on 12 Nov 2009 14:36
Andy wrote:
> On Nov 12, 1:09 am, "Phil Allison" <phi...(a)tpg.com.au> 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
From: Phil Allison on 12 Nov 2009 18:04

"daestrom"
>
> 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

** Fraid you have got that all wrong.

The way to *guarantee* very large inrush surges ( with transformers and
transformer based PSUs) is to switch on at the zero crossing of the AC
voltage.

Cos doing this generates the maximum degree of magnetic saturation in the
core.


> 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 :-)


** Fluke make no specific claims about the accuracy of their "inrush surge"
detection circuitry.

But I would not doubt is does the job required, as far as motors and
circuit breakers are concerned.


..... Phil


From: Shaun on 12 Nov 2009 20:08

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:7m3ik3F3fri91U1(a)mid.individual.net...
>
> "daestrom"
>>
>> 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
>
> ** Fraid you have got that all wrong.
>
> The way to *guarantee* very large inrush surges ( with transformers and
> transformer based PSUs) is to switch on at the zero crossing of the AC
> voltage.
>
> Cos doing this generates the maximum degree of magnetic saturation in the
> core.
>
>
>> 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 :-)
>
>
> ** Fluke make no specific claims about the accuracy of their "inrush
> surge" detection circuitry.
>
> But I would not doubt is does the job required, as far as motors and
> circuit breakers are concerned.
>
>
> .... Phil
>
*******I'm afraid that YOU have it all wrong

Switch on power at or close to the voltage maximum on the AC sinewave
creates the biggest transient ( surge current) in equipment with a magnetic
core. Think about it.

Shaun


From: Phil Allison on 12 Nov 2009 20:28

"Shaun"


> In inrush current measurements first off it's a triggered measurement and
> measures for a very short period of time and it doesn't depend on samples,
> I think it's an analog approach.


** I see no sign of thought here whatever.




..... Phil



From: Proteus IIV on 12 Nov 2009 21:26
On Nov 11, 3:11 am, notme <no...(a)notme.org> wrote:
> Fluke clamp current meters have 2 features that seem similar: peak and
> in-rush. The older models have Max (some: Peak). The recent advent in Fluke
> clamps is "In-rush".
>
> How do these differ? Isn't in-rush current the short, max current at
> motor-turn on? Shouldn't meters with a Max feature capture this accurately?
>
> Compare, for example, my old Fluke 36 (Max):
>
> <http://assets.fluke.com/manuals/36______iseng0000.pdf>
>
> and the 334 (In-Rush):
>
> <http://us.fluke.com/VirtualDemos/330shock.asp>
>
> (click "Explore" then "Selection Guide").
>
> How do Max & In-rush differ? Only in the marketing department?
> Or is there a real-world difference?
>
> Thanks,
> Dave

THERE IS NO IN-RUSH APPLICABLE WITH CLAMP ON METERS

WHAT ARE YOU A TROLL FISHING FOR COX TOO ?

I AM PROTEUS
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