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From: Andy on 13 Nov 2009 07:35
On Nov 12, 4:04 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote:
> "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

Shaun is correct in his assessment of inrush current being at a
maximum if the breaker is closed at a zero crossing of voltage.
Because the magnetization current is inductive, the current lags by 90
degrees. So, if the breaker is closed at V=0, and current is lagging
by 90 degrees, then this timing corresponds to a max value of current.
From: Archimedes' Lever on 13 Nov 2009 21:09
On Thu, 12 Nov 2009 18:26:26 -0800 (PST), Proteus IIV
<proteusiiv(a)gmail.com> wrote:

>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


The meter READS the inrush signature, you idiot. Not create it.
From: ehsjr on 14 Nov 2009 18:11
Paul G. wrote:
> On Sat, 14 Nov 2009 08:07:37 +1100, "Phil Allison" <phil_a(a)tpg.com.au>
> wrote:
>
>
>>"Shaun"
>>
>>>I looked up inrush current for transformers and I stand corrected, it's
>>>maximum will occur at the voltage zero crossing point. I thought it was
>>>the same as an L R circuit in which case if switch closes as the peak of
>>>the ac waveform it causes the maximum transient.
>>
> A bit off topic....
> At first thought, it seems counter-intuitive that zero-crossing is
> the worst case for inrush current,

To make it more intuitive, consider that starting at 0 crossing
provides more time charging the inductance, so higher I.

Ed

so I used LTSPICEIV to simulate the
> situation. As usual Phil is correct! here's the .asc file for the
> simulation, note that you must set the inductor current to zero for
> the initial condition.
From: Phil Allison on 14 Nov 2009 19:05

"Paul G."
"Phil Allison"
>"Shaun"
>>>
>>> I looked up inrush current for transformers and I stand corrected, it's
>>> maximum will occur at the voltage zero crossing point. I thought it was
>>> the same as an L R circuit in which case if switch closes as the peak of
>>> the ac waveform it causes the maximum transient.
>>
> A bit off topic....
> At first thought, it seems counter-intuitive that zero-crossing is
> the worst case for inrush current, so I used LTSPICEIV to simulate the
> situation. As usual Phil is correct! here's the .asc file for the
> simulation, note that you must set the inductor current to zero for
> the initial condition.
>
( snip listing)
>
> In the sim above, is a 1H inductor in series with 5 ohms, with
> 110vac (160v peak) applied. The voltage is set to start at zero
> crossing.

** Your simulation is of an inductor - and NOT a AC supply transformer
primary as the question requires.

The differences are many and great and the switch on transient behaviour
very different - mostly because a transformer's laminated iron core will
saturate hard when a frequency just a little lower than it is designed for
is applied.

I doubt that LTSPICE IV can even do such a simulation.

Its why I said to TRY it !!


> Phil has pretty good advice, but he sure gets irritated if you
> don't know what you're doing. You'll get absolutely nowhere biting him
> back, unless you can decently support what seems to be your mistake.

** Yep - there is nowhere to stat demolishing a fallacy if no sensible case
is posted that supports it.

Posting inane drivel like " .think about it " or " I am surprised you don't
know this " is just pouring fuel on the fire.



...... Phil




From: Phil Allison on 14 Nov 2009 19:21

"ehsjr"
>
> To make it more intuitive, consider that starting at 0 crossing
> provides more time charging the inductance, so higher I.
>

** But we are discussing the behaviour of a AC supply TRANSFORMER - where
core saturation IS the cause of inrush current surges.

The simplest way to think about it is to first note that the core of nearly
any AC supply tranny operates on the edge of saturation ALL THE TIME when
mains power is applied.

( Proof of this is that if one raises the applied voltage by about 20% or
lowers the AC frequency by the same - the magnetising current drawn by the
tranny will dramatically increase. )

The note that switching the supply on at a zero crossing means the *average
value* of the wave will not become zero until a whole cycle has passed -
while switching on at a peak means only half a cycle need pass for the same
result.

The second case produces a very much smaller inrush surge as a result.

Interestingly, if the AC voltage applied to a supply tranny is half or less
its rated operating voltage - inrush current surges barely exist.


...... Phil





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