On Nov 12, 4:04 pm, "Phil Allison" 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.