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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Strange behaviour - core saturation puzzle
Date: Fri, 17 Jan 2003 07:37:09 GMT
NNTP-Posting-Date: Thu, 16 Jan 2003 23:37:09 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Klaus Kragelund wrote:
> I've been doing some common-mode coil saturation tests:
> One side of the coil is shorted and on the other side a DC voltage is
> applied. The current ramps linear up until the core saturates at which
> point the coil di/dt will rise significantly due to the reduced
> permeability. Taking the data from the scope - doing some number
> crunshing will allow me to plot the inductance as a function of the
> coil current.
> My problem is that the test is done perhaps once a second and the
> saturation current reaches some level - lets say 10amps. But if I
> increase the frequency of the test - the saturation level moves down
> to perhaps 8 amps. Does anyone have an explanation for this?
> I've allready checked the current when the ramp starts - and it is
> zero - so there is no energy stored in advance. Moreover I have a
> flyback diode accross the coil to absorbe the flyback voltage.
> Perhaps some flux imbalance from the hystersis curve?
There are a few "minor" problems:
1) If you truly have DC on one winding, the state of the other winding
is not relevant ("shorted").
2) You state current ramps up; therefore neither the voltage nor the
current is constant; ie: *no* C.
3) You are using a transformer with a shorted secondary.
4) There is a fair amount of leakage inductance as well as inter-winding
5) Losses in any transformer core tend to increase as frequency
6) Just because the current you see as being zero, does not mean there
is no energy stored somewhere.
7) A flyback diode will not and cannot absorb all of the energy, and
certainly does *not* "absorb" voltage!
In fact, the diode will help the current continue thru the transformer
winding, building up a magnetic field opposite to the original
You most likely will see a partly damped "oscillation" if the duty
cycle is low.
8) That shorted winding will reflect a rather low impedance (a short if
the transformer were perfect).
You are probably driving the leakage inductance of the transformer.
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