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From: "Klaus Vestergaard Kragelund"
References: <firstname.lastname@example.org> <3E27B2AA.3F7C851C@earthlink.net>
Subject: Re: Strange behaviour - core saturation puzzle
Date: Sun, 19 Jan 2003 17:07:00 +0100
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Organization: TDC Internet
"Robert Baer" wrote in message
> 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?
> > Thanks
> > Klaus
> 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").
The DC is switched in at each cycle - to let the current rise at a rate
depending upon the inductance at that instance. If the other coil was not
shorted - the inductance would be hundreds of times larger
> 2) You state current ramps up; therefore neither the voltage nor the
> current is constant; ie: *no* C.
Hmm - I don't know what you mean by that. For a coil: dI/dt = V/L. So if
the voltage is DC and the coil is perfect the ramp will be perfectly linear
> 3) You are using a transformer with a shorted secondary.
Yes - se the my other post - that explains why
> 4) There is a fair amount of leakage inductance as well as inter-winding
Yes - exactly - thats what I measuring :-)
> 5) Losses in any transformer core tend to increase as frequency
Also - se the other post
> 6) Just because the current you see as being zero, does not mean there
> is no energy stored somewhere.
The current probe is in series with the coil (coil energy is ½ x L x V^2)
And the duty-cycle is very low - so parasitics should not have anything to
say - but I'll check that current again to make sure it is indeed zero
> 7) A flyback diode will not and cannot absorb all of the energy, and
> certainly does *not* "absorb" voltage!
Absorbe voltage is a bad expression - but it should be able to dissipate all
> In fact, the diode will help the current continue thru the transformer
> winding, building up a magnetic field opposite to the original
> "charging" field.
> 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.
Yes - what I aimed for :-)
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