Envelope-to: dave@picks.force9.co.uk
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From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
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Date: Thu, 15 Sep 2005 07:48:19 +1200
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Subject: LF: Re: Current Transformers (off topic, but expertise probably here)
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Andy G4JNT,

Having a "bend" in the transfer characteristic that changes with level 
certainly means some non-linearity.  If you look at spectra (in the 
frequency domain) instead of RMS level (in the time domain), you may observe 
what harmonics grow as the level is increased (assuming things get worse 
with drive level).  That would be one way to evaluate nonlinearity, but as 
you want to use it for measurements then it would be irritating to need to 
correct for self-distortion in the probe.  One possible explanation is that 
magnetostriction dynamically modulates the (micro) air gaps between EI 
laminations?

Current transformers I have seen are invariably of toroidal construction. 
That suggests that leakage reactance is part of the quality of making a good 
current transformer, but leakage is "linear" and that alone would not 
explain why you have a "bend" in the transfer characteristic using an EI 
core.  A toroidal core would seem to be less prone to magnetostriction 
affects?

Have you got a toroidal transformer to try?  There is no need to strip off 
turns as it is fairly easy to arrange a 1 turn primary when the toroid has a 
decent internal diameter ;-)

73, Bob ZL2CA