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Re: Mains Current Monitoring
Tesla List wrote:
>
> Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
>
> Thanks Marco,
>
> > Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> >
> > For complete data (PDF format) about all Philips soft ferrites materials, I
> > recommend this address:
> >
> > http://www.passives-dot-comp.philips-dot-com/cgi-bin/hwhsql/sql/euprod.htm?518
>
> Actually, I have their databook MA01. I've been designing switchmode
> transformers for quite a while now.
>
> > Basically you should keep yourself below 150 mT (1500 Gauss) and you can be
> > sure it won't saturate.
>
> Depends on the ferrite grade but in a lot of apps you can do
> reasonably well up to 250mT. I was simply curious to know why ferrite
> is considered by some to be a non-starter for current transformer
> applications. In fact it is used in those applications at high
> frequencies. I suppose the short answe boils down to :you need rather
> more ferrite mass than iron mass at 50Hz. But I thought the object of
> the exercise was to measure the oscillating primary currents, not the
> charging current or am I missing something? I know the core *must* be
> ungapped for this to work correctly.
>
> Malcolm
> <snip>
Malcolm, Marco, all:
One thing to keep in mind is that the excitation current of
the core, whether ferrite or steel, determines the accuracy of
the current transformer. The higher the excitation current,
the less accurate the measurement. The larger the amount
of core, the larger the excitation current needs to be to
excite the core to a given flux density (known as excitation
VA per pound). At 50/60 Hz, I would recommend a steel core
designed to run at a low flux density. One can easily achieve
1% accuracy using this method.
David L. McKinnon
D&M's High Voltage