Potential Transformer
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From: John Larkin on 19 Jan 2006 22:32 On Fri, 20 Jan 2006 12:49:09 +1100, "Phil Allison" <philallison(a)tpg.com.au> wrote: > >"John Larkin" > >> >> Use a resistibe voltage divider. Or a series resistor into a small >> audio-type transformer, operated in current mode. Or any ole 240-to-6 >> or whatever transformer. You'll probably have to to a phase tweak >> somewhere in the system anyhow. > > > ** Using "any ole" transformer is bad advice. > >Small E core mains transformers have high and non linear magnetising >currents - so the output waveform is distorted and does not follow input >voltage changes linearly. There is often significant phase shift as well. The magnetizing current waveform couples weakly to the secondary voltage waveform. And any electric meter winds up needing some phase tweak somewhere anyhow, as noted. >Small mains toroidal generally have no such issues. What makes a toroid different? John
From: Phil Allison on 19 Jan 2006 22:39 "John Larkin" "Phil Allison" > >>> >>> Use a resistibe voltage divider. Or a series resistor into a small >>> audio-type transformer, operated in current mode. Or any ole 240-to-6 >>> or whatever transformer. You'll probably have to to a phase tweak >>> somewhere in the system anyhow. >> >> >> ** Using "any ole" transformer is bad advice. >> >>Small E core mains transformers have high and non linear magnetising >>currents - so the output waveform is distorted and does not follow input >>voltage changes linearly. There is often significant phase shift as >>well. > > The magnetizing current waveform couples weakly to the secondary > voltage waveform. ** Not true of most small E-cores. Your knowledge of them is sadly lacking too. " Small E core mains transformers have high and non linear magnetising currents - so the output waveform is distorted and does not follow input voltage changes linearly. " > And any electric meter winds up needing some phase > tweak somewhere anyhow, as noted. ** Not much help when the phase shift is varies with applied voltage. >>Small mains toroidal generally have no such issues. > > What makes a toroid different? ** Very low Imag when used within ratings. ......... Phil
From: John Larkin on 19 Jan 2006 22:50 On Fri, 20 Jan 2006 14:39:38 +1100, "Phil Allison" <philallison(a)tpg.com.au> wrote: > >"John Larkin" >"Phil Allison" >> >>>> >>>> Use a resistibe voltage divider. Or a series resistor into a small >>>> audio-type transformer, operated in current mode. Or any ole 240-to-6 >>>> or whatever transformer. You'll probably have to to a phase tweak >>>> somewhere in the system anyhow. >>> >>> >>> ** Using "any ole" transformer is bad advice. >>> >>>Small E core mains transformers have high and non linear magnetising >>>currents - so the output waveform is distorted and does not follow input >>>voltage changes linearly. There is often significant phase shift as >>>well. >> >> The magnetizing current waveform couples weakly to the secondary >> voltage waveform. > > >** Not true of most small E-cores. > > Your knowledge of them is sadly lacking too. > How many electronic electric meters have you designed and sold? I'm running about 5000 so far. >" Small E core mains transformers have high and non linear magnetising >currents - so the output waveform is distorted and does not follow input >voltage changes linearly. " > > > >> And any electric meter winds up needing some phase >> tweak somewhere anyhow, as noted. > > >** Not much help when the phase shift is varies with applied voltage. AC line voltage hardly changes. It's the CT that has the serious phase-versus-current problem, which can be electronically compensated to some extent. > > >>>Small mains toroidal generally have no such issues. >> >> What makes a toroid different? > > >** Very low Imag when used within ratings. > Any core can be run at low Imag. The shape doesn't matter. John
From: Phil Allison on 19 Jan 2006 23:16 "John Larkin" "Phil Allison" >>> >>>>> Use a resistibe voltage divider. Or a series resistor into a small >>>>> audio-type transformer, operated in current mode. Or any ole 240-to-6 >>>>> or whatever transformer. You'll probably have to to a phase tweak >>>>> somewhere in the system anyhow. >>>> >>>> >>>> ** Using "any ole" transformer is bad advice. >>>> >>>> >>>>Small E core mains transformers have high and non linear magnetising >>>>currents - so the output waveform is distorted and does not follow >>>>input >>>>voltage changes linearly. There is often significant phase shift as >>>>well. >>> >>> The magnetizing current waveform couples weakly to the secondary >>> voltage waveform. >> >> >>** Not true of most small E-cores. >> >> Your knowledge of them is sadly lacking too. >> > > How many electronic electric meters have you designed and sold? ** Big noting yourself like this does not work with me. In fact, it makes you look like a pathetic jerk. >>" Small E core mains transformers have high and non linear magnetising >>currents - so the output waveform is distorted and does not follow input >>voltage changes linearly. " >> >> >>> And any electric meter winds up needing some phase >>> tweak somewhere anyhow, as noted. >> >> >>** Not much help when the phase shift is varies with applied voltage. > > > AC line voltage hardly changes. It's the CT that has the serious > phase-versus-current problem, which can be electronically compensated > to some extent. > > >> >>>>Small mains toroidal generally have no such issues. >>> >>> What makes a toroid different? >> >> >>** Very low Imag when used within ratings. >> > > Any core can be run at low Imag. The shape doesn't matter. ** You are quite wrong, yet again, plus missing the point entirely and as usual. The point related to using " any ole " transformer". Remember those words ?? How quickly you do forget. ......... Phil
From: John Larkin on 19 Jan 2006 23:58
On Fri, 20 Jan 2006 15:16:21 +1100, "Phil Allison" <philallison(a)tpg.com.au> wrote: >>> Your knowledge of them is sadly lacking too. >>> >> >> How many electronic electric meters have you designed and sold? > > >** Big noting yourself like this does not work with me. Why would I care how I look to you? What matters is that the money's still in the bank. >>>" Small E core mains transformers have high and non linear magnetising >>>currents - so the output waveform is distorted and does not follow input >>>voltage changes linearly. " >>> >>> >>>> And any electric meter winds up needing some phase >>>> tweak somewhere anyhow, as noted. >>> >>> >>>** Not much help when the phase shift is varies with applied voltage. >> >> >> AC line voltage hardly changes. It's the CT that has the serious >> phase-versus-current problem, which can be electronically compensated >> to some extent. >> >> >>> >>>>>Small mains toroidal generally have no such issues. >>>> >>>> What makes a toroid different? >>> >>> >>>** Very low Imag when used within ratings. >>> >> >> Any core can be run at low Imag. The shape doesn't matter. > > > >** You are quite wrong, yet again, plus missing the point entirely and as >usual. You might be a tad more specific. So how does the shape of the core affect the available Bmax? > The point related to using " any ole " transformer". > > Remember those words ?? Sure. Most any AC power transformer will make a decent PT for metering. Even a little distortion doesn't matter much, if you do the math. Besides, if it's approaching magnetic saturation, and if its primary copper loss is so high that significant secondary distortion results, it will run hot without load. I suppose there may be wall warts this bad, but it would be pretty rate. And this is a school project... it doesn't have to be qualified to ANSI C12, as my meters were. John |