On Sat, 28 Jan 2012 19:30:48 -0800, josephkk <joseph_barrett@sbcglobal.net> wrote:>On Sat, 28 Jan 2012 18:12:46 -0800, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>On Sat, 28 Jan 2012 18:01:11 -0800, josephkk >><joseph_barrett@sbcglobal.net> wrote: >> >>>On Tue, 24 Jan 2012 21:47:41 -0600, "vkj" >>><tranquine@n_o_s_p_a_m.n_o_s_p_a_m.gmail.com> wrote: >>> >>>>>On 2/12/2011 11:57 AM, Tim Williams wrote: >>>>>> Actually Jim, that's exactly the problem. >>>>>> >>>>>> A high impedance load will work perfectly. >>>>> >>>>>Only as long as the primary current doesn't drive the core into >>>>saturation. >>>>> >>>>>Sylvia. >>>>> >>>> >>>>Lets not confuse the focus of the issue and consider air cores only, thus >>>>eliminating saturation. >>>> >>>>Lets say your current transformer presents an impedance of 1 milli-Ohm. >>>>Lets say your current is 20A. The prim. voltage is then 20mV. As someone >>>>has pointed out the turns ratio of a typical CT could be 1000:1, so the >>>>sec. open circuit volt is still just 20V. Hardly disastrous. >>>> >>>>vkj >>> >>>Wrong and wrong. The primary impedance applies _ONLY_ when the secondary >>>is terminated with a proper load (resistor). Otherwise it becomes an >>>ordinary voltage transformer, and the primary voltage is no longer in the >>>mV range, but in the volts range, with corresponding kV across the >>>secondary. >>> >> >>If I may borrow a quotation, "Wrong and wrong." >> >>http://en.wikipedia.org/wiki/Rogowski_coil > >Not a "normal" current transformer in case you hadn't noticed. > >?-)We were discussing air-core CTs. -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Current transformer
Started by ●November 30, 2011
Reply by ●January 28, 20122012-01-28
Reply by ●January 29, 20122012-01-29
"John Larkin" wrote in message=20 news:mqg9i7dg6l9s9op7okcvtdi0rtbq4coedg@4ax.com...> On Sat, 28 Jan 2012 19:30:48 -0800, josephkk > <joseph_barrett@sbcglobal.net> wrote:>> Not a "normal" current transformer in case you hadn't noticed.> We were discussing air-core CTs.The discussion started with "ordinary" CTs, but morphed to air-core = Rogowski=20 coils after "vkj" argued that an unloaded CT would produce a useful and=20 non-extreme voltage. Rogowski coils and other current measurement means = such=20 as Hall effect devices are rare in the usual application on electrical=20 systems for current and power monitoring and protective devices. Paul=20
Reply by ●January 29, 20122012-01-29
On Sat, 28 Jan 2012 17:52:15 -0800, josephkk <joseph_barrett@sbcglobal.net> wrote:>On Mon, 23 Jan 2012 22:29:51 -0500, DJ Delorie <dj@delorie.com> wrote: > >> >>"vkj" <tranquine@n_o_s_p_a_m.n_o_s_p_a_m.gmail.com> writes: >>> Why? I dont understand why one cant simply use the open ckt voltage on the >>> secondary as a measure of the primary current. Heres my argument: >> >>Because a current transformer is normally in the 1000:1 range. Do you >>really want to try to measure 120,000 volts? >> >>(also: this is why it's important to hook up the load resistor *before* >> you put the current transformer on the primary wire) > >That is not all, maybe not even the most important. The flux in the core >when operated properly in the CT mode is nearly zero, and if the secondary >is open that is NOT true, and the core could easily saturate. BTW the >relevant IEEE standards make a distinction between metering and >protection/relaying CT and their performance properties in overload. > >?-)The physical implimentation of both current and voltage transformers is a compromise determined by material characteristics and cost. The important thing is that the final physical iteration performs repeatably as required, within a practical budget, with a full understanding of it's limitations under non-optimal conditions. No transformer is actually useful under 'ideal' short circuit or 'ideal' open circuit conditions. RL
Reply by ●January 29, 20122012-01-29
On Saturday, January 28, 2012 6:33:12 PM UTC-8, John S wrote:> On 1/28/2012 7:52 PM, josephkk wrote: > > On Mon, 23 Jan 2012 22:29:51 -0500, DJ Delorie<d...@delorie.com> wrote: > > > That is not all, maybe not even the most important. The flux in the core > > when operated properly in the CT mode is nearly zero, > > Not true. There is flux in the core no matter what. It is what gives > rise to a secondary voltage across a resistor.Irrelevant. The flux is proportional to the voltage drop, both ON THE PRIMARY and on the secondary. Your intent is to have zero voltage drop on the primary, SO all the current transformer design criteria are to make that flux (and d(flux)/dt) as small as practical. To make a CT design truly ideal, it's frequently useful to use op amps to make a zero-ohm (or negative resistance) load for the secondary. Low resistance load resistors are not just a compromise, they are a USEFUL compromise. Don't think they're irrelevant.
Reply by ●January 29, 20122012-01-29
On Sun, 29 Jan 2012 08:23:40 -0800 (PST), whit3rd <whit3rd@gmail.com> wrote:>On Saturday, January 28, 2012 6:33:12 PM UTC-8, John S wrote: >> On 1/28/2012 7:52 PM, josephkk wrote: >> > On Mon, 23 Jan 2012 22:29:51 -0500, DJ Delorie<d...@delorie.com> wrote: >> >> > That is not all, maybe not even the most important. The flux in the core >> > when operated properly in the CT mode is nearly zero, >> >> Not true. There is flux in the core no matter what. It is what gives >> rise to a secondary voltage across a resistor. > >Irrelevant. The flux is proportional to the voltage drop, both ON THE >PRIMARY and on the secondary. Your intent is to have zero voltage >drop on the primary, SO all the current transformer design criteria >are to make that flux (and d(flux)/dt) as small as practical. > >To make a CT design truly ideal, it's frequently useful to use op amps >to make a zero-ohm (or negative resistance) load for the secondary. >Low resistance load resistors are not just a compromise, they are a >USEFUL compromise. Don't think they're irrelevant.Just realize that IxR drop on the copper does not contribute to volt-seconds on the core, (B) , which is why drawing current on a winding of an unregulated voltage transformer, like a wall wart, will actually lower the flux density. boB K7IQ
Reply by ●January 29, 20122012-01-29
On 1/29/2012 10:23 AM, whit3rd wrote:> On Saturday, January 28, 2012 6:33:12 PM UTC-8, John S wrote: >> On 1/28/2012 7:52 PM, josephkk wrote: >>> On Mon, 23 Jan 2012 22:29:51 -0500, DJ Delorie<d...@delorie.com> wrote: >> >>> That is not all, maybe not even the most important. The flux in the core >>> when operated properly in the CT mode is nearly zero, >> >> Not true. There is flux in the core no matter what. It is what gives >> rise to a secondary voltage across a resistor. > > Irrelevant. The flux is proportional to the voltage drop, both ON THE > PRIMARY and on the secondary. Your intent is to have zero voltage > drop on the primary, SO all the current transformer design criteria > are to make that flux (and d(flux)/dt) as small as practical. > > To make a CT design truly ideal, it's frequently useful to use op amps > to make a zero-ohm (or negative resistance) load for the secondary. > Low resistance load resistors are not just a compromise, they are a > USEFUL compromise. Don't think they're irrelevant.Okay. I have on hand a Triad CSE187-L. You can look up the specs, if you like, but here are the pertinent ones: .1 to 30 amps at 50 to 400 Hz Np/Ns = 1:500 (has one turn on the sense, so the secondary is 500 turns) Suggested burden resistor: 60 ohms Sense DCR: 21 ohms I measured the core center leg dimensions as .2" x .2" The secondary winding resistance and the burden resistor are in series. 30A/500 through 81 ohms gives a secondary voltage of 4.86V. My calculations say the flux density is just under 17 kiloGauss at 50 Hz. I don't think instrumentation current transformers run the flux that high because of linearity issues. Also, if an op-amp is used as you suggest, the secondary voltage (as far as flux is concerned) will reduce the flux density to about 6 kiloGauss at 50 Hz and 30A. A vast improvement, but not necessarily ideal. John S
Reply by ●January 29, 20122012-01-29
On 1/29/2012 4:13 PM, boB wrote:> On Sun, 29 Jan 2012 08:23:40 -0800 (PST), whit3rd<whit3rd@gmail.com> > wrote: > >> On Saturday, January 28, 2012 6:33:12 PM UTC-8, John S wrote: >>> On 1/28/2012 7:52 PM, josephkk wrote: >>>> On Mon, 23 Jan 2012 22:29:51 -0500, DJ Delorie<d...@delorie.com> wrote: >>> >>>> That is not all, maybe not even the most important. The flux in the core >>>> when operated properly in the CT mode is nearly zero, >>> >>> Not true. There is flux in the core no matter what. It is what gives >>> rise to a secondary voltage across a resistor. >> >> Irrelevant. The flux is proportional to the voltage drop, both ON THE >> PRIMARY and on the secondary. Your intent is to have zero voltage >> drop on the primary, SO all the current transformer design criteria >> are to make that flux (and d(flux)/dt) as small as practical. >> >> To make a CT design truly ideal, it's frequently useful to use op amps >> to make a zero-ohm (or negative resistance) load for the secondary. >> Low resistance load resistors are not just a compromise, they are a >> USEFUL compromise. Don't think they're irrelevant. > > > Just realize that IxR drop on the copper does not contribute to > volt-seconds on the core, (B) , which is why drawing current > on a winding of an unregulated voltage transformer, like a wall > wart, will actually lower the flux density. > > boB > K7IQYes, and the secondary voltage as well.
Reply by ●January 29, 20122012-01-29
"John S"> > Okay. I have on hand a Triad CSE187-L. You can look up the specs, if you > like, but here are the pertinent ones: > > .1 to 30 amps at 50 to 400 Hz > Np/Ns = 1:500 (has one turn on the sense, so the secondary is 500 turns) > Suggested burden resistor: 60 ohms > Sense DCR: 21 ohms > > I measured the core center leg dimensions as .2" x .2" > > The secondary winding resistance and the burden resistor are in series. > 30A/500 through 81 ohms gives a secondary voltage of 4.86V. > > My calculations say the flux density is just under 17 kiloGauss at 50 Hz.** That means the core is heavily saturated. The makers specs say the unit is not usable beyond 20 amps at 50Hz. http://datasheet.octopart.com/CSE187L-Triad-Magnetics-datasheet-79667.pdf> I don't think instrumentation current transformers run the flux that high > because of linearity issues.** Gobbledegook. Say what you mean and stop being a smartarse. .... Phil
Reply by ●January 29, 20122012-01-29
On 1/29/2012 6:36 PM, Phil Allison wrote:> "John S" > >> >> Okay. I have on hand a Triad CSE187-L. You can look up the specs, if you >> like, but here are the pertinent ones: >> >> .1 to 30 amps at 50 to 400 Hz >> Np/Ns = 1:500 (has one turn on the sense, so the secondary is 500 turns) >> Suggested burden resistor: 60 ohms >> Sense DCR: 21 ohms >> >> I measured the core center leg dimensions as .2" x .2" >> >> The secondary winding resistance and the burden resistor are in series. >> 30A/500 through 81 ohms gives a secondary voltage of 4.86V. >> >> My calculations say the flux density is just under 17 kiloGauss at 50 Hz. > > > ** That means the core is heavily saturated.Yeah, it's what I would consider high and into saturation for most materials.> The makers specs say the unit is not usable beyond 20 amps at 50Hz. > > http://datasheet.octopart.com/CSE187L-Triad-Magnetics-datasheet-79667.pdfNo, they say it is 70% useable. Based on the plot, it only 80% useable at 60 Hz.> > Say what you mean and stop being a smartarse. > > > > .... PhilSound advice, Phil. You should pay attention to your own advice.
Reply by ●January 29, 20122012-01-29
"John S" Phil Allison wrote:> "John S" >> >>> >>> Okay. I have on hand a Triad CSE187-L. You can look up the specs, if you >>> like, but here are the pertinent ones: >>> >>> .1 to 30 amps at 50 to 400 Hz >>> Np/Ns = 1:500 (has one turn on the sense, so the secondary is 500 turns) >>> Suggested burden resistor: 60 ohms >>> Sense DCR: 21 ohms >>> >>> I measured the core center leg dimensions as .2" x .2" >>> >>> The secondary winding resistance and the burden resistor are in series. >>> 30A/500 through 81 ohms gives a secondary voltage of 4.86V. >>> >>> My calculations say the flux density is just under 17 kiloGauss at 50 >>> Hz. >> >> >> ** That means the core is heavily saturated. > > Yeah, it's what I would consider high and into saturation for most > materials. > >> The makers specs say the unit is not usable beyond 20 amps at 50Hz. >> >> http://datasheet.octopart.com/CSE187L-Triad-Magnetics-datasheet-79667.pdf > > > No, they say it is 70% useable.** Utter, smartarse bullshit. All you are any good for. ... Phil
