Larkinisms
in [Design]
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From: George Jefferson on 21 Jul 2010 17:24 "Tim Williams" <tmoranwms(a)charter.net> wrote in message news:NvH1o.44762$Ls1.25777(a)newsfe11.iad... > "George Jefferson" <phreon111(a)gmail.com> wrote in message > news:i27dat$vr2$1(a)news.eternal-september.org... >> The magnetic flux density, B, is proportional to i for all >> inductor/transformer configurations. > > Incorrect. Flux is more fundamental than current, at least for the > purposes of transformers. Really? How so? I guess most transformers do not need current to work? > So, how much peak flux is applied by a 120V 60Hz sine wave? What is the > resulting flux density in the core? What is the resulting flux when the > secondary is heavily loaded? > > At any rate, even when using H, you somehow forgot to count the amp-turns > of primary *and* secondary, which is just silly. > > Tim You obivously don't understand the difference between magnetic flux and magnetic flux density.
From: Tim Williams on 21 Jul 2010 18:04 "George Jefferson" <phreon111(a)gmail.com> wrote in message news:i27oi0$ner$1(a)news.eternal-september.org... >> Incorrect. Flux is more fundamental than current, at least for the >> purposes of transformers. > > Really? How so? I guess most transformers do not need current to work? Correct. The current is only a byproduct, and in general it is quite nonlinear. Permeability can be made arbitrarily high, so magnetizing current can be made abritrarily small. A several-kW power transformer made with stripwound permalloy could be driven beyond full operating voltage, into saturation, with a transistor radio. Well, assuming transistor radios had response down to 60Hz. >> So, how much peak flux is applied by a 120V 60Hz sine wave? What is the >> resulting flux density in the core? What is the resulting flux when the >> secondary is heavily loaded? > > You obivously don't understand the difference between magnetic flux and > magnetic flux density. Evidently you never passed highschool algebra. This is the same question as "a*x = 5, solve for x". Are you dismayed because it's not a number, but a function? Tsk tsk... I should know better, trolls don't answer questions... Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Ian Iveson on 21 Jul 2010 18:49 John Larkin wrote: >>> What, George? Is it supposed to be untrue? Explain how. >>> >>> Tim >>> >> >>The magnetic flux density, B, is proportional to i for all >>inductor/transformer configurations. >> >>B = uH, u is approximately constant. >> >>H = N*i/(2*pi*r) for a toroid. >> >>At 0 current the magnetic flux density is 0. Simple as >>that. > What "I" are you talking about? A transformer has two > windings. And > each winding has resistance. > > Take a typical power transformer. Connect the primary to > the AC line > and leave the secondary open. Measure or compute the core > flux > density. Now short the secondary. At least before it > catches fire, > shorted-secondary flux density will be about half of the > unloaded > value. > > Loading the secondary of a transformer reduces flux > density... just > look at the directions of the primary and secondary > winding currents. > It's surprising how many people get this wrong, and think > that a > transformer "saturates" if you short its output. Yes. The flux arising from secondary current is in opposition to that arising from primary current. The two currents are related, so the situation is more complicated than George's Theory appreciates. For the purpose of calculation, it's handy to consider that, for a given frequency, saturation will occur when the effective voltage across the magnetising inductance reaches a particular value. With respect to the primary winding, its resistance is in series with its inductance, so the effective voltage across the inductance reduces as current increases. Because it does so in proportion to the winding resistance, the ratio of inductance to resistance becomes important in mains transformer design. For audio purposes, the mains transformer should arguably have the lowest possible resistance, to minimise regulation under widely-varying load, and highest inductance, to ensure that the core remains within the most linear region of the BH curve, so minimising higher frequency mains harmonics. Such a transformer would, however, be very unforgiving of abuse, and less safe for general-purpose use. I agree that George's Theory is common, perhaps because saturation is normally associated with flow rather than force. In this case, it's the difference between two opposing flows that counts. A perfect transformer with infinite inductance and no winding resistance would be totally transparent to the circuit. The inductance disappears, together with the flux, so it can't saturate no matter how great the current. George's Theory would predict saturation with any current, no matter how small. On a practical level, it's important to realise that the input impedance of a transformer reduces as current is drawn from the output! George's Theory would get this wrong, too. The ensuing extra primary current must be going somewhere other than through the primary inductance, and so cannot cause saturation. All of this becomes apparent, George, if you look at the equivalent circuit of a transformer commonly used for purpose of simulation. It should be a proper road-to-Damascus moment. The primary inductance, in series with its resistance, is in parallel with the output. Consequently, transformed current flowing from input to output has no effect on flux, other than to *reduce* it because of the voltage drop across the inductance, caused by the primary winding resistance. AFAIK most mains transformers are designed so that they are close to saturation with an open secondary. When under rated load Bmax is reduced. Would you expect mechanical transformer noise to reduce, or increase, George, when a secondary load is connected? Ian
From: Tim Williams on 21 Jul 2010 21:30 "Ian Iveson" <IanIveson.home(a)blueyonder.co.uk> wrote in message news:8QK1o.42476$AS4.24234(a)hurricane... > Yes. The flux arising from secondary current is in > opposition to that arising from primary current. Not fluxes. They share identical fluxes, less leakage inductance. It's the equal flux that forces equal voltages and *opposite* currents to flow: EMF = -dPhi/dt. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 21 Jul 2010 23:47
On Wed, 21 Jul 2010 23:49:36 +0100, "Ian Iveson" <IanIveson.home(a)blueyonder.co.uk> wrote: >John Larkin wrote: > > >>> What, George? Is it supposed to be untrue? Explain >how. >>>> >>>> Tim >>>> >>> >>>The magnetic flux density, B, is proportional to i for all >>>inductor/transformer configurations. >>> >>>B = uH, u is approximately constant. >>> >>>H = N*i/(2*pi*r) for a toroid. >>> >>>At 0 current the magnetic flux density is 0. Simple as >>>that. > >> What "I" are you talking about? A transformer has two >> windings. And >> each winding has resistance. >> >> Take a typical power transformer. Connect the primary to >> the AC line >> and leave the secondary open. Measure or compute the core >> flux >> density. Now short the secondary. At least before it >> catches fire, >> shorted-secondary flux density will be about half of the >> unloaded >> value. >> >> Loading the secondary of a transformer reduces flux >> density... just >> look at the directions of the primary and secondary >> winding currents. >> It's surprising how many people get this wrong, and think >> that a >> transformer "saturates" if you short its output. > >Yes. The flux arising from secondary current is in >opposition to that arising from primary current. The two >currents are related, so the situation is more complicated >than George's Theory appreciates. An easy way to measure the flux density in a transformer core is with a probe coil, namely any old 3rd winding on the same core. So get a power transformer with one primary and two secondaries. Connect the primary to the AC line. Measure the voltage on secondary 1 and switch a load on/off secondary 2. If flux density increases with load, the voltage on the sec1 winding will go UP as sec2 is loaded. Of course, it doesn't anything that stupid. It goes down. At least AlwaysWrong has a new friend. John |