Larkinisms
in [Design]
|
From: Tim Williams on 22 Jul 2010 02:05 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:u7ff4619g56kvnjhclcm7v9anmru46fa3h(a)4ax.com... > 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. Ironically, switching power supplies do this. But they have feedback, so that's simply inversion of the feedback network. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Ian Iveson on 22 Jul 2010 07:41 Tim Williams wrote: > 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. ***The two sum to one, obviously, because there is only one core. It's the equal flux that forces equal voltages and *opposite* currents to flow: EMF = -dPhi/dt. ***Opposite? How are the currents opposed? Only in the sense that the flux arising from one is in opposition to the flux arising from the other. Ian
From: Ian Iveson on 22 Jul 2010 08:28 John Larkin wrote: >>> 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. ***Careful. Easy to measure the *rate of change* of flux with a coil. If you try to explain to someone who doesn't know, it's important to be rigorous. > 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. ***Good, although the anthropomorphism may not be helpful. Also, if you wish to explain to someone who is unsure of the relationship between coil current, voltage, and flux density, an explanation that depends on an assumption of that relationship will appear as a circular argument. > At least AlwaysWrong has a new friend. ***Learn to read and think before you squabble. Ian
From: Nunya on 22 Jul 2010 09:13 On Jul 21, 3:49 pm, "Ian Iveson" <IanIveson.h...(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. > > 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 This is on the money. Jefferson's lack of further response duly noted.
From: m II on 22 Jul 2010 09:17
Ian Iveson wrote: > ***Good, although the anthropomorphism may not be helpful. Interesting word. You're assuming that the possession of, or being in, a state of 'stupid' is a purely human trait. I know of a poster here who uses over seventy aliases, with every single one aspiring to be as thick as a brick. Ergo, if a step upwards, to the level of a brick, advances the quality of the state of stupid, we can not claim the condition to be a purely human one and the 'anthropomorphic' label is being misapplied. On a side note, does Mister Larkin really deserve this *obviously* misguided MacCarthyistic attack on his beliefs? I think not. Let's not surrender so easily to the darker side of human nature. mike |