- #176
jim hardy
Science Advisor
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dang it whole post disappeared **!@@@{(*TYEW$^#%#! I HATE (PRESENT PARTICIPLE ) WINDOWS
back to the picture
were we using this one ?
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That's alright, you only left off "23.72V" under the 70V on your load line picture didn't you? I think I get that, if so.jim hardy said:i guess i should add volts across load to the curve ?
Thanks.tim9000 said:I feel we're making progress...you'd better read this whole post through before putting anything to paper, I've been editing it all bloody day.
tim9000 said:That's alright, you only left off "23.72V" under the 70V on your load line picture didn't you? I think I get that, if so.
tim9000 said:Now would probably be a good time for me to pester you about explaining what you meant by:
" volt-seconds per turn defines a core's flux capability
volt seconds defines the ability of a winding on a core to not saturate when asked to block current"
I'm not sure I completely get that...
H'mm, I'm just thinking 'the more turns you have the less current it takes to saturate the core' but I also understand inductance proportional to n squared. Well looking at that equation I'm assuming dt is just the amount of time so far, because e is a constant so it's just like ∫dt, the flux will ramp up at the gradient -e/njim hardy said:If it had only one turn,
volt-seconds applied to that one turn would be the flux at which it saturates
e = -n*dΦ/dt
Φ = -1/n * ∫edt , n=1
when it saturates it can no longer block current
if n>1 it can hold off more volt-seconds before it saturates and impedance collapses
Ah, that is actually a really good idea I hate reading off a screen,jim hardy said:I printed out your last post, took 3 pages, but that way i can study it without flipping screens which wears me out.
Yep, they were, and yep they did change, you'll get to that part.jim hardy said:Incandescent lamps change resistance about tenfold cold to hot so i'd have expected more like 100 ohms cold - maybe your 40 was both in parallel ?
tim9000 said:I should have put up more precise data before, I would have if I had of known I'd struggle the concept so much, since we need it I'll include some better numbers than eye-balling the graph. (sorry I realized I was doing this for 70V pk, not pk-pk)
tim9000 said:I do however think I understand the load-line idea and I really like it...with a 'but':
SO what you said was for those two series 200 Turn coils when we excite with 70 V pk-pk at 50 Hz, that will get us with a peak flux density B at the knee point, with a pk-pk current of around 110mA, when no DC control, telling us the impedance of the Amp + bulbs = Zbetween knees = 70 / 0.11 = 636 ohms. I completely agree with that method.
tim9000 said:But say I want to go past the knee point t 50V*√2 = 70V pk = 141 V pk-pk.
At 70V rms excitation with zero DC I got 67.3V rms across the mag Amp and only 66mA rms through the load. So I think it can hold off more current than you estimated:
Yes , it seems to hold off 70 volts RMS pretty wellSo I think it can hold off more current voltage? than you estimated:
tim9000 said:Say we were using 70V rms. At a VDC = 424mV (not shown below) then you would get the 23.72 V across the mag amp. So I think the impedance collapses more than you reckon, see:
tim9000 said:Looking at my data for 50Vrms*√2 = 70V pk, (which is less extensive data than at 70V rms data set):
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The supply would fluctuate a bit, you can see as VL + V3 are sometimes greater than V1 (VL + V3 = V1 ideally). You can see that the voltage across the amp went as low as 775mV rms and the hold off current was as low as 38mA rms! (not bad eh?)
I take it that's 0.038rms*2√2 = 107.5mA pk-pk.
Great, thanks, because I really want to be able to generate a load line and/or inductance line, for the impedance or inductance of the mag amp, Vs. control voltage.jim hardy said:Then i'll try to figure out how to add DC to it...
tim9000 said:And on that note of you saying Z is the slope of the redline intersecting Zero to the curve (post #185), I thought you said that it was the tangent gradient to a point on the curve?
(you can see in the data of the curve cut-&-pasted in post #184) Certainly looks like it has a positive second derivative at first, if I had to describe it I'd say it has a 'less slow liftoff than the mumetal, and that the liftoff was more prolonged' so that the transition looks smoother than with the mumetal.jim hardy said:As of this minute I'm puzzled by rising behavior between 0 and ~35 ma
do you think your core might have a slow liftoff like this mumetal (post 52) ?
for the 200 Turn and the (corrected) 840 turn impedance:tim9000 said:![]()
Also please see the Edit to my last post.jim hardy said:wow I'm going to have to print your data
it's almost 2am here , i think I'm about done for the day
back tomorrow
jim
tim9000 said:Also could you explain to me again why it's the tangent to the curve, and not just the value of V/I ?
from 184...tim9000 said:(you can see in the data of the curve cut-&-pasted in post #184)
184 said:I can't even really remember what I did the other day that well, but from memory I ended up using measured current, to calculate the impedance, but instead I wanted to come up with was, was very much like you're curved load-line: some sort of impedance curve VS. DC control, for a set excitation voltage, determined by the BH curve, to from that determine the load current.
I suppose that sort of mag amp load line would look something like this
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tim9000 said:made from the BH curve...
The data of one of the V I curves used in the plot was:
Energising one half small coil V:
0.8
2
5
10
13.4
15.2
20
23.77
28.2
32.2
33.99
35.15
37.4
39.17
40.22
Corresponding current:
I (mA)
2
4.8
10
17
21
23.2
30
36
44.4
57
71.5
85.5
150
260
390
Glad to have you back safe and sound...and with a new driver, though I'm not sure I get the 'silver hammer' remark.jim hardy said:After a dozen false starts I'm back. Sorry, just couldn't settle on a direction and got frustrated at myself.
A new touchpad driver saved this computer from Maxwell's Silver Hammer...
Bang! Bang! Maxwell's silver hammer
Came down upon his head.
Bang! Bang! Maxwell's silver hammer
Made sure that he was dead.
Glad I asked, I just youtubed it. Yeah it was alright. I really only like 'a day in the life' and 'dear prudence'.jim hardy said:Maxwell's Silver Hammer is a weird 60's Beatles song
i think it's about how life seems to beat on us with its unexpected events
okay will look after breakfast..jim hardy said:[just moving it to this thread...
QUOTE="tim9000, post: 5208207, member: 480143"]No the current doesn't, I know the voltage will start to move from being over the ideal TX to be over the resistance of the coil (as the current spikes), so V.s on the core won't be sineusoidal as the core saturates. But you can see from the graphs in post # 198 that the shape of the N.s/N curve is identical to the BH curve, so what does that mean?
Also, I don't see why B = Vrms/(2*π*f*N*Area) From the EMF of a TX equation (which is essentially Faraday's equ.)
But if: Φ = V.s/N
than why V.s/(N*A) = Vrms*√2*2*/(N*π*50*Area) = VPK*2*/(N*π*50*A) isn't equal to B?
159.256 is soooo close to 50Πtim9000 said:After calculating V.s/N as (Vrms*√2*2) / (π*50)
the V.s/N is smaller than B calculated via EMF equ. By a factor of 159.258 (top picture)
tim9000 said:Just to double check, V.s = V/(2*pi*freq) right??
Why not just V/Hz?
Because time period = 1/f
so V/f would be [V.s]
I have to check. I remember so little I'm condemned to a life of figuring out basics.Just to double check, V.s = V/(2*pi*freq) right??
When I do V.s/N = 2*√2*Vrms/Njim hardy said:159.256 is soooo close to 50Π
it's got to be simple.
In addition to not knowing teh causality.tim9000 said:When I do V.s/N = 2*√2*Vrms/N
it equals B!???
I was doing V.s/N = 2*√2*Vrms / (N*π*50)
which was 159 times bigger.
What the hell is going on?
Hmmm maybe there's not such a fundamental difference after all, provided we are mindful of what our meters are telling us.tim9000 said:And I'm still confused as to why Z = ΔV/ΔI at a point, and not just V/I (which is the gradient from zero to point on curve)
i.e. what the difference in compared methods of plotting Z curve in posts #190 and #192 between tangent Z and direct Z = V / I
But even fundementally, even if we had perfect equipment, I still don't understand the tangent slope, or the red line slope, what the difference between the two means, or rather, specifically why the tangent to the curve is V / I, any more than the red line is Z.jim hardy said:Hmmm maybe there's not such a fundamental difference after all, provided we are mindful of what our meters are telling us.
Here the meters will report volts and amps , and below the knee volts and amps will both be decent sinewaves.
Impedance should be pretty linear below the knee because the red and blue lines don't diverge very much hence volts/amps should give fairly constant Z.
Beyond the knee current increases rapidly., the red and blue lines diverge , so what are our meters reporting?
The ammeter is now handling a very nonsinusoidal wave
voltmeter still sees a sinusoid provided there's no load impedance , which is how i think you (dammit italics stuck on again) took your readings
so volts/amps gives a number that's related to imedance . Impedance is some weighted average of the slopes along the blue line
but we can't see that relation with just meters
so we work with what we have, bearing in mind we're limited by using meters rather than oscilloscopes.
I always have to go back to basics , that's why i;m so slowtim9000 said:When I do V.s/N = 2*√2*Vrms/N
it equals B!???
I was doing V.s/N = 2*√2*Vrms / (N*π*50)
which was 159 times bigger.
What the hell is going on?
tim9000 said:In that last post, there is an implicit understanding of mine that flux = V.s/N and I'll throw some numbers in for colour: Take the knee point flux density;
Vrms*√2*2 / (π*f*A*N) = 296*√2*2 / (π*50*0.00111*840) = Vpk*2 / (π*50*0.00111*840) = 5.7163 T
But we previously determined using the EMF equ that the knee point was about 1T
tim9000 said:last point before I go to bed, I was thinking I should be able to get a load line of the mag amp using the inductance relation:
Z = 2*pi*freq* N2*A*µ / length = 2*pi*freq* N2*A*(B/H) / length
that's Zmagamp , 2pi X freq X inductance
which would have the same shape as the µ curve, which is proportional to the gradient of the B curve. ?? It's just a number not a curve unless you make something a variableBut what has been irking me is that I can't seem to figure out (probably because I dont' understand Volt-seconds well enough)
volt-seconds is simply what the integral vdt evaluates to given some starting and ending point,
it has the subtlety (fortuitous quirk if you will) that it happens to numerically equate to amount of flux induced in an inductor that's been subjected to that voltage. I think they're not so important for AC operation excpt to understand why saturation occurs.
how to get some sort of impedance per turn value curve for the core. Like a load line derived from the BH curve, to give me the impedance of the core at a specific flux density, then I can just work out what the impedance of the core will be for a given control DC current.