Electromagnetic Theory

Tesla23 · Jun 30, 2008

arrie said:
Sorry to barge in on your topic, but it's kinda related.
Here follows a quiz, one I'm not quite solving yet:
So you connect a 220V 125Hz supply across a coil, the current in the coil reaches 692mA after 3.2ms, and then the circuit is switched off, probably using a switch between the supply and coil - hehehe......
What would the final steady value of current be?
Any educated guesses?

The final steady state current for any open circuited coil is zero. All the rest of the information is spurious. In practice something nasty happens when you open the switch (arcing typically), but ultimately the current is interrupted and drops to zero.

rjvh · Jun 30, 2008

The Question is not verry clear

there is a AC voltage suplied so the current would also be alternating

that means there is no steady current only an changing one and out of phase of the voltage

could it be that they ask if the curent is 692 mA after 3.2 ms

you have to calculate the inductance from there you have to calculate the coil resistance and then you have to calculate the staedy current in case the field breakes down deu to the resitance of the coil

is it not that if the power is switched off the frequency will be 0Hz and the field can be seen as a in/decreasing dc voltage and with the risistance of the coil you will have a current (altought i don't think it's a steady one)

Just guesing

Robert-Jan

arrie · Jun 30, 2008

Thanks guys.
All you have said makes sense.
the university came back to me stating that this question is incorrect, and they are reviewing all questions now to short-circuit such problems.
Apparently I must use the formula i(t) = I(max) (1-e (-t/T)) to work out the final steady current if it could exist, etc.
This is a bit silly since the whole thing is wrong, but I'll do for now.
It must be one of those imaginative conditions.
Go well all

Space Varmint · Jul 15, 2008

Bob Scott said:
I thought it would be easier to design a saturating switching power supply transformer with a different equation for B.
The existing B=V*10^8/(4.44*A*N*f) uses AC voltage and frequency. I think current is more suitable.

So I've come up with some new ones:

B=0.1415*Al*N*I/A

B is in Gauss
Al is the core's inductance in milliHenries at 1000 turns.
N is the number of turns that pass through the core.
A is the core's cross sectional area in cm^2.

I've also come up with:

Al=µ *0.89*A/Le

µ = the core material permeability.
A is the core's cross sectional area in cm^2.
Le is the core's magnetic path length in cm.

This way you can easily see that the Al value is dependent only on core material and core geometry.

You see I had some trouble verifying the published specifications of an Amidon toroidal core FT240-77 made of type 77 material that has a characteristic permeability of 2000. The specs in Amidon's leaflet said it had an Al value of 3130. Their catalog said it was about 2750. My own test with an oscillator to determine inductance found the real thing to be closer to 1690.

I can elaborate on the derivation of the formulas but only if you're interested.

That's a big core! What is that...2.4"? I use Amidon all the time.

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