Electromagnetic Theory - Page 2 - Electronic Circuits Projects Diagrams Free

rjvh · 28th June 2008, 02:15 AM

Quote:

Originally Posted by arrie

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?

this is what i think
if the power is switched off than there is no current flowing so the created field is dying out
but if the curent is flowing and you don't change the frequency and or windings of the coil than it will stay the same only the curent is out of phase with the voltage

Robert-Jan

arrie · 29th June 2008, 06:59 PM

Thanks Robert-Jan
My thinking leans towards this.
Since the circuit is made up of basically two components, namely the AC supply and the coil, as soon as the supply is taken out no circuit exists. There is thus no potential to maintain the charge that was already stored in the coil, and as previously mentioned no circuit means no current flow.
I'm just not sure exactly how the potential stored in the coil will manifest itself after the supply is removed, will it go out with a bang, e.g. arc at switch on instant of switch-off, or bleed off at a slower rate.
I've been told it will definitely arc when the supply is switched off, at the switch.
The thing that puzzles me is that twice a reference is made to the amount of current after the supply was switched off.
This is an Electrical Engineering university question.

Sceadwian · 30th June 2008, 04:07 AM

You haven't supplied enough information, there are a LOT of questions about how to determine what you ask. Namely, the inductance value you're switching on and off, and the switch itself. After the supply is removed the magnetic field will collapse and create voltage across the inductor. If the inductor after being switched off is at it's peak current, the DC resistance is low and the overall isolation of the circuit is high the voltage can be incredibly high. If the switch is triggered during a zero crossing (current through the inductor not AC voltage) exactly nothing will happen.

arrie · 30th June 2008, 10:31 AM

Thanks Sceadwian

Unfortunately that is all the information that is provided, just the supply voltage at freq. and the current after certain interval.
Nothing more. I've assumed there to be a switch to disconnect the supply from the coil.
I also think something is wrong with this question, but still have to provide calculations regarding some values about it, e.g. R of coil, L of coil and E stored in coil.
It is an assignment question for my studies.
I've contacted the tutor about this, but seems she is just as dumbstruck as I am.
I'm starting to think that I'm to old to be wanting to study something at a University again.

mdwebster · 30th June 2008, 11:24 PM

Badly worded question.

Steady state current with no supply is always zero. There's always a few milliohms of resistance in the best of traces and they'll eventually bring down any circuit to a no-flow condition if there's no supply to keep things moving.

Just about any inductor will arc when contact is yanked suddenly. How much depends on the amount of current it was carrying at the time and the inductance. A big enough inductor with a large enough current will fry switch contacts if you don't have some sort of kickback protection in place (say a zener diode across the inductor with a breakdown voltage 30% higher than expected load).

Thinking on the question, the only way it would make any sense is if the questioner considered "switched off" to mean a short circuit across the supply. Then you would have a case to make for a steady state current, but only if completely ignoring resistance (which is, sadly, often the case in classroom environs).

Tesla23 · 1st July 2008, 12:43 AM

Quote:

Originally Posted by arrie

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 · 1st July 2008, 12:53 AM

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 · 1st July 2008, 05:04 AM

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 · 16th July 2008, 02:34 AM

Quote:

Originally Posted by Bob Scott

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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29th June 2008, 06:59 PM	(permalink)
arrie	Thanks Robert-Jan My thinking leans towards this. Since the circuit is made up of basically two components, namely the AC supply and the coil, as soon as the supply is taken out no circuit exists. There is thus no potential to maintain the charge that was already stored in the coil, and as previously mentioned no circuit means no current flow. I'm just not sure exactly how the potential stored in the coil will manifest itself after the supply is removed, will it go out with a bang, e.g. arc at switch on instant of switch-off, or bleed off at a slower rate. I've been told it will definitely arc when the supply is switched off, at the switch. The thing that puzzles me is that twice a reference is made to the amount of current after the supply was switched off. This is an Electrical Engineering university question.

30th June 2008, 04:07 AM	(permalink)
Sceadwian	You haven't supplied enough information, there are a LOT of questions about how to determine what you ask. Namely, the inductance value you're switching on and off, and the switch itself. After the supply is removed the magnetic field will collapse and create voltage across the inductor. If the inductor after being switched off is at it's peak current, the DC resistance is low and the overall isolation of the circuit is high the voltage can be incredibly high. If the switch is triggered during a zero crossing (current through the inductor not AC voltage) exactly nothing will happen. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

30th June 2008, 10:31 AM	(permalink)
arrie	Thanks Sceadwian Unfortunately that is all the information that is provided, just the supply voltage at freq. and the current after certain interval. Nothing more. I've assumed there to be a switch to disconnect the supply from the coil. I also think something is wrong with this question, but still have to provide calculations regarding some values about it, e.g. R of coil, L of coil and E stored in coil. It is an assignment question for my studies. I've contacted the tutor about this, but seems she is just as dumbstruck as I am. I'm starting to think that I'm to old to be wanting to study something at a University again.

30th June 2008, 11:24 PM	(permalink)
mdwebster	Badly worded question. Steady state current with no supply is always zero. There's always a few milliohms of resistance in the best of traces and they'll eventually bring down any circuit to a no-flow condition if there's no supply to keep things moving. Just about any inductor will arc when contact is yanked suddenly. How much depends on the amount of current it was carrying at the time and the inductance. A big enough inductor with a large enough current will fry switch contacts if you don't have some sort of kickback protection in place (say a zener diode across the inductor with a breakdown voltage 30% higher than expected load). Thinking on the question, the only way it would make any sense is if the questioner considered "switched off" to mean a short circuit across the supply. Then you would have a case to make for a steady state current, but only if completely ignoring resistance (which is, sadly, often the case in classroom environs). Last edited by mdwebster; 30th June 2008 at 11:27 PM. Reason: Added thought

1st July 2008, 12:53 AM	(permalink)
rjvh	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

1st July 2008, 05:04 AM	(permalink)
arrie	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