Exciting current

[Firefly]

Hi, I've been trying to do a question on exciting current for about a week now and I can't for the life of me think what's the right equation to use.. Every equation I know I have tried and none give me the right answer And knowing me it's probably something really simple and I'm just over thinking it or something!

Well the question is about a ring which has had a gap cut in it. I have values for the flux density, the length of the cut, the original length of the ring, the number of turns, and the original cross sectional area of the ring.

Any help on this would be much appreciated as I have an exam coming up..

 

[physis]

Re: Exciting current

I have an incomplete picture. Is it an incomplete toroidial coil? You said there are turns.

I'm no genious so the engineering guys might know exactly what your descriding but I have a couple questions.

What are you exciting? Is the current a threashold of some kind?

 

[Firefly]

Re: Exciting current

Well I'm only in my first year of study so I'm hoping it's nothing too complicated!

I'll write a out the question for you because that will probably be easier to understand than me trying to explain it!

'The mean diameter of a steel ring is 50 cm and a flux density of 1 T is produced by 40 amp-turns per cm. If the cross section of the ring is 20 cm^2 and the number of turns is 500, find a) the inductance in Henries, and b) the exciting current and the inductance when a gap of 1 cm length is cut in the ring, the flux density being 1 T. Ignore leakage and fringing.'

I have managed to do part a and come up with the right answer it's part b that is giving me problems. Maybe I have misunderstood the question..

 

[physis]

Re: Exciting current

That's better. Now I"ll have to see if I have enough memory and text material to even start on it. It's outside of my expertise but that's usually not enough to stop me.

 

[bob]

Re: Exciting current

Firefly
You made me look back, way way back for the following:
using metric units
Rc = reluctance of the core
Lc = length of the flux path
Ac = area of the core in M?
Ag = area of the gap in M?
u = relative permeability of the material
u1 is for core material
u2 is for air
Rc = Lc/(u1 x Ac)
Rg = Lg/(u2 x Ag)
Rt = Rc + Rg amp turns/weber
gap flux = amp turns/(Rt)
flux density = gap flux/Ag

Hope this is of some help. You could probably do a google search and get this information.

[ January 09, 2005, 03:14 PM: Message edited by: bob ]

 

[Firefly]

Re: Exciting current

I originally started doing the question with reluctance however I then wasn't sure how to find the exciting current from doing that..

 

[physis]

Re: Exciting current

Well, I'm not doing so good on it either.

Let me ask you something, since you have some texts. (An assumption but one I'd wager on)

When you cut the ring you loose the toroid properties. Does it now act like a wire or a rod?

 

[Firefly]

Re: Exciting current

I have tried looking in my text book however I found nothing similar to this question.. So to be honest I have no idea. I know what the answer is supposed to be so at the moment I am just using trial and error and getting no where.

All my lecturers have a habit of doing this.. They go over the 'simple' things which are fairly straight forward to understand once you read about them and then throw us in the deep end with questions which we have no idea how to solve.. They don't even tell us with chapters we should look at for background information and considering this was our first semester at university it would have been nice if they gave us a little help getting used to the new way we have to learn!

 

[physis]

Re: Exciting current

I personally strogly agree with that meathodology. The most valuable educational asset is the ability to find answers with no assistance. Strictly speaking, it may not be an engineering principal but I guarranty it's something you'll need.

Look at me, just because I have no more library I'm nearly impotent at answering this really really simple question.

 

[Firefly]

Re: Exciting current

I know it's a good way of teaching but I whine because I am lazy!

 

[physis]

Re: Exciting current

Wait till you're 40.

Editted because I can't even get three words and a number right!

[ January 09, 2005, 05:30 PM: Message edited by: physis ]

 

[al hildenbrand]

Re: Exciting current

posted by Sam, January 09, 2005 04:29 PM

When you cut the ring you loose the toroid properties. Does it now act like a wire or a rod?

Hey Sam,

The flux in the steel ring is still in a ring shape, the flux is still completing its magnetic circuit. The flux density is 1 T. There are two magnetic permeabilities in the flux circuit, that of the steel, and that of the air gap.

In a simple sense, the steel is the conductive copper wire of a circuit (low resistance) and the air gap is a load resistance. The flux density of 1 T is the equivalent of a current flow in the electrical circuit. . .The current that flows is based on the sum of the resistance of the wire and the load.

 

[physis]

Re: Exciting current

Yeah, but isn't the diffence in the iron core and the air gap dramatic? And if that's not enough, don't you loose the internal currents of the core, I mean to the extent that you simply no longer have a toroid?

 

[physis]

Re: Exciting current

I see what you're saying Al. So do you mean you can put a reluctance value on the two matterials and treat it as a ratio?

 

[rattus]

Re: Exciting current

If I remember right, you must compute the mmf. Then you must compute the reluctance of the core and of the air gap. Then the flux is computed by dividing the mmf by the total reluctance. The flux is the same in the core as in the air gap. Don't ask me to remember any constants though.

 

[physis]

Re: Exciting current

For me the only constant is recolectic dislexia.

That sounds better rattus, a sum.

 

[al hildenbrand]

Re: Exciting current

Sam,

don't you loose the internal currents of the core?

Do you mean the eddy currents?

The eddy currents will still be the same, air gap or no. You can apply the Right Hand Rule to the direction of the flux in the steel ring and the currents will be going around the 20 cm? cross-section of the ring. Remember how core laminations run? The thin laminations keep the cross section currents from adding together.

 

[physis]

Re: Exciting current

Well then help me out here Al. What are the benefits of the toroid and what happens when you cut the ring?

 

[rattus]

Re: Exciting current

Toroids offer the shortest path for minimum reluctance. They are typically made from powdered iron and a binder which tends to reduce the eddy currents. They are Hell to wind, but they can be made very small. The "core memories" of early computers consisted of an array of tiny cores laced with address lines and sense lines.

 

[physis]

Re: Exciting current

Wow Rattus, I member those post ENIAC cells.

I think the effect of cutting that ring is more than a series sum.

I just cant look it up.