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#206065 - 05/17/12 10:16 AM Which bright spark can help me here?
SparkyLeeeee Offline
New Member

Registered: 05/17/12
Posts: 2
Loc: West Sussex
going through a practice question paper and theres a few questions I am struggling to find awnswers for in my notes, not sure which formula I need here but one of the questions is;

A self induced e.m.f. of 60 Volts is produced by a current of 6 amperes in 0.4 seconds. What is the inductance of the inductor?

any help much appreciated.

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#206067 - 05/17/12 10:42 AM Re: Which bright spark can help me here? [Re: SparkyLeeeee]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
It's been a while but I think it is

V = L A/T so you have 60=L 6/.4 or 60=Lx15 or L=60/15 (4 henries)

Scott or one of the other engineers here can grade my work. wink
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#206071 - 05/17/12 12:15 PM Re: Which bright spark can help me here? [Re: SparkyLeeeee]
SparkyLeeeee Offline
New Member

Registered: 05/17/12
Posts: 2
Loc: West Sussex
Thanks Greg that looks familiar, theres another question im unsure of if anyone can help;

a 15H coil (inductor) carries a current of 7A. What amount of energy is stored in the magnetic field?

Thanks again

Lee

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#206073 - 05/17/12 02:53 PM Re: Which bright spark can help me here? [Re: SparkyLeeeee]
sparkyinak Offline
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Registered: 07/08/07
Posts: 1286
Loc: Alaska
welcome aboard from across the pond!
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#206074 - 05/17/12 05:08 PM Re: Which bright spark can help me here? [Re: SparkyLeeeee]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
I think we need the voltage to figure that out.

It has been something like 48 years since I actually used this so I may be forgetting something.
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#206095 - 05/20/12 02:34 AM Re: Which bright spark can help me here? [Re: SparkyLeeeee]
Scott35 Offline

Broom Pusher and
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Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Quote:

a 15H coil (inductor) carries a current of 7A. What amount of energy is stored in the magnetic field?


Need to know the Frequency applied to this Inductor, to figure Xl.
(BTW: "pi" = Greek lower case "pi", or 3.141592653589...)

Formula is: XL = 2pi FL

If F= 60Hz, then XL =:

2 * 3.141592653589 * 60 * 15.0 = 5,654 Ohms

Find Voltage: E=IR

I = 7.0 Amps,
R = 5,654 Ohms
7.0 * 5,654 = 39,578 Volts.


Find Reactive Power: VAR= E * I

E = 39,578 Volts,
I = 7.0 Amps
VARs = 277,046 VARs.

--- Personal Note:

I think the value of the Inductor is quite large - 15H.

Substituting 0.0015H (15mH) for the Inductor results in a much more reasonable Reactive Component: 277.046 VARs, with a Potential Difference of 39.578 Volts (rounded to 40 Volts).

-- Scott (EE)

---let's see if these spec. characters work now!---


Edited by Scott35 (05/20/12 12:26 PM)
Edit Reason: Nope, symbols will not work!
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#206096 - 05/20/12 10:03 AM Re: Which bright spark can help me here? [Re: Scott35]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
If you are looking for the energy stored in the magnetic field, I thought the formula was:

W = 1/2 x L x I^2

W = .5 x 15 x 49

W = 367.5 Watts

Larry

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#206097 - 05/20/12 10:20 AM Re: Which bright spark can help me here? [Re: LarryC]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Link to where I found the equation:

Physics page

Larry

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