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Which bright spark can help me here? #206065 05/17/12 12:16 PM
Joined: May 2012
Posts: 2
S
SparkyLeeeee Offline OP
New Member
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.

Work Gear for Electricians and the Trades
Re: Which bright spark can help me here? [Re: SparkyLeeeee] #206067 05/17/12 12:42 PM
Joined: Jul 2004
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G
gfretwell Offline
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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


Greg Fretwell
Re: Which bright spark can help me here? [Re: SparkyLeeeee] #206071 05/17/12 02:15 PM
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SparkyLeeeee Offline OP
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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

Re: Which bright spark can help me here? [Re: SparkyLeeeee] #206073 05/17/12 04:53 PM
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sparkyinak Offline
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welcome aboard from across the pond!


"Live Awesome!" - Kevin Carosa
Re: Which bright spark can help me here? [Re: SparkyLeeeee] #206074 05/17/12 07:08 PM
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gfretwell Offline
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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.


Greg Fretwell
Re: Which bright spark can help me here? [Re: SparkyLeeeee] #206095 05/20/12 04:34 AM
Joined: Oct 2000
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Scott35 Online Happy
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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!---

Last edited by Scott35; 05/20/12 02:26 PM. Reason: Nope, symbols will not work!

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Which bright spark can help me here? [Re: Scott35] #206096 05/20/12 12:03 PM
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LarryC Offline
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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

Re: Which bright spark can help me here? [Re: LarryC] #206097 05/20/12 12:20 PM
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LarryC Offline
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Link to where I found the equation:

Physics page

Larry


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