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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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So, i had a lengthy discussion on the General Science and Electronics board about coil efficiency. As it turns out, as you pump progressively more and more power into a coil, you start to get less of a magnetic field per watt of electricity. However, there is apparently a non-linear relationship between magnetic field (and thus, power supplied to coil) and the force exerted on a coilgun bullet.
So, are smaller, lower power coils more efficient on coilguns? Is that why multistage coilguns exist, because many small coils are more efficient than one big one?
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Joined: Thu Jun 03 2010, 01:25PM
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Yanom wrote ...
So, i had a lengthy discussion on the General Science and Electronics board about coil efficiency. As it turns out, as you pump progressively more and more power into a coil, you start to get less of a magnetic field per watt of electricity.
Yes, but the force a solenoid imparts on a magnet or ferromagnetic object is not linear with the field strength ... it's a quadratic function. Look here :
Lets say we can ignore the depth and winding stacking height of solenoids for a moment. We have an iron plate and we pull on it with these two set ups at distance g :
- 1 solenoid with C copper, A area, using P power to generate F ampere turns
- 2 solenoids each with 1/2 C copper, A area. using 1/4*P power to generate 1/2*F ampere turns each (separated from each other of course, because if you stack them they simply ARE the first solenoid!). They generate more ampere turns per Watt just like you say ...
What would be the total force imparted on the plate?
For the first it will be F^2*u0*A/(2*g^2), for the second 2*(1/2*F)^2*u0*A/(2*g^2) ... only half as much!
To generate just as much force the two solenoids would have to use 1/2*P power to generate 1/2*(sqrt(2))*F ampere turns each ... so combined exactly as much power as the first solenoid. As I said in the first post in the former thread, TANSTAAFL.
Now in practice you can not ignore the length and stacking height of the windings of the solenoid ... and THAT is why two solenoids might be better than one (as well things like pull back, inefficiency when not close to the slug etc). But there is no theoretical advantage in the way you think.
PS. hope no one read that as I was editing it for 5 minutes to get the bugs out :)
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Low power coils more efficient?
