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Parallel coil-layers?

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mogallin

Thu Jan 01 2009, 06:20PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Having too much spare time atm and being too bored here comes another thing i've been thinking about;

Longer wire gives higher inductance, right?

Higher inductance gives longer discharge-time, right?

So connecting each layer in a coil in parallel would decrease the inductance and thereby the discharge time?

If so, what may be the problems with this?

Marauder709

Thu Jan 01 2009, 07:33PM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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The discharge time would decrease but not because of any change in inductance. If you couple coils in parallel with the caps you can drastically reduce resistance of the circuit. If you reduce the resistance you increase your circuits frequency and thereby decrease the pulse length.

Theoretically you could hook up every single turn in parallel but you pulse would would me unbelievably short and the feild might not "grab" the projectile for long enough.

Self induction on the other hand is based on the coil's overall geometry, not how it is electrically hooked up.

mogallin

Thu Jan 01 2009, 07:43PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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ok. Thanks.

badastronaut

Thu Jan 01 2009, 09:48PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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If you connected each layer in the coil in parallel, you'll end up with lower inductance. Two in parallel = half the inductance.

This is equivalent to using thicker wire. No free lunches lulz.

Inductors add in series and parallel just like resistors. It matters how they are connected.

Marauder709

Thu Jan 01 2009, 10:08PM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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The quantitative definition of self inductance for a solenoid is: (N^2*u*A)/l (not exactly but close enough)

There is no I or B or R or any other quantity that is related to how the circuit is wired. You may be right though, I hadn't considered that inductors add in series. It would be interesting to see how the maths work out on that.

uzzors2k

Thu Jan 01 2009, 11:19PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
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Marauder709 wrote ...

If you reduce the resistance you increase your circuits frequency and thereby decrease the pulse length.

Not quite. Resistance has no impact of the frequency of an LCR circuit, just the damping of it. Lower resistance will cause the current to swing at a higher amplitude and take longer to die out. This isn't necessarily good in a coil gun since the negative pulse will be clamped wasting all of that energy. Ideally you want a critically damped LCR circuit, consisting of one pulse.

badastronaut

Fri Jan 02 2009, 04:05AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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I think that is the ideal solenoid inductance equation and is a gross oversimplification of the problem. It is not the definition of inductance. The definition of inductance is the ratio of the voltage change to the rate of change of current through the inductor. It is also the ratio of flux to current.

In the case of the solenoid equation that you have posted, the number of turns is the effective number of turns that are in series. The ones in parallel won't count. It basically says how dense the number of turns are.

Marauder709

Fri Jan 02 2009, 06:58AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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I don't think its a gross oversimplification. It's certainly the easiest way of calculating self inductance.

The maths, as far as I can tell work out like this:
-L(di/dt) = (cEMF). L is constant for a given circuit, no matter how you wire it. di/dt however does change quite drastically depending on the wiring.

When they say that inductance adds in series it assumes that the inductors have separate fluxes, separate magnetic fields. In mogallin's case however, the inductors share the same magnetic feild and thus must be treated as a single inductor. If you want quantitative proof: cEMF = -N(dphi/dt) where N is the number of turns through which the flux, phi, passes. Because the flux passes through all the turns they must all be counted as a single inductor.

blackgrunge

Fri Jan 02 2009, 06:49PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Its not a gross simplification at all. It works with low voltage models such as 24v capacitor banks EXTREMELY well. I've done it my self to lower inductance and increase magnetic field strength. It also cuts back on resistance significantly (R= r^-1 + r^-1.....)

Its a great concept that hasn't been used all that often but I've experimented with it and it yields great results at lower voltages. It becomes uncontrollable at higher voltages because the discharge time plummets and you end up with a minuscule discharge time that become difficult to work with. In a way inductance is what helps people work with higher voltages and ultimately higher energy levels.

If anyone has the money or owns one, a car audio capacitor works excellent using this approach and if you build the coils as mentioned then you will see some great results working with those lover voltages. The bonus of using lower voltages with this method is that you can gain capacitance very fast. When I built this setup I put together a 6 stage in about 3 days because there was no need for high power SCRs, power MOSFETs did just fine. The one 1.2 Farad capacitor @24v served as a great central power supply for the coils.

badastronaut

Fri Jan 02 2009, 08:57PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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That's what I meant, putting the layers in parallel will reduce the inductance, just as you have said.

You're only increasing the magnetic field strength because you are increasing the peak current. Buy an LCR meter and measure it yourself. Or if you have a good voltmeter and a function generator, you can use phasor analysis to find the change in inductance.

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