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I agree. Essential is a fast turn off of the current. That is the reason for my suggestion of applying a reverse voltage to the coil, e.g. by some RD>0. A large RD will have the drawback of requiring a higher voltage transistor. On the other hand, an RD=0, as your tables imply, leaves you at the mercy of the coils internal resistance to reduce the current. That involves thin wires and eventually leads to heat up.
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
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Posts: 58
to Uspring:
On the other hand, an RD=0, as your tables imply, leaves you at the mercy of the coils internal resistance to reduce the current
Well, I didn't suggest Rd=0 or any another value of the damping resistor. I only said that I didn't include this process into my investigation (see remark 1 to fig. 1).
to Sulaiman:
Maybe you hint on how much energy dissipates on snubber circuitry? It is interesting question. As all the energy dissipated after turn-off is stored in magnetic field of the coil, it usually makes no more than 10.....50 % of the whole energy wasted per shot. Then it is dissipated in Rd and RL in proprotion to their resistances.
I was wondering, why the tables show the highest possible velocities, when they are thinnest. I believe that can be traced your requirement of tmax < t0. tmax is about L/R, so a small t0 implies a big R and therefore a thin coil. That is a bit strange, because a thicker coil with the same L but a smaller R will provide more acceleration. A drawback of such a thicker coil will be the higher current draw of the first stage, where the max current is limited by the resistance and not the inductance.
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
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Posts: 58
I believe that can be traced your requirement of tmax < t0. tmax is about L/R, so a small t0 implies a big R and therefore a thin coil.
Yes, you are absolutely right.
That is a bit strange, because a thicker coil with the same L but a smaller R will provide more acceleration.
Well, there is not a simple dependence between thickness and acceleration efficiency (while keeping a constant R). It has some broad maximum indeed, but it is not strictly tied to thicker coil - a thinner one can be more efficient depending on initial velocity of projectile, its length etc. In my investigation the additional problem is that all the coils must be identical - it means that even if for some of them we find the efficiency to be higher for thicker winding (it corresponds to nearly triangle pulseform), the next (similar) coils would have low (and decreasing with the stage nubmer) influence on projectile acceleration, as their current cannot reach any substantial value during the accelerated body is moving inside them (although their absolute efficiency could be really high). That is a root paradox of the gauss-building - decreasing the power (current) in a coil increases an efficiency, but yields less acceleration. The point is that we must get some final velocity not only with some limited energy reserve (which implies considerable efficiency, indeed) , but within a fixed length of acceleration, too (wich means some minimum speed increment delivered to the projectile at any stage).
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Maximum speed in multistage accelerator with identical windings
