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DerAlbi

Thu Sept 24 2015, 08:29PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Increasing voltage will not necessarily increase performance. It may increase the current slightly, but that may only cause deeper saturations.. so not *much* gained. Your LiPos will have a current limit and having more in series only increases the chance of having a bad one in series and that will ultimately limit the current. Even if all LiPos are fine, its not the voltage that transfers the power, its the current...
The only way to get away with higher voltage is to also increase turn-count in your coils. This will lower the current but keep the turns*current constant.
That would be totally fine then, but it wont increase your power transfer.. it basically would just increase the LiPo lifetime due to lower peak current.

Increasing the stages would be the way to go. But you have to check, if you can do that actually (with a proportional energy increase). The faster the projectile gets, the more power (current*voltage) must be provided or in other words: the same energy, but shorter time.
As i see the current in your design as a limiting factor it might be that the additional stage wont push as much energy as the 2nd or 3rd stage.
It really sucks, that you cant do a good measurements currently (how is the oscilloscope going?

).. it would be really beneficial to see how close your LiPos are allready to dying to get some starting point for a debate thats more based on facts instead of fear and believe. So currently i just can give you my opinion based on gut feeling..

If no schematic, can you tell me the topology? Have you used a Halfbdige, or do you use the typical reverse diode across the coil?

ben5017

Mon Sept 28 2015, 12:34PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Good points albi. (BTW I plan on ordering a scope this week)

I am using paralled bi-directional tvs diodes for clamping

I have decided to stay on 12s and add 2 stages.

I got most of the modifications roughed out on CAD this weekend.

Removed the fuse box and the injector, replaced gear motor/encoder with a stepper motor, This allowed the drum mag to be moved back enough to fit 2 more stages with only increasing overall length an additional 3/8"

Yandersen

Tue Sept 29 2015, 08:56AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Ben, paralleling TVS is not good - due to a variance in breakdown voltage most of the power may be dissipated by the TVS with lowest BD voltage. Better to put TVS with smaller breakdown voltages in series.

I also do not agree with Albi about the batteries. The more cells you put in series the better, but the coils must be rewound as well - more turns with thinner wire. After all, your input power is U*I, and it is better to aim for higher U. The pull force is I*turns, therefore the more turns the coil has the better, but you need more U to ensure that current can rise fast enough.

DerAlbi

Tue Sept 29 2015, 12:02PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Nice form of disagreement. Just repeat, what i say

The only way to get away with higher voltage is to also increase turn-count in your coils. This will lower the current but keep the turns*current constant.

ben5017

Tue Sept 29 2015, 02:39PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Yan,

Thanks for the heads up on the TVS diodes. Makes perfect sense after you educated me. I will be ordering new ones this week.

I am interested to see what gains in performance i can make after rewinding better and tighter(with epoxy) coils and tuning them for the new 50v setup.

(performance numbers given on previous page were using same loose and sloppy coils i made & tuned for 25v. With Delta KE of approx 5j/STAGE)

Even an increase in KE/stage of 10% would add up, especially with additional 2 stages.

30j KE might be possible

DerAlbi

Tue Sept 29 2015, 04:04PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I think you should wait until you get the oscilloscope to measure if/how your swtiching and everything works on your setup right now. I think if you really do the effort to rewind the coils you should maximize the learning effect. That works best if you do good before- and after-measurements.
(for some reason i dont believe that TVS are really effective at such currents, since they absorb the energy.. but not in such orders..., yeah.. i know.. thats not based on facts.. sry)

ben5017

Tue Sept 29 2015, 04:31PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Albi,

I agree. Should have the scope here in a couple weeks. Might as well get one while my electronics are still in breadboard stage..

Remaking the coils will be the last upgrade done once it is shooting consistent with 6 stages.

I still have to modify the v3 frame and cut new pieces on router. + additional electronics and coding for 2 more stages + stepper motor control.

It will be a good bit before I focus on tuning coils.

As far as the TVS diodes, I have put around 2-3 hundred rounds through with no issues. However I am always open for ways to improve my design.

ben5017

Thu Oct 01 2015, 12:54AM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Started drawing up the modifications for the V3 mark 2.

First pic shows what I will be reusing once I get done removing the back half.

Last 2 pics show 2 possible configurations for the mark 2

I can either

1) add two more stages with coils using similar geometry currently used and shown measured approx 5j/stage

2) split coils in half for a total of 12 shorter stages. I have the room for the extra FETs would just be alot more work.
I could use the arduino to play with the timing and sequencing to have multiple coils on at the same time for more of a rolling mag field.

1443660847 3315 FT147269 V3 Mark 2 Modifications

Yandersen

Fri Oct 02 2015, 06:44PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Uh, I love that last option. :) Though the shorter the coils the more precise the triggering must be and you will have to deal with higher currents which is not good considering your choosed a battery power supply. Better to maximize the length of the barrel and use the same long coils.

There is no way a battery-powered gun can be efficient, so don't bother about it and aim for light weight and rapid fire to compensate this disadvantage.

And I really don't like your reloading mechanism - it's a bulky solution. Why not using a conventional spring-pushed ammo holder with magnetic plunger to push out projectiles? It can store more ammo, feed it faster, require less space and eliminates a need for a rotation motor.

And a quick tip in case you will go with longer barrel - you may need a cap on the far end of it. The longer the power rails the higher the voltage swing could be due to their increased inductance.

BigBad

Sat Oct 03 2015, 03:25AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Yes, last one looks good to me too.

Short, fat coils often tend to work well.

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