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Sup guys, I'm building a proper coilgun, and will be looking into the effect of hysteresis on the coilgun, both theoretically and experimentally. <no longer relevant> To make things a little interesting, I was considering using a permanent magnet as the projectile, please give your thoughts on this idea, I see it as an advantage as it should remove the losses in realigning the iron's field. Anyway, I have a 450V 2400uf capacitor (243J), and expect it to output between 1000A and 1500A maximum. What is the lowest max current rating that I could get away with (IGBT's are expensive!)? <as explained below, I am increasing the resistance which will lengthen the pulse and reduce the maximum current to a level which is switchable on a budget> Cheers
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
1. It has long been discussed that the use of permanent magnets would be a poor idea - as their fields will be re-magnetized and would 'saturate' too quickly. Their efficiency would only be seen in use of ultra-lower power cg's.
2. The capacitor looks great for beginning in CG's. Have you used Barry's CG RLC simulation yet? 1000A is not unrealistic - but your wire gauge will be pretty thick. As you thin the wire gauge, resistance and inductance increases which lowers your peak amperage but and increases your pulse time.
3. The link to the IGBT's - Those are very unacceptable. First, they are SMD components - very very limited current handling capabilities. Have you ever built a CG before? I would recommend starting with a thyristor unless you can reasonably design a gate driver for the IGBT.
A lot of people have sampled the 70TPS12 with varying luck. 1000A is too high for this thyristor, but would work at lower currents. Note: many users have noted that this thyristor does not like to be 'in-front' of the coil.
Also, your three-head-eye horse picture is disturbing.
"...70TPS12... 1000A is too high for this thyristor" - 40TPS80 handles that much without problems - me tested on 300us pulse, so 70TPS may handle even more. It is short non-repetitive single pulse, nothing is going to burn. Just take care of a gate current during triggering, as I zapped few 40TPS80 overcurrenting their gates.
"Note: many users have noted that this thyristor does not like to be 'in-front' of the coil" - total nonsense. Please, stop spawning this rumor over the forum.
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
@Yandersen, I'm not trying to be a dick - but he's not the only one whose experienced the failure:
"I've also found (possibly because of how the back emf/quenching works) that these scr's will fail if they are used at the positive side of the capacitor-coil circuit, so put them on the negative/ground side." -Forty (Posted Here)
Since you're eliminating the reverse voltages with the non-polar caps, this may account for not having fried any of yours.
I would rather warn a CG'er of potential-part-failure and let them make the call than entirely not warn them at all.
Thanks for your comments guys! Thanks for your SCR suggestions, but for my experiment I need to be able to turn off the switch mid-pulse, and the methods I found for doing this on the internet with an SCR seemed rather complicated. Now that you mention it however, I don't fully understand how an IGBT is driven, can't I just put 15 volts through the gate from a MOSFET? <I worked this out from looking up design documents, basically the gate is a capacitor, it must be driven on and off, here is a good beginners guide > I should have said this before, but I plan to release the energy in 3 stages, so each IGBT only sees between 50 and 100J. I found an IGBT with a decent current rating (80A continuous (http://australia.rs-online.com/web/p/igbt-transistors/7616876/ or RJH60F5DPQ-A0)), would this be acceptable? <to begin with I will be undervolting to 310V, this means that I can evenly distribute between 30 and 40J to each stage> I have done simulations, and am aiming for a 1000A peak pulse over about 2ms, cut up by the IGBT into three pulses. <target is now 200-300A> I guess I may as well just use iron as the projectile, I may try a magnet later, but my supervisor told me to keep things simple. <I am doing this project as my Extended Essay in HL Physics> Cheers
Regarding SCR failure if connected anode to the positive cap's terminal and cathode to to the coil ("high side"). Knowing your level of understanding what are you guys doing, I may assume that in this case cap's negative terminal was connected to ground as well as battery cell's minus, while SCR was triggered by touching it's gate to the cell's plus (hope via resistor). In this case once SCR opens, it's cathode flies to the positive rail while gate is held just 1.5V over the ground - this results in a huge negative voltage between cathode and gate which is allowed to be no more than 10V according to the datasheet. Was it the case? Did I predicted your monkey actions correctly? If so, don't blame SCR and high side connection. I trigger SCR via decoupling microtransformers, where secondary coil connected between cathode and gate and primary coil get pulse from a little cap.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yandersen wrote ...
Regarding SCR failure if connected anode to the positive cap's terminal and cathode to to the coil ("high side"). Knowing your level of understanding what are you guys doing, I may assume that in this case cap's negative terminal was connected to ground as well as battery cell's minus, while SCR was triggered by touching it's gate to the cell's plus (hope via resistor). In this case once SCR opens, it's cathode flies to the positive rail while gate is held just 1.5V over the ground - this results in a huge negative voltage between cathode and gate which is allowed to be no more than 10V according to the datasheet. Was it the case? Did I predicted your monkey actions correctly? If so, don't blame SCR and high side connection. I trigger SCR via decoupling microtransformers, where secondary coil connected between cathode and gate and primary coil get pulse from a little cap.
Jhackulon wrote ...
but my supervisor told me to keep things simple.
Would not the simplest solution here be to put the SCR on the low side? Are there any disadvantages in doing this?
No, it just doesn't matter if SCR is triggered via a decoupling transformer. I just can't stand watching a counter-science believe to born and spawn. Saying that "SCR doesn't like to be connected high side" instead of saying "it is easier to trigger SCR connected low side" make non-sence in which newbies are ready to believe.
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IGBT ratings/magnetic projectiles
