coilgun coils - Page 1
Forum section: electronics / beginners

author: eeps
author_rank: Newbie
timestamp: October 07, 2020, 09:19:08 am
content: I'm trying to figure out high voltage coils for a coilgun application.If I have a coil and apply for example 40kv across it, would i need to use some special magnet wire with insulation rated for 40kv? or are there some special tricks I could use to increase the breakdown voltage using some more "normal" magnet wire insulation thicknesses?

author: Manul
author_rank: Super Contributor
timestamp: October 07, 2020, 05:24:46 pm
content: Not necessarily. There are two things to be concerned about. Voltage differential between adjacent turns and voltage differential between any turn of wire and core if it is conductive.So for example if a coil is one layer and 100 turns, and there is 100V at the ends of it, the differential voltage between adjacent turns is just 1V. You can also imagine and calculate differential voltages for multi layer coils (voltage difference between layers).Voltage between core and winding may be a full voltage so needs extreme attention. For example - isolated core.Also there is back emf, caused by switchoff of current, and it may be very high. So isolation needs to be good for that also.But, in my opinion, the idea of applying 40kV to a coil for accelerating projectiles is totally wrong, mad, dangerous and will give no result, not even worth trying.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2020, 04:21:54 am
content: The winding pattern needs to minimize the voltage between layers, or provide for extra insulation.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2020, 02:58:53 pm
content: 40kV does sound redicilous, but I'm no expert in coil guns.High voltage transformers fr for example CFL lights usually have multiple chambers next to each other for better isolation.To expand on maul:You can add an extra isolation layer after each layer of wire, to further separate the voltage, but just neatly wiring also already helps. Make sure there are no gaps in the 5th layer, so the 6th layer does not get near the 4th layer.I have seen those youtube vids where they snap soda cans in half by the eddy currents generated by a single winding of copper. I think they usually use a big capacitor with not too high a voltage, but extremely high currents. Some of those video's have good explanations and views of the components used.The same method is also used for "shrinking coins" with high currents. I think these need a (few) order(s) of magnitude more power to get the job done.But such coils having many windings is not very likely. Many windings result in high inductance, which results in slow rise times of the current. Inductance of an inductor is quadratic with the number of windings. So if you double the turns, you need a 4x higher voltage for the same performance.Also, don't forget the mechanical strength.After "MikesElectricStuff" tested a simple regular circuit breaker with his special test device, he found pieces of the copper winding embedded in his workbench and in the wooden ceiling.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 08, 2020, 03:14:53 pm
content: Quote from: Doctorandus_P on October 08, 2020, 02:58:53 pmCould you overcome that problem by winding a coil with multiple layers of windings but having each layer either in parallel with the others or, maybe better yet, making each winding (and it's associated power supply) electrically isolated from all of the other layers? If the later then the problem might then become how to switch each winding on at exactly the same time. I don't know how critical that might be but OTOH each switching circuit and power supply would only have to handle the current for a single winding instead of all of them. Multiple layers that are electrically isolated from each other should also prevent problems due to voltage differences between layers.Agreed, 40kV sounds ridiculous. Arcing across the windings would be a nightmare unless you used a LOT insulation but then cooling may become a problem and thick insulation would reduce the space available for more windings.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2020, 03:26:57 pm
content: Another reason why 40kV sounds as a bit overdone.It's not about the voltage, it's about the energy, and how to deliver that energy efficiently to a piece of copper wire. Capacitors that have both High voltage ratings and are able to be discharged quick enough without destroying themselves are next to the vat of unobtanium.

author: Brumby
author_rank: Unknown
timestamp: October 08, 2020, 10:08:38 pm
content: I'm curious .... Where did the 40kV figure come from?

author: Bud
author_rank: Super Contributor
timestamp: October 08, 2020, 10:43:28 pm
content: The concern here should be how to isolate the shooter from the gun

author: Brumby
author_rank: Unknown
timestamp: October 08, 2020, 11:02:00 pm
content: Darwinism in action?Let's hope not.

author: Brumby
author_rank: Unknown
timestamp: October 08, 2020, 11:03:07 pm
content: In regard to voltages in transformer windings, here's a graphic I put together a while back to illustrate.....This is a very basic construction, but shows a 10kV transformer which has 2kV maximum potential between adjacent layers. This should give you an idea.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2020, 12:31:29 am
content: At higher voltages, the winding will typically be broken into sections.Also, do not forget that the winding needs to be isolated from the core, which becomes nontrivial at high voltages.