Write up I did a long time ago on how to rewind solenoids, way past time to share it. PDF version here: https://rcwarshipcombat.com/resources/solenoid-rewinding.254/ Solenoids are an integral part to this hobby (poppets are out-dated, get over it), they are also one of the more expensive parts of any battleship. This is in large part due to the de facto standard of using ~6V for main power in our ships. Unfortunately for us there aren't that many industrial systems that use 6V, they tend to be 24V or even 120VAC with a smattering of other voltages here and there. Since there isn't much demand for 6V solenoids there is a corresponding lack of supply. At 12V there are some decent options, Clippard for instance has a rather nice 12V solenoid (MME-2PDS-W012) that has served me well in my VDT for a number of years. They don't work well at 6V but I never had an issue with them at 7.2V, I've heard reports of the newer ones not liking 7.2 V either but haven't been able to verify that. While I am a large proponent of moving to higher voltages (my VDTs were at 7.2 V and my Bismarck now runs at 13.2 V) I know there are a lot of people in the hobby that like ~6V for whatever reason. This makes the availability and cost of 6V solenoids an important issue. I think that Battler's Connection has done the hobby a great service by being able to make larger orders of the custom 6V KIPs that are the hobby standard and stocking them to help combat the availability problem. Unfortunately there probably isn't anything they can do about the cost. Fortunately for us it is possible to use solenoids that were built for higher voltages with only a minor amount of effort. Solenoids and Electromagnets Before I get to how to rewind solenoids for different voltages a little background in solenoids seems appropriate. A Solenoid Actuated Valve is the proper name for what we common just call a solenoid. The Solenoid part of that comes from the name for the coil of wire that current passes through when we energize the circuit. When current is passed through a coil of wire a magnetic field forms in the center of the coil. Depending on what you put in that center you can increase the magnetic field significantly. By putting an iron (or otherwise ferromagnetic) core inside the coil you can produce a fairly strong electromagnet. It is this electromagnet that pulls the plunger inside the valve up and opens the valve when current is applied to the coil. (There is a spring on the plunger that returns it to the closed position when the coil is de-energized, once closed the gas pressure keeps it closed.) The magnetic field (B) in a solenoid with a given geometry and core material is determined solely by the current passing through the windings (I) multiplied by the number of windings (N) in the coil. In equation form this is: B=N x I x (some constants we dont care about because they are constant, assuming the geometry/core material is the same) Voltage doesn't directly matter! There is a physical limit on B depending on those constants we didnt care about but that is outside the scope of this article and is left to the reader to determine if so interested. There are two electrical equations that everyone should know, basic high school physics type stuff: V=IR Voltage = Current x Resistance and P=IV (or P=I^2R same equation just different forms) Power = Current x Voltage (Power = Current x Current x Resistance) Take a standard 6V KIP solenoid and use your handy multimeter to measure the resistance across the leads, you should get roughly 5 ohms (the one I measured was ~5.1). So using V=IR we would expect a solenoid to draw about 1A at 6V, and they do. If you read the label on the Solenoid you'll see that it says it is rated for 7 watts of power, remember P=IV, the solenoid is using about 6 watts of power. The slight mismatch between rated power and actual power is expected. In my Bismarck I run 6V KIPs at 13.2V, this works because the resistance of the coil doesn't change so if I increase the voltage I also increase the current that the coil consumes. B=NxI, if I goes up then of course B will still be strong enough to function the valve. The downside of doing this is the wire that makes up the coil can only carry so much current, put too much current through it and you'll burn/melt something causing the current to drop to zero and you lose the magnetic field so the valve can't open. In the VDT I was able to run 12V rated coils at 7.2V. There is/was enough design margin in the Clippard solenoids that the 40% drop in the current didn't weaken the magnetic field enough to prevent the valve from functioning. If we can change the number of winds and/or the current through the coil we can make the solenoid work at whatever voltage is convenient for us. This brings us to the main point of this article. Winding Solenoids We could go out and blindly buy some magnet wire and rewind some solenoids and determine what works and what doesn't but if we do a little more math first we can make our initial attempts much more likely to work. If we knew the number of winds on a 6V KIP and the gauge of wire used we could simply copy that to wind our own, which would work just fine if all you want to do is wind 6V coils. I don't really have any desire to rip apart a perfectly good 6V solenoid and then count the number of turns in the coil while I rip it apart, that doesn't sound like a fun evening to me. Plus since haven't run a boat at 6V since 2006 that isn't super helpful for me. One thing I did notice is that a lot of surplus KIP solenoids available at other voltages have the same power rating as the 6V ones we use. If we keep the power the same and then use those handy equations I described earlier you can determine the resistance needed in the coil for whatever voltage you desire. (and atleast at 6V and 12V these calculated values match up with the actual resistance in a manufactured coil.) Resistance in the coil is determined by the gauge and length of the wire used. (Since we're forming a coil you can think about the length as a number of turns, x ft is equivalent to y turns.) Magnet wire is well standardized and finding the resistance per foot is an easy lookup and from that you can get the number of ft required for the proper resistance. Smaller gauge wire has more resistance per foot so you need less length/turns but it can't carry as much current so you run the risk of burning it up, thicker wire can carry a lot more current but you need more which takes up more space and may not fit inside the solenoid housing. There is a sweet spot for what gauge to use, luckily there is a decent amount of flexibility around that sweet spot. The tricky part here is how much wire you use when rewinding, since I don't have a rewinding machine (yet, they seem simple to make) I've just been winding as much wire as I can fit of each gauge onto the spool, it is rather simple to unwind and cut off wire as needed to trim the resistance to a precise value if desired. By doing this you can rather easily take a surplus solenoid and rewind it to function at a different voltage. From my calculations and testing on Series 2 KIP valves (same as what we typically use) rewinding using 28 AWG magnet wire results in a coil that function at the voltages we use. 30 AWG would match the specs for a 12V valve better. What follows is a step by step pictorial explanation of the process: Surplus KIP Valve bought on ebay for ~$15. Note that it is built for 120V AC, we'll make it work on 6VDC: Step 2 - Disassembly, they just unscrew: Step 3 - Carefully remove the coil from the housing that is sits in: Step 4 - Remove tape and leads from the coil so you can unwind it: Step 5 - Unwind the existing coil without damaging the spindle the wire is wrapped around. I rigged it up so that my drill press would do the work and I didnt have to sit and unwind it by hand. I'm lazy. Step 6 - Attach magnet wire to spindle for rewinding, I just used some electrical tape to hold it in place: Step 7- Time to actually wind a new coil. I used a wooden rod with a dowel cross ways through it in a vise to hold the coil and let it spin. (If you just lay it flat on the table it doesn't unwind as smoothly and you can kink the wire which is bad.) I ran a bolt through the spool with a wingnut (what I had laying around) to clamp the spool to the bolt and then put that into my cordless drill. I'm sure you could wind this by hand but remember I'm lazy. Step 8 - When winding the coil try and keep the winds as neat as possible for as long as possible. Sooner or later I mess up and then typically give up and just try to have the coils as even as possible. Step 9 - Finished Coil, give it a wrap of electrical tape to keep it from unwinding and make sure you have access to both ends of the wire.
Step 10 - Strip a little of the enamal insulation off of the end of each lead and check the resistance of the coil. Increasing the resistance is hard but it is easy to unwind and kep checking until you get the resistance you want. (This coil has 7.5 Ohms of resistance which is right in the correct range for the voltages we use.) Step 11 - Solder new leads onto the ends of the coil. You can reuse the ones that came with the solenoid or replace them with your own wire. (most magnet wire has an enamal coating that you have to scrape off before soldering to, however some has a coating which acts a flux so just be aware what type you have and solder accordingly.) Step 12 - Attach the leads to the coil and make sure they are close together (but not touching) so they can both exit through the original opening in the housing.: Step 13 - Put the new coil back into the housing with the leads coming out the original hole in the side. I've had some were the rubber grommet was destroyed when I disassembled it but you can always fill that hole with goop or scothkote to seal it up. (Solenoids don't really need to be sealed since the wire is all coated water doesn't have a huge effect on them.): Step 14 - Reassemble the Solenoid: Step 15 - Test the solenoid. This is the power supply powering the solnoid. I have it hooked up to 150 PSI CO2 and you can see the solenoid is drawing slightly less current since the coil resistance is a bit higher than the stock 6V coil. : Overall rewinding solenoids is pretty easy to do. Just be careful when buying surplus solenoids as the physical geometry of the coil does play a role and that isn't easily modifiable. I picked up some surplus solenoids at a surplus store around here and rewound those but the dimensions were slightly different which gave different results. The picture below compares one of the parts from the KIP (left) and a part from the surplus solenoid on the right. The diameter of the shaft on the right is slightly smaller which modifies those constants we ignored earlier and changes the math for determine how to rewind it. I was still able to rewind them and get them to function on lower voltages than they were designed for but only by significantly increasing their power consumption. Ebay so far has been a decent source of surplus KIPs for me and I've found them listed on a couple other websites as well. When looking for surplus valves the stock voltage isnt important but the Max Operating Pressure Differential (MOPD) and the orifice size are. (also 10-32 ports aren't common but it is easy enough to use a 1/8"npt or 1/4"npt) MOPD should be at least 150 since that is the pressure we intend to control (note: the hobby standard KIPs have a MOPD of 250) and 3/32" is the standard orifice size we seem to use. Depending on how your guns are set up and how efficient they are going with a smaller orifice could degrade their performance. (You'll notice that the solenoid I used here has a 1/16" orifice, luckily the surplus solenoids that I have which don't have the same internal geometry to the coil and core do have bases which are compatible with KIPs so I'll switch that base out to something with a larger orifice.) Hopefully this helps some of you guys that like to scrounge up parts and make them work or would rather stick with the KIP form factor than move to Clippards or the Spartan Scientific solenoids. I dont know about the Spartan Scientific Solenoids but the Clippard solenoids dont come apart like the KIPs (I think the coil might ever be potted) so rewinding those would be a more significant undertaking, however the same basic theory applies. Good luck and see you on the water!
