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Thread: DC Contactor Coil Winding
01-25-2013, 08:04 AM #1
DC Contactor Coil Winding
(This is an article Joe Williams and I wrote for the Home Metal Shop Club)
There was a need to obtain a replacement coil to restore a DC contactor for a Monarch 10EE lathe of 1943 vintage. The coil was part of a pre-built assembly of multiple contactors and relays made by Struthers Dunn (RPXA595A). The original coil must have failed at some point in the past, and a modern solenoid coil had been installed – and then burned up (it measured a dead short), along with the Reverse selector switch.
It would not be good to use another modern coil like the one that burned up. Although an identical coil could be found, it was likely the wrong coil in the first place. And forget about finding any repair parts.
Fortunately, the design employed two identical coils – one for Forward and one for Reverse. Only one burned up. The good coil was removed for testing. There was a very faint number fragment on the good coil, but it didn’t provide any useful information.
It was decided to make a replacement coil. First, the physical parameters of the coil were measured:
Hole in center of spool: 0.50" x 1.22” long
Wire on spool from: 0.625 ID to 1.42" OD (including tape around outside)
Length of space for wire: 1.08"
Then, the electrical values were measured after removing the iron core from the coil:
Value: 3.25 H (Using a Sencore capacitor/inductor analyzer)
R: 2183 Ohms
Wheeler's approximations from 1928 can be used to calculate coil parameters. His approximation for a multi layer air core coil with a rectangular cross section is:
L (uH) = 0.8 * a^2 * n^2 / (6*a + 9*b + 10*c )
a = average radius of windings
b = length of the coil
c = difference between the outer and inner radii of the coil
n = number of turns
all dimensions in inches.
Other formulas are necessary to calculate other parameters, but fortunately, there’s a web page that lets you calculate coil parameters easily.
Input information is the Inductance, the coil ID and length, and the wire gauge. The wire size was unknown, but several of the output parameters were known. It was possible to guess the wire size and see if it resulted in something reasonable.
The most important output parameter is Resistance. A low resistance would likely result in another burned up coil because of the relatively high actuation voltage.
Assuming 36 AWG wire gives:
Coil OD: 1.46"
R: 1539 Ohms
This is probably not right, because R is too low and the coil OD is too high
Assuming 37 AWG wire gives:
Coil OD: 1.33"
R: 1882 Ohms
L: 3676', 68 layers of 212 turns
This looks good!
Assuming 38 AWG wire gives:
Coil OD: 1.2"
R: 2360 Ohms
This looks like too big for R and too small of a coil
So, the best guess is that the original coil used 37 AWG wire.
The next thing to check is to see if the wire selected can withstand the expected current. The nominal voltage for a 10EE DC panel is 115 VDC. The current through the wire is calculated as follows:
I = V/R = 115/1882 = 61mA.
Various sources list the ampacity of 37 AWG wire as about 28mA, assuming 700 circular mils per Amp. The calculated current is more than twice this amount.
This results in a dilemma. 34 AWG is the smallest wire that can handle the current. But as seen above, larger wire won't give you the right parameters for coil size, inductance and resistance. This won't work. A series resistor could be used to reduce the current. It would likely be a multiwatt, ceramic resistor, and there was no evidence of this on the original design. So the decision was made to wind the coil as calculated, test it after installation, and hope for the best.
The new coil form was made on a lathe from a block of Nylon. Other insulators could have been used, but this was on hand. A 4,000’ reel of #37 AWG copper wire rated to 200C for the coil was obtained inexpensively on eBay.
No special equipment was available to wind a coil. Instead, a small lathe was used to turn the coil. A small motor could have been used, or even a hand drill if it was mounted in a vice and didn’t turn too fast.
For this tiny wire, it’s not necessary to wind the wire so that it’s precisely spaced, so no special mechanism is needed. Electrically, the coil will have the same value whether the wire is perfectly spaced or not. A side note about the wire – it’s about 5 mils in diameter. Human hair ranges from 1.5 to 5 mils in diameter! This is very small wire.
01-25-2013, 08:11 AM #2
Coil Winding Step By Step
Following are descriptive pictures and text that show the entire coil winding process.
The coil form is being prepared to insulate and anchor the start of the winding process. The tape being used is fiber glass high temperature electrical tape. The first thing I do is to double back a short section of the wire to provide additional support for the starting end. This is brought out and taped to the outside of the spool . An arbor was used and was a reasonably close fit to the spool but not enough to insure it would not slip. A few parallel chisel marks were placed on the arbor, and this provided enough grip to keep the coil from turning. The assembly was placed in a Sherline lathe with a off/on foot switch. This is important as you need both hands to manipulate the small wire.
The winding has started and the wire is slowly fed to the spool in a reasonable manner. Normally I would park a small reel on a rod placed across my legs. This very fine wire did not work that way.
Initially, the wire was allowed to come off the spool from the position shown. I’ve used this for larger wire, but unfortunately for this fine wire, it tended to kink and catch.
After a small quantity of wire was installed the dreaded thing happened - the wire broke. This required carefully scraping off the insulation with a pocket knife, twisting the wires together, and soldering. Be careful not to nick the wire. This required some persistence, as the wires wrap and hold about as easily as two hairs would wrap – not easily at all.
There are other methods to remove the insulation, such as paint thinner or 400 grit sandpaper. Wire is available where insulation can be removed with the heat of soldering.
01-25-2013, 08:14 AM #3
Coil Winding Step By Step
The splice is made and checked for continuity from the initial end taped to the spool being wrapped to the inside of the supply spool. Make sure that you have a good splice before going on!
The wire is placed in a folded over section of tape and secured to the coil with the same piece of tape. This insulates it on both sides.
After two breaks, and after trying various methods to feed the wire, a short trip to the shop yielded a better method to handle the supply reel. It employed a small plastic bushing of the correct size with a metal center hole. The axle is a drill blank. The axle was clamped on to a pair of machinist’s V blocks, which allowed it to clear the table and also provided some weight to hold the reel. Problem solved! It worked flawlessly for the remainder of the wire, and no more breaks happened.
The small reel of wire contained 4,000 feet of wire. This was slightly more than what was called for to exactly match the old coil, but because of the concern on high current, it was all used. Another advantage to using the entire reel was that it was not necessary to measure the wire length directly with a gauge or by counting turns, or indirectly by the coil OD.
01-25-2013, 08:21 AM #4
Coil Winding Step By Step
The winding process continues. I am using my left hand to guide the wire onto the spool and my right hand acts as a ‘dancer’ to control the tension and provide a little time should a problem arise.
The supply reel is empty. The coil has its finished hourglass shape. I wound a little more wire on each end as the reel became full to provide a recess for the lead wire termination.
The start of the lead wire installation. You can see the start wire end with the twisted wire and the single finish end. Before going on, it’s time for another Ohm meter check. The final check shows the coil resistance at 2112 ohms. This is very close to the original coil’s 2183 ohms. That was good news. A later check showed the inductance to be 3.86 H, a bit higher than the original. As the only effect of this is a slightly stronger electromagnet, it was deemed close enough.
01-25-2013, 08:25 AM #5
Coil Winding Step By Step
The lead wires have been soldered to the coil wire and are being insulated and taped down to the coil.
The next step in securing the lead wires is to tape them securely in place.
I prefer to bend back the lead wires and wrap tape over them to firmly anchor the wires. That way, if they are pulled and slip, they likely won’t break the tiny wire. At this point the coil is finished and should be tied with lacing cord or plastic ‘tie wraps’ and varnished. This is to protect the coil in later years, when the glue on the tape dries out. The product traditionally used for this is made by Glyptal. It is too expensive for a single use.
01-25-2013, 08:30 AM #6
Coil Winding Step By Step
The coil being installed in the contactor. The contactor is a dual section Forward/ Reverse unit with DC Blow Out coils at the contact area. The original coil can be seen in the background. The Blow Out coils create an electromagnetic field across the spark created when the contacts open, and force the arc away from the contacts down a ceramic chute.
The completed unit installed in the lathe. The replacement coil is the blue thing center right. Final measured values for the coil were 3.86 H and 2112 ohms. The calculator predicted 3.83 H and 2050 ohms for 4,000’ of 37 AWG wire. The lathe was powered up, and the coil worked!
Because the current calculations for the coil showed a high current for the wire size, I did some testing on how fast the coil heated up during use. For my coil, I expected:
I = V/R = 115/2112 = 54 mA
My measurement of the current through the replacement coil was 51 mA.
For the test, I put the spindle in Forward, which energized the original coil. I took temperature readings for twenty minutes with a non-contact infrared thermometer. I then put the spindle in Reverse, and repeated taking temperature readings for the replacement coil. Here are the results:
As you can see, the temperature rise for the two coils is essentially identical. I’m satisfied that the coil I made is nearly identical to the original coil. Additionally, since the Reverse on a lathe has only infrequent, short use (like backing up while threading, or making a cut-off on with the tool on the back side of the cross slide), it will likely never heat up very much at all.
01-25-2013, 12:28 PM #7
Nice work and write up! I wonder if a fishing reel could be used to wind the wire? Good job.
06-12-2013, 02:14 PM #8
06-12-2013, 08:02 PM #9
An exceptional contribution to this Forum.
06-12-2013, 11:11 PM #10
Too kind guys.
I'll just add in here that I'm winding a new coil because my 115v machine seems to be fitted with a 230v pull-in coil. As a spare part this has got to be rarer than hens teeth, so if you are visiting this thread because you have a 230v-exciter 10EE with a dead fwd/rev contactor - send me a PM and it'll be yours for postage.
06-13-2013, 12:01 PM #11
Could you post a dimensioned sketch of the coil form? What did you make it out of?