Rewinding Solenoid Coils

Have you ever came across a great deal on a DC relay for your project and it turned out to be not such a good deal because the coil was the wrong voltage?

I have had good success rewinding coils for, "new in the box", Albright contactors that I purchased for less than 10% of their usual cost because they had 24, 36/48, 60 or 72 Volt coils. I've also rewound a few of those small military antenna relays.. you know, the 28 Volt ones. I "approximated" the wire size on those because they were very intermittant duty and cheap to replace.. hi hi.

Here is the information that you may have been looking for. . . how to rewind the coil for a voltage that fits your needs. Or simply how to turn an undesireable coil into a desireable one.

Solenoid strength, (power), is calculated simply by the current through the coil times the number of turns, or known as Ampere-Turns.
To change the voltage of the coil the diameter of the wire must change in order to change the resistance and have the power remain the same.

The first deep dark secret to rewinding a coil is this, "the power remains relatively constant for any size wire that fills the same space on the coil form."

That means that we only need to know the power of the existing coil in order to have it remain close to the same after rewinding for a different voltage.
Get out the VOM and measure the coil resistance.

Now we have the Voltage and the Resistance of the coil and it only takes a simple calculation to find the power.
To find the Power simply square the Voltage and divide by the Resistance. IE: for a 24V coil with 158 Ohms of resistance... 24V*24V/158 Ohms=3.6W
To find the required Resistance needed for the "new" coil simply square the Voltage and divide by the Power of the old coil. Let us assume we want to rewind it for 12 volts. IE: 12V*12V/3.6W=40 Ohms. That means that our new coil needs to have about the same amount of wire and have a resistance of 40 Ohms.

Now, measure the diameter of the wire. And remember that it may have a couple of thousandths of coating.

For the existing coil we now know the voltage, diameter of the wire, the power and the resistance. This, along with an AWG, (American Wire Guage), chart will give us all we need to know about the coil.

I like the chart that I found here:

The AWG wire chart will allow us to calculate the length of the existing coil of wire and calculate the area of that coil of wire. . . But wait! . . what about those of us who are always looking for an easier way to do things?
Well it just so happens there is. Which brings up the second secret to rewinding solenoid coils. . ."The weight of copper magnet wire remains fairly constant for the area of the coil form"
That means no slide rules or calculus is involved, only simple math. You don't even need a pencil.. well.. maybe to write down a few things while you use the charts. Simply weigh the coil, then use the chart to find a size wire that weighs just as much at the desired resistance. You can double check it by using the resistance for the length of wire. It should be within about 5% of the resistance by weight. It may not be exact because your existing coil may be wound with a wire size that is not AWG, it may be British Imperial Standard (S.W.G), Washburn & Moen or Birmingham - Stubs, but if made in the USA, it will more than likely be AWG. The resistance per lb is really "close enough" no matter what the manufacturer labels the wire size.

Remember we aren't building a space shuttle, we're just rewinding a solenoid coil to operate at a desirable voltage while still maintaining very close to the level of power that it had at our undesirable voltage.

**For those on a real tight budget, or are "cheap" like me, remember that a good inexpensive source of magnet wire is a transformer. You may have to disassemble a few to find the right size wire that you need, but in this case they don't even need to be working transformers, although working ones usually smell a lot better.
When you get the transformer stripped apart you can measure the resistance, weigh the coil of wire, measure the diameter and use your chart to find if it will be "close enough" to get this project one step closer to completion.

The mechanical part of dissasembling the contactor and unwinding/rewinding the coil is left up to the rewinder because the factory assembly methods vary greatly.
A bit of advice would be to carefully take it apart and snap a photo or two to serve as a reassembly guide. It could take some time to rewind your coil and even the best technician may not remember how some parts fit back together.
I used a Dremel tool and ground off the crimped part of the yoke on the Albright contactors, then used a file to smooth up and fit the ears back into the slots. Once they were re-assembled, I drilled, tapped and countersunk small screws into the ears to hold the yoke onto the upper plate. They could be welded or even staked in the same manner that the factory did.
* Note for Albright coils: When finished winding the coil solder longer stranded insulated leads onto the coil wire and feed them back through the small slotted hole in the outer cover of the coil. Then simply cut off the excess and solder them onto the spade lugs.

Here are some before and afters for different Albright contactor coils that I have rewound.

SW80-B-4 (24V) factory wound with S.W.G. #28 wire. resistance was 48 Ohms and weight was .58 lb. Pulled in at 16V and dropped out at 6V.
Rewound SW80 (12V) with AWG #23 wire salvaged from a transformer. Resistance now 7.5 Ohms and weight was .61 lb. Pulls in at 8V and drops out at 5V. (I did say we weren't building a space shuttle, didn't I?)

(2) SW202-269 (36/48V) factory wound with S.W.G. #28 wire. resistance was 79/80 Ohms. They weighed 1 lb each, pulled in at 24.4V & 25V and dropped out at 8V & 7.5V
After rewinding with AWG-22 wire, Resistance was 8.5 and 8.6 ohms, weight was 1.02 and 1.03 lb. Pulled in at7.4V and 7.8V and dropped out at 2V and 3V. (I'll let you do the heavy math to obtain the percentages.)
Stock SW202's varied between 8 and 8.5 ohms. Weight was 1 lb and pulled in at 6 to 7V and dropped out at 2 to 3V.

I would caution anyone to be very careful using a variable speed drill clamped in a vise to rewind the coil. I've done it but had a difficult time getting the drill to run at a slow enough speed.
If you've gotten this far I'm sure you can handle that part. Just be careful, work safely and take your time. A job well done is very satisfying, especially when you think of the money that you just saved.

I could always go out and buy a suitable contactor, but where is the challenge in that. If I did that I wouldn't have anything to keep me out of the pool halls now would I?

That reminds me. . . I still have a 1500A albright contactor with a 72 Volt coil that is collecting dust in the corner of the shop. Anyone interested in making a trade, maybe for some magnet wire?

"Good Luck to all those backyard engineers."

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Background is a partial view of the antennas on top of Shaw Butte in the middle of Phoenix. Photo by Don Shelley - W3TV