Very old electromagnet advice needed

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You don't need to experiment by trial and error to design the replacement coil.

If you first select the correct wire size according to your desired operating voltage, the number of turns is the outcome of rebuilding the coil to its original dimensions.

For American Wire Gauge, the following gets you very close: (New AWG) = (Old AWG - 10 log (Original voltage/New Voltage))
 
Hi All

Shortbus, seen those and read through with interest and found many useful hints.I will join the forum.

Mneary, I played around a little in the past as I have many plans in my head.The math is out there fort all to follow what I have to sort out is the winding and making it neat.

I want to make a good coil winder that when it's set off running it will wind on the bobbin in neat turns with no over lapping and keep retuning once to the end.
I have looked on youtube but I don't see anything that fits the bill for me to copy.

Today I'm take some measurements, wire size, turns. bobbin dimensions and post the results.


Cheers all.
 
Right my brain is now hurting!!

I measured the gauge of the wire and I have to say it's an approximation as I don't have a wire gauge so I used Vernier.I measured it at-
0.10mm think this comes out at around 38awg and 42swg ?? (thinner than hair)

I also did a no. of windings count and again an approximation is-

8000 turns on full coil

I wound the coil on to square tube measuring 30x30mm giving 120mm in length.

120mm x 8000 turns = 960,000mm or 960meter of wire.


The square bobbin that formed the coil had approx 20 layers comprising of- (200 turns + 200 turns one on top of the other then some insulation paper=400) x20 =8000 turns in total.

Not sure how that equates to the 11000ohm total coil resistance, advice please.

kind regards
 
Last edited:
Hi Eric

mmmmm!! wish I could... unfortunately a couple of time through the unwinding the wire broke as I just tied together the two ends and carried on.Because the wire is coated it won't test.

If necessary I will unwind and solder the ends together and test as you say.

Just shows with a little foresight I would have thought of this!!

Cheers
 

hi Mark,
Use some fine emery paper to remove the enamel, an old match box works, or a sharp blade edge.
 
Hi Eric

Given up that trial!! finding and soldering the broken parts easy enough trouble is I end up with more and more to repair!! the wire is so vulnerable.

I know it was a 11000 ohm coil originally as it written so I'm going to have to stick to calculations made from my findings to make the new winding-

Total length of wire approx 960m or 960000mm
Diam of wire 0.10mm AWG.38 or SWG.42
8000 turns

This was full coil.

I'm struggling with the math now on how to get this down to a 24v ish coil. Am I right in thinking that I will need-

New coil on the same size bobbin using AWG. 20 or 0.914mm approx. and 4000 turns to give 24v use???

Steep learning curve these coils!!
 

hi Mark,
Can you confirm that 0.10mm and 0.914mm is the CSA [cross sectional area] of the wire or is the diameter.??
 
ericgibbs said:
hi Mark,
Can you confirm that 0.10mm and 0.914mm is the CSA [cross sectional area] of the wire or is the diameter.??
Click to expand...

EDIT:
what is the CSA of the metal core that the former fits over.???
Roughly the same size as the hole in the former/bobbin
 
Last edited:
wombweller said:
Hi Eric
0.10mm is the actual diameter measured with my Vernier caliper. 0,914 again is approx 10x the diameter!!
Click to expand...

hi Mark
Assuming your AWG values are correct, the AWG 38 has current rating of 0.0228A and the AWG 20 is rated at 1.5A,,, a AWG of 28 is rated at 0.226A and would be OK.

You need approx 850 turns on the bobbin, you will never do that with AWG 20, I would say look at AWG 27 or 28

With the original 8000 turns for 230Vac thats 35t turns/volts, so for 24Vac its approx 840 turns.
 

hi Mark,
I make it approx 800 turns based on your numbers and a AWG of 28 for 24Vac operation.

A point to note as its a low voltage winding and modern insulation is good, the insulation between the layers is not required.
 
For my benefit so it's in one place and I'm clear.

0.0228A x 10=0.228 so AWG.28 is closest at 0.226A (giving me 10 times the factor of original wire)

8000/230v = 34.7 (35v per turn) 35x24=840 turns.

AWG.28 with 840 turns.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

How do I calculate the new full coil ohm value? should it be same as original?
Assuming I'm not 100% correct with my original turn count for the new coil I will go over on the 840 turns as calculated and test and work back if required. Does this sound right???/

Cheers
 
ericgibbs said:
hi Mark,
I make it approx 800 turns based on your numbers and a AWG of 28 for 24Vac operation.

A point to note as its a low voltage winding and modern insulation is good, the insulation between the layers is not required.
Click to expand...

Glad to hear that Eric although without insulation between layers It's harder to see the next layer above.

Although I want to do a neat job is it imperative that each turn follows the last without no overlapping?
 
wombweller said:
Glad to hear that Eric although without insulation between layers It's harder to see the next layer above.

Although I want to do a neat job is it imperative that each turn follows the last without no overlapping?
Click to expand...

hi,
Tight careful winding is the only way to be sure you will get all the turns on.
 
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