Solenoid wrapped with 2 wires?

[flynnr2]

TL;DR Summary

Circa 1905 solenoid wrapped with 2 wires on each of 8 layers ...1 wire is not connected to any circuit ... any ideas why this would be done?

I have a bit of a mystery with a circa 1905 solenoid and not sure where to turn, hoping someone here might have some insight.

The particulars

• Circa 1905 solenoid
  (application is an electromagnet in a self-winding clock)
• 8 layers of windings of 2 different cotton sheathed copper wires
  • paper in between each layer
  • each layer consists of 2 different wires
    • 21 AWG (0.0285") wire
    • 26 AWG (0.0159") wire
  • 32-35 turns on each layer for both wires
    (so the thinner wire is not closely packed on the spool, while larger wire is)
• Only the larger wire is attached to the circuit
  (multiple examples of this, so seems intentional)

Can anyone suggest why this might be the case - does this have any particular electric effect, &c.?

Cheers in advance

 

[berkeman]

Welcome to PhysicsForums.

When you say "circa 1905", you mean it was made in around year 1905? Do you have any pictures of it, or some part number or something?

It sounds like you are describing a bifilar wound coil, with the 2nd winding not attached to anything (in this application). Is the spool an air core, or is there some magnetic material filling the spool/bobbin?

Do you have a schematic of the circuit that this connects to?

 

[flynnr2]

FWIW, I’ve been designing (OpenSCAD) & 3D printing replacement parts for this clock ...

Additional detail
- manufactured in 1905 (so no useful part number)
- connected to 3V dry cell battery
- spool has some sort of steel core (a bit rusty these days)
- was connected in parallel with another electromagnet, side by side
(though in other examples of this clock, the two are in series ... mine appears to be an early, possibly pre-production example while they were still working out some kinks)
- correct, that second winding is not attached to anything
(maybe just a filler on the spool to lower the windings->length->resistance of the connect wire?)

Spool


Partially unwound, showing 2 wires


Fully wound


Map of circuit (sort of)


2 coils side-by-side

 

[flynnr2]

Top of electromagnet

 

[Keith_McClary]

One use of dual-wound solenoids in old pinball machines (from the 1940s through the early 2000s) .

 

[berkeman]

Paging @dlgoff -- our local vintage electronics restoration expert.

Thanks for the extra info and pictures -- that is helpful. One thought for the extra/unused bifilar winding might be for production testing and fine-tuning the timing circuit. Maybe some connection to that extra coil helps in the testing, but that's a lot of embedded cost and complexity just for manufacturing test.

One other thought is that the same coil is used in another product where both of the windings are used, and the coil is being used in this product for convenience or volume-cost advantages.

Both of those are pretty lame guesses on my part. Hopefully others have better ideas...

 

[flynnr2]

No timing circuit here per se - completely mechanical operation which completes the circuit, causing the electromagnets to fire and "wind" the clock (about once every 2 minutes ... it makes a bit of a racket frankly).

Not sure about the testing, frankly I think they were wining it rather a lot at this manufacturer. I have that same guess - I suspect they bought the coils from a different manufacturer and they were possibly intended for another application. To be clear, most self-winding clocks of this period have coils with a single winding.

Thanks for the taking the time to look!

 

[dlgoff]

Paging @dlgoff -- our local vintage electronics restoration expert.

Thanks for the compliment Mike, but I'm not too knowledgeable when it comes to old coils. Sorry about that.

 

[berkeman]

Thanks for the compliment Mike, but I'm not too knowledgeable when it comes to old coils. Sorry about that.

No need to be sorry. I'm thinking that this may be the right guess:

I have that same guess - I suspect they bought the coils from a different manufacturer and they were possibly intended for another application.

@flynnr2 -- Check out the thread by @dlgoff pinned at the top of the EE forum. You'll see a lot of stuff that is similar to what you are working with.

 

[Klystron]

The untapped second coil rings a distant bell; something about inhibiting hysteresis loss in soft-core solenoids? Though not connected directly to the circuit, the second coil lags the induced EMF in the main coil?

I learned vacuum tube tech in my youth but 1905 electronics predates that old tech by several decades. Perhaps @Baluncore can contribute?

Researching early coil winding methods. Could the untapped coil be left there for circuit testing?
[I now read that Mike already suggested leftover testing coil.]

 

[256bits]

- connected to 3V dry cell battery

Or two 1.5 v batteries
- were they series or parallel connected circa 1900s.

Assuming series,
And if one had a 6v battery the thinner wire could be used.

 

[Tom.G]

The resistance ratio between 26ga and 21ga is just over 3:1. (41.6Ω vs 13Ω per 1000ft.)

3V on the 21ga implies 9V would be appropriate for the 26ga winding for the same current.

I'll let someone else figure the magnetic force ratios for series and parallel connections.

Overall, I think the remark by the OP is the most likely:

I suspect they bought the coils from a different manufacturer and they were possibly intended for another application

Cheers,
Tom

 

[flynnr2]

edit: missed the link on the pdf originally!

#159 on page 274 (page 303 in this pdf) perhaps? Discovered by a co-conspirator last night.

The “unconnected” wire does have its two ends connected together.

(Not saying this theory is correct, but presumably this book represents current thinking when the coil was made ...)

 

[Klystron]

[snip]
#159 on page 274 (page 303 in this pdf) perhaps?
The “unconnected” wire does have its two ends connected together.

Good catch! I extracted and lightly edited the relevant text from the PDF.

This winding consists of two similarly insulated wires wound simultaneously side by side. Only one of the wires is used for exciting purposes, however, the other wire having its inside and outside ends connected together, thus forming a closed circuit.
It is extensively used where sparking, due to self-inductance, is detrimental to the contacts of the controlling device. When the circuit of the exciting winding is opened, the short-circuited winding absorbs the magnetic energy which would, otherwise, cause a momentary high voltage in the exciting circuit

While reading the archaic tech-speak, I remembered this circuit was manufactured the same year Albert Einstein worked in a patent office preparing publication of 5 physics papers 116 years ago.

 

[artis]

hmm interesting , so it could then be that the second short circuited winding was used as a "medieval" flyback diode back in times with no flyback diodes/diodes or semiconductors in existence for that matter.
@flynnr2 can you make a schematic of how the insides are wired?

 

[flynnr2]

Not sure I can, but I'll give it a go at some point.

In words
- The thicker wire winds up & down the core 8 times and the two ends are connected to the circuit, exiting though 2 separate holes in the bottom
- The thinner wire winds up & down the core 8 times as well, in layers with the wire which activates, but the 2 ends are soldered together in a single hole on the bottom, not connected to anything else

 

[berkeman]

I don't understand this now. Adding a shorted turn to a coil generally would effectively short out the other coil in the bifilar pair. Is there a resistor somewhere to give the 2nd coil some finite impedance?

 

[flynnr2]

Just want to be clear on terminology, I worry I might be confused by what you mean by "2nd coil" above
- there are 2 coils around solid cores
- wired in parallel (though some examples are apparently in series)
- each with 8 windings
- of the 2 different cotton sheathed copper wires
- the larger wires are connected to the circuit (3V)
- the larger wires are just a closed loop on each coil
- there are no resistors, etc.

The way the circuit works is
- as the clock ticks
- 2 contacts are allowed to come closer together & finally connect
- this completes the circuit, which activates the coils
- the electromagnet pulls a plate down when it fires
- which breaks the circuit and tensions the spring
- lather, rinse, repeat

will try to sketch something out (so not my strong suit!)

 

[Keith_McClary]

Adding a shorted turn to a coil generally would effectively short out the other coil in the bifilar pair.

When the DC is turned on, there will be a current induced in the shorted turn, which will decay.

 

[berkeman]

Oh right. I keep thinking this is an AC circuit for clock timing generation, but OP said it is just a DC circuit.

No timing circuit here per se - completely mechanical operation which completes the circuit, causing the electromagnets to fire and "wind" the clock (about once every 2 minutes ... it makes a bit of a racket frankly).

So maybe the shorted turn is a sort of soft-start feature?

 

[Baluncore]

So maybe the shorted turn is a sort of soft-start feature?

I think it is more likely to be a slow release technique. The current continues to flow in the shorted secondary when the primary is disconnected. Another way to slow the relay would be to use a bigger lump of solid iron, that will take longer to release.
It may be there to demagnetise the core?

 

[Tom.G]

Hey you two, read posts 14 and 15; and the link in 14.

 

[flynnr2]

First off, thank you again to everyone for engaging - I suspect this isn't the normal fare for this forum and my ability to describe it accurately is clearly challenged!

Now to a few additional bits of information
- recall this is a old application, circa 1905, early on in low voltage applications, etc., so folks were experimenting & discovering a lot along the way would be my guess

- solenoid is solid core

- both wires are ribbon wound, side by side, 8 layers X 35 turns = 280 turns in total

- schematic drawn by a co-conspirator, smaller wire is not grounded or shorted, just a closed loop within the coil connected to nothing (shown on right)


- #159 referenced above from:
UNDERHILL, CHARLES R (1910). SOLENOIDS ELECTROMAGNETS AND ELECTROMAGNETIC WINDINGS. D. VAN NOSTRAND COMPANY , NEW YORK, PG 274-275
159. Differential Winding This winding consists of two similarly insulated wires wound simultaneously side by side. Only one of the wires is used for exciting purposes, however, the other wire having its inside and outside ends connected together, thus forming a closed circuit. It is extensively used where sparking, due to self-in ductance, is detrimental to the contacts of the control ling device. When the circuit of the exciting winding is opened, the short-circuited winding absorbs the mag netic energy which would, otherwise, cause a momentary high voltage in the exciting circuit. This winding is calculated by the methods described in Art. 158. Owing to the fact that only one half of the total winding space is available for the exciting coil, it is very inefficient.

 

[berkeman]

Hey you two, read posts 14 and 15; and the link in 14.

I didn't understand #15 when I read it, and was too lazy to follow the link.

 

[Klystron]

Research indicates multi-meters became common after the 1920's. Engineers circa 1905 could measure resistance using a Wheatstone bridge but would not have CRT's to view waveforms. One imagines they saw new circuits as cutting edge physics despite odd-to-us theories and terminology.

Wheatstone bridge circuit diagram for testing unknown resistance.

 

[flynnr2]

So if I am understanding the emerging consensus here (plus the reference, etc.) it seems like the looped, closed circuit wire may not serve any real purpose, so I could rewind that coil with just the wire connected to the circuit and have a perfectly serviceable electromagnet?

Cheers in advance again ...

 

[Keith_McClary]

serve any real purpose

Just to prevent sparking if you control it with a switch.

 

[flynnr2]

Empirically, arcing across the contacts which activate the circuit & electromagnets is a definite problem (not to mention some questionable metallurgical choice, which combined are probably one of the reasons this clock maker quickly disappeared), even with this second looped winding.

I have a couple of Zener diodes across the contacts to suppress this along https://www.electronics-tutorials.ws/power/transient-suppression.html:



Not quite period correct for 1905 perhaps, but works a treat.

 

[flynnr2]

I have a related question to all this which someone might be able to point me in the right direction on. I had to unwind one of the coils to repair a break. To the extent that the looped, closed circuit wire isn't effective or serving much of a function (I am suppressing arcing with more modern tech as described above), if I rewind the coil with only the activating wire do I need to be concerned about changes in the pulling force (the absence of that second wire will change the relationship of the other wire to the solid core, on average making it closer than before if that makes sense)?

Put another way, can I calculate the difference in pulling force between the two different styles of winding? I've seen the basic formulas for this, but most seem to assume negligible insulation or sheathing of the wire and don't allow for multiple wires, etc.

Cheers in advance

 

[flynnr2]

BTW for those who are interested, I may not have done a decent job describing the mechanism, so here goes (some photos below as well) ...

• As the clock ticks a few things happen with each tick (it is mechanical)
  • Two contacts which complete the electromagnet circuit get closer to connecting
  • A metal plate mounted on a pivot which the electromagnet attracts gets further away from the coils
  • A spring which drives the clock pendulum looses tension
    (the spring is also pulling down on the opposite side of the pivot from the plate)
• When the contacts finally meet (about once every couple of minutes ...)
  • The electromagnet activates (obvs.)
  • Which attracts the metal plate to them
  • Which has the effect of both tensioning the spring as well as breaking the circuit by forcing the contacts apart (and causing that annoying arc, fouling the contacts, hence the 2 Zeners I have now in place)
• Lather, rinse, repeat

Basic (very) circuit schematic


Overall shot


Close-up of plate, notice circuit contacts on lower left of plate
(spring is out of sight, between the 2 brass plates)


Zener hack (test rig, not very pretty)

 

[Baluncore]

In post #28 you show pair of back-to-back Zener diodes on an AC supply. You also show data for a MOV = metal oxide varistor.
Since you have a DC supply you only requires one power diode to suppress the flyback arc.

A single diode would keep the current flowing for longer after the contacts open because the reverse voltage across the coil would be less than with a MOV or the Zeners. Just make sure the diode and battery polarity are correct so it does not normally conduct.

The shorted coil would waste energy while turning on and extend the field while turning off. If you disconnected or removed the shorted coil, the single diode would go some way to compensating for the removal of the shorted coil.

 

[cmb]

Perhaps it is to get rid of DC flux when driven by a directly connected (non-capacitive) connection? One would use a serial capacitor now as a DC block, but I imagine wire was cheaper than capacitors in those days.