Solenoids

[cfellows]

Didn't want to highjack Allthumbs' thread so thought I would start another. I've been doing some calculations and experiments with solenoids the past couple of days. I decided to build a solenoid 2" long and 1" OD. The inside diameter of the coil is .45". As shown below, I made the bobbin out of 1" diameter black delrin. I bored an 11/32" hole, 2 inches deep and epoxied a piece of thin walled brass tubing inside. This makes the bobbin stronger and, perhaps, wear better without having any affect on the magnetic properties.

Then I turned the inner part to .45" diameter and parted off the finished bobbin.

I drilled a very small hole in one of the flanges to feed the start of the wire through. Then I mounted the bobbin on a 5/16" bolt with a rubber washer over the end with wire protruding, started the wire, and chucked it in my lathe.

Here's a picture of the 8lb spool of wire

I wrapped about 350 feet of #28 wire on the bobbin and secured it with black electrical tape.

The measured resistance was about 20 ohms and the wire is rated at a maximum of about .32 amps.

Continued on next Post...

 

[cfellows]

I tried the solenoid with a 5/16" plunger which fit nicely inside the brass liner using 12 volts. I wasn't particularly happy with the amount of pull force. I figured it was barely enough to run a small, single cylinder engine. I could increase the pull proportionately by increasing the voltage, but I wanted to use 12 volts or less to run any future solenoid engines I build.

I unwound all the 28 gauge wire and rewound it with 24 gauge wire. 24 gauge is rated at about .81 amps and even though I got significantly fewer turns, the resistance was a lot lower at only 5.7 ohms for the finished coil. This time, with 12 volts applied, I got a lot stronger pull. That's because with the lower resistance, I was pushing 3 times as many amps through the coil. The coil started getting pretty warm after about a minute so I couldn't run the solenoid at 100% duty cycle for very long. However, I think at a 50% duty cycle, which would be 1/2 of every revolution, it would be fine.

The finished coil has 14 layers of tightly spaced turns. The total amount of wire is about 150 feet which is under 1/4 pound.

I've also learned a few things about physical coil parameters based on calculations:

1. If you use smaller gauge wire on the same size bobbin, your pull force will be less. So, if you use smaller wire, you need to use a smaller bobbin because the resistance goes up faster than the number of turns.

2. If you make the overal diameter larger resulting in more turns, the pull force will also be less, because, again, the resistance goes up faster than the affect from increased turns.

3. If you increase the inside diameter of the coil, you'll also get less pull force.

4. If you make the coil longer, increasing the number of turns, but using the same size wire, the same OD and the same ID, the pull force will remain the same. This is because the increased resistance is exacly balanced out by the increased number of turns. However, the current (amps) will be less and the effective stroke can be longer.

Encasing the solenoid in iron or steel doesn't much affect the pull force until the plunger gets close, maybe 80% in, to the end of the stroke. Then the pull beings to increase really dramatically.

That's all for now. More information as I go along.

 

[AllThumbs]

Thank you. That is helpful.

E

 

[zeusrekning]

Very good information here. Made some attemps at this a couple weeks ago with little luck. Could you go into more detail about the winding. Did you wind each layer left to right, right to left? Or did you wind left to right then come back to the left and wind left to right again? I couldn't think of a better way to word that. Hope it makes since. I was actually making a simple electro-magnet, should the principles be the same? I wanted my EM to repel magnets for a hall effect motor but I couldn't get it to repel a rare earth magnet. The magnet would still stick to my steel core.
Keep the info coming Cfellows.
Tim

 

[DICKEYBIRD]

Great info Chuck!
Where did you find your wire? I reckon the high price of copper brings the price of an 8 lb. roll of wire up there pretty high.

Using my meager knowledge of ohm's law, the amperage is roughly 2 amps? If so, what are you planning to power your motor with? That's a bit above the average wall wart. Maybe an old computer power supply?

Keep the info flowing! ;D
Milton

 

[tattoomike68]

I am no electrical wizzard but I did used to make many tattoo machines. the solenoids are much like what you are building. adjusting the points while the machine was running would light a person up good. ;D

For a power supply I used a tyco HO scale train set transformer.

 

[cfellows]

Very good information here. Made some attemps at this a couple weeks ago with little luck. Could you go into more detail about the winding. Did you wind each layer left to right, right to left? Or did you wind left to right then come back to the left and wind left to right again? I couldn't think of a better way to word that. Hope it makes since. I was actually making a simple electro-magnet, should the principles be the same? I wanted my EM to repel magnets for a hall effect motor but I couldn't get it to repel a rare earth magnet. The magnet would still stick to my steel core.
Keep the info coming Cfellows.
Tim

Starting on the left side, I just went back and forth winding the turns as tightly as I could. It's not terribly important to get the windings in nice even layers, but it does let you get more turns in a smaller space.

The principles for solenoids and electromagnets are basically the same. The only difference is that in a solenoid the steel or iron core is the plunger whereas it's usually fixed in an electromagnet.

The problem you've got with your rare earth magnet is that it's attraction to the steel core is stronger than the repulsion from the electromagnet. Increasing the voltage to the EM might work. It might also be worth trying it with the steel core of the EM removed.

Chuck

 

[cfellows]

Great info Chuck!
Where did you find your wire? I reckon the high price of copper brings the price of an 8 lb. roll of wire up there pretty high.

Using my meager knowledge of ohm's law, the amperage is roughly 2 amps? If so, what are you planning to power your motor with? That's a bit above the average wall wart. Maybe an old computer power supply?

Keep the info flowing! ;D
Milton

I had bought the wire on eBay several years ago and it's been sitting on a shelf. You can still get pretty good prices on eBay. Just search for magnet wire.

I calculate the amps to be a little over 2 (Volts / Ohms). I have a number of Variacs with full wave rectifiers in the shop since some of my tools are powered with large DC motors. For those of you who aren't familiar, a Variac is a variable transformer than runs on AC, usually 120 volts but some are rated for 240 volts. The transformer has a rotary knob that lets you adjust the output voltage from zero to about 140 volts. These are availabe from about a 2 amp rating up to 30 amps or more. There's usually a lot of them for sale on eBay. The voltage has to be changed from AC to DC, but this is easy to do with what's called a bridge rectifier. These are a simple little device with two poles for AC in and two poles for DC out. They cost less than 5 bucks. Just make sure they a high enough voltage and current rating.

Ultimately I plan to run the motor on a 12 volt rechargeable batter. These are available on eBay and hobby shops for RC models.

 

[AllThumbs]

Mr Fellows,

Good information. I have been fiddling too. You are a lot smarter in that you are fiddling before building an engine, I am fiddling after building parts, so my coil is now limited by what I can squeeze into what I have built so far.

An observation/question. Do you filter the rectified DC out of the variac? if not, this will affect your testing, since the DC of of the variac will be choppy. This will add inductive reactance, (increasing the impedence) to the electrical circuit, causing calculated current to be more than actual current (ohm's law doesn't apply to AC or straight rectified DC when we are talking coils).

In other words, for coils, 6V rectified AC does not behave like 6 V DC (battery) unless it is filtered with a large capacitor.

Eric

 

[cfellows]

Eric,

No, I don't filter the DC voltage. I do understand that the rectified DC is choppy, and that the actual current is probably less than calculated. When I get around to powering the engine with a battery, I plan to use a small, electronic controller to vary the speed, so hopefully the increased current won't be a problem.

Thx...
Chuck

 

[cfellows]

I just uploaded an Excel spreadsheet that will let you change physical parameters of a coil such as wire size, coil length, outside diameter, inside diameter, and voltage. The spreadsheet will calculate the length of wire required, the resistance in ohms, the current in amps, and the amp turns which is a gauge of magnetic force.

Chuck

 

[zeusrekning]

:bow:

 

[rickharris]

The output of the bridge rectifier won't be all that choppy as the bridge rectifies both sides of the AC voltage, The inductance of the coil will have some smoothing effect anyway.

Sources for wire can include transformers from older electrical equipment, electric bells, motors and similar. So called Magnetic wire means enamel coated rather than plastic insulation. You should find 2 different gauges in the primary and secondary coils.

You must wind the coil in the same direction as this effects the direction of the magnetic field and changing direction will produce a field that opposes it's self and so reduces the over all effect.

If you want a stronger reaction replace you iron core with your rare earth magnetic, Switching a coil with reed switches or mechanical switches is no problem but be cautious when using electronics with the coil as the collapsing magnetic field creates a strong reverse electrical flow (Back EMF) this can distroy electronics connected to drive the coil - usually it is cut off by using a flywheel or snubbing diode across the coil.

 

[AllThumbs]

By choppy I mean from 0 to max voltage 120 times a second. ;D

Just to clarify, it makes no diference if you wind from left to right and back or right to left and back. What you don't want to do is wind half the spool clockwise and the other half counterclockwise.

E

 

[cfellows]

I wound another coil tonight to see if the diameter of the plunger had much affect. The construct of the coil was the same as the last one, #24 wire on a delrin spool with a thin wall brass tube pressed into the center. However, where the last one took a 5/16" diameter plunger and was 2 inches long, this one took a 7/16" plunger and was 3 inches long. I had to make this coil longer with fewer layers to get approximately the same number of amp turns. Running both coils on 12 volts, I didn't notice any appreciable difference in the pull. While the diameter of this plunger is only about 40% larger, the cross sectional area, which is what matters, is nearly twice as large. Perhaps a more objective test using a scale would measure some difference but I couldn't feel any.

From a technical standpoint, I think this means that the smaller plunger was more than adequate to conduct the magnetic lines of force without saturating and using a larger diameter plunger didn't really improve anything. As you increase the voltage and/or amps, the magnetic lines of force are increased and eventually the core (plunger) becomes saturaged. Once you've saturated the plunger, increasing the amps or voltage will not make any further increases in the pull strength on the plunger. Stated another way, I believe if the plunger is not saturated, increasing its diameter will not make any difference.

The three inch long coil I made tonight did provide pull over a longer stroke, so for a longer stroke engine, longer coils are the order of the day. I also discovered that if you insert a short bolt into the opposite end of the coil, the pulling force increases dramatically as the plunger gets closer to the end of the bolt inserted from the other end. This, however, does shorten the effective stroke.

Of course, the real test would be building a motor and seeing how well the coils perform. Too much pull and the engine is going to run too fast. But then, I guess I'd have to wonder how that could be a problem? :

By the way, as a disclaimer, I'm far from an expert in these matters and welcome any challenges to my proclamations!

Here is a link to a 5 page description of solenoid construction, how they work, and the different types. Very good reading.

http://www.sal.wisc.edu/pfis/docs/archive/public/Product%20Manuals/bicron/soldesop.pdf

Chuck

 

[cfellows]

I started on an engine for my solenoid this weekend. I plan to use a larger diameter flywheel in the finished engine, mostly for looks.

I used the 2" solenoid, wound it with 24 gauge wire. The finished coil has 14 layers of tightly spaced turns. The total amount of wire is about 150 feet which is under 1/4 pound. The bobbin was made from 1" diameter delrin, with the inside of the spool turned down to .45 inch.

Not the best pictures, but you get the idea.

The aluminum cylinder shroud is 2" tall and 1.25" diameter. The stroke on this engine will be around 5/8 inch.

 

[Bogstandard]

Lovely engine coming along there Chuck,

If it runs as well as it looks, it will be a stunner when finished.

It's always nice to see homebuilds emerging from the ashes.

John

 

[DICKEYBIRD]

Sigh, you guys make this stuff look so easy.

Chuck, is that an o-ring at the top? Destined to hold the outer "cylinder block" in place? Clever.

 

[AllThumbs]

You wisely did some testing before building. A question or two: Does the piston ever come right out of the coil? Also, what will be the shape of your piston? I am thinking a "T" shape would be the best. Is that what you are planning?

E

 

[cfellows]

You wisely did some testing before building. A question or two: Does the piston ever come right out of the coil? Also, what will be the shape of your piston? I am thinking a "T" shape would be the best. Is that what you are planning?

I plan on using 5/16" plunger from an existing solenoid which is just a solid rod with a slot in the bottom to attach the connecting rod to. I would be worried about the lateral forces on the piston if the full diameter part were too short. What do you think would be the advantage of a T shaped piston?

I will have to experiment some with the length of the plunger and the connecting rod. My home wound solenoid is not encased in iron or steel, so it behaves differently than one which is. In a standard, steel encased solenoid, the pull on the plunger increases dramatically as it reaches the end of the stroke. Conversely, the amount of pull exerted by a coil not encased in steel drops off as the plunger approaches the centered point.

So, the short answer is, I'm not sure if the piston will come all the way through or not...

Chuck