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### Topic: Solenoid type (Read 858 times)previous topic - next topic

#### Yaro

##### Jun 12, 2014, 10:37 am
Hi, i've a question about solenoids. I have found two type of solenoid. Here's an image:

The first type, why this type is not diffused?(i see more II type) It's easyer to set stroke to millimeters, what can be defects?

The second type, from where i need to calculate stoke? form last winding to the wall of plunger?
The plunger can't be long as coil because i need to attach a hook, so how much this influence solenoid force? Or i can have an infinite length on one side and other need to be distant(stroke) from the last winding?

Thank you.

#### jackrae

#1
##### Jun 12, 2014, 10:43 amLast Edit: Jun 12, 2014, 10:46 am by jackrae Reason: 1
Type 1 has a problem in that the plunger (or slug) is not within the main magnetic field so the pull force is relatively low.  The nearer the slug gets to the coil armature the greater the force experienced

Type 2 plunger is the armature and is contained within the magnetic field so the pull action is much higher.  The plunger will attempt to "balance" itself  to the centre of the magnetic field.  Providing the plunger is longer than the coil you can get it to extend beyond the coil end.

#### MarkT

#2
##### Jun 12, 2014, 01:37 pm

Type 1 has a problem in that the plunger (or slug) is not within the main magnetic field so the pull force is relatively low.  The nearer the slug gets to the coil armature the greater the force experienced

They both have the problem that the magnetic circuit is not closed so all the forces will
be relatively small.  Usually solenoids have a steel frame that completes the magnetic circuit

Type I has a large piece of iron in the coil, the fact the other piece isn't in a coil
isn't a problem since its close by and is attracted just as a piece of iron is to a permanent
magnet.
Quote

Type 2 plunger is the armature and is contained within the magnetic field so the pull action is much higher.  The plunger will attempt to "balance" itself  to the centre of the magnetic field.  Providing the plunger is longer than the coil you can get it to extend beyond the coil end.

The balance effect applies to a single iron rod in a coil and is a weak force - here there's
a lump of iron at the end of the winding, so again there is a magnetic attraction between
an electromagnet and a lump of iron (stronger force).  Normally this more common type
has a frame completing the magnetic circuit so that their is only one gap and the forces
are a lot higher still.

Magnetic circuits are strongly limited by the gap or gaps as these have far higher
reluctance than the iron or steel (by factors of 1000 or so).  Calculating detailed magnetic
forces in the presence of ferromagnetic materials is a rather involved bit of calculus,
alas...  The forces on the coil are a lot less than the forces on the iron, since the bound
currents in the iron are many times larger than the free currents in the coil(*). The large
forces are always between two bits of iron/steel, never between the coil and something else.

(*) bound currents are due to electron spin in ferro/ferri-magnetic materials and
are quantum mechanical in nature.  For a material with a permeability of 1000 the
bound currents can be upto 1000 times the free currents in a coil around it.

For instance neodymium iron boron magnets have bound current density of
about ~500kA/m, which is close to the maximum achievable in any ferromagnetic
material.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Yaro

#3
##### Jun 13, 2014, 09:42 am

But for my solenoid i didn't build external structure. I found the first type on a Matlab, and the second is a common type, so i didn't know what type is better.

So, they have equivalent principle of operation and the second type is easyer to build.

Can you explain me how external steel frame need to be?, near to the last wire right? how distant from the upper wires?
Also, it need to be connected to something? ground or other connections?
I have stainless steel and aluminum aviable as material, but they have low magnetic permeability to build a frame?

I've noticed that the metal is magnetized after i turn off the solenoid, but it'll stuck to the iron frame, there is a method to unmagnetize it?

#### MarkT

#4
##### Jun 13, 2014, 03:04 pm
Wikipedia "reluctance" and google "magnetic circuit" for all you could ever want to know.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Yaro

#5
##### Jun 13, 2014, 04:47 pm

Wikipedia "reluctance" and google "magnetic circuit" for all you could ever want to know.

Ok, it's bit complicated but i've read something, but i have one last doubt(maybe).

I found this:

Back stop is only to stop plunger, but how much need to be inside the core? or this have no influence? i've seen someone is inside other's outside.

#### jackrae

#6
##### Jun 13, 2014, 07:25 pm
Read my input  -  slug endeavours to reach a balanced position at the mid-point of the coil.
The position of the stop does affect the final pull value with it being zero at the point where the slug centre is at the coil centre.  In your illustration this will only occur if the slug is shorter than the coil

#### michinyon

#7
##### Jun 14, 2014, 06:58 am
The important bit you left out of that drawing,  is the spring.   The magnet will only pull the armature one way.

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