Solenoid with a rod magnet instead of an iron plunger

Thread Starter

cmartinez

Joined Jan 17, 2007
6,467
Question, wouldn't it be more efficient to use a cylindrical rod magnet (say, neodymium) instead of a soft steel rod as plunger for a solenoid magnet?

The way I understand it, current traveling through the solenoid creates a magnetic field, which in turn attracts inside the rod sitting on the outside. But wouldn't the force (and efficiency) be much greater if a rod magnet were used instead? That way both the solenoid's field and the magnet's would interact, instead of just the iron rod being magnetized by the solenoid's field alone.


 

crutschow

Joined Mar 14, 2008
23,361
Normally a solenoid is surrounded by magnetic material which the magnet would be attracted to.
So that attraction would likely negate any advantage of using a magnet for the rod.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,467
Normally a solenoid is surrounded by magnetic material which the magnet would be attracted to.
So that attraction would likely negate any advantage of using a magnet for the rod.
I was thinking about the solenoid being wrapped around a non-magnetic tube. Like plastic, for instance.
 

nsaspook

Joined Aug 27, 2009
6,281
I was thinking about the solenoid being wrapped around a non-magnetic tube. Like plastic, for instance.
You would need some sort of pole piece for the coil magnetic field generation.

I would think most PM magnet materials would saturate (most are already near the saturation point for max strength) rather quickly limiting the amount of induced magnetism from the coil (the effective attractive force) vs what's possible with soft iron.
http://www.shinetsu-rare-earth-magnet.jp/e/design/mandc_zoom_05.html

I'm sure it's possible but I don't see much of an advantage unless you need some sort of latching effect.
https://standexelectronics.com/wp-content/uploads/Form-B-Latching-Reed-Relays-Sensors-Training.pdf
 
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Thread Starter

cmartinez

Joined Jan 17, 2007
6,467
You would need some sort of pole piece for the coil magnetic field generation.

I would think most PM magnet materials would saturate rather quickly limiting the amount of induced magnetism from the coil (the effective attractive force) vs what's possible with soft iron.
Ok ... this is embarrassing ... I'm realizing that maybe I don't understand the basic principles of magnetism here ... doesn't the coil itself create a magnetic field by itself, without needing a paramagnetic material?
 

nsaspook

Joined Aug 27, 2009
6,281
Ok ... this is embarrassing ... I'm realizing that maybe I don't understand the basic principles of magnetism here ... doesn't the coil itself create a magnetic field by itself, without needing a paramagnetic material?
Sure, it would generate a weaker magnetic field due to the reduced inductance of the coil if the pole piece was missing and much of the concentrated pulling force to the engaged position would be lost.



Contactor type
 
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crutschow

Joined Mar 14, 2008
23,361
doesn't the coil itself create a magnetic field by itself, without needing a paramagnetic material?
Do you mean a magnetic material?

Yes, a coil creates a magnetic field, but it's very weak unless is has a soft magnetic material to concentrate and carry the field.
Thus a coil wound on a steel nail makes a much stronger field than the same coil in air.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,467
So what you guys are saying, is that a magnetic material acts as a carrier of the magnetic field. Even though it doesn't create it itself, nor does it add more energy to it (how could it?)

So the iron rod acts as a "funnel" or "concentrator" to what's already there?
 

Alec_t

Joined Sep 17, 2013
10,372
A solenoid coil generates a magnetic field H, but the magnetic flux B in a material inside the core = μH, where μ is the magnetic permeability of the material.
It is likely that soft iron and a permanent magnet have magnetic permeabilities of the same order of magnitude (I haven't checked), so using either as a core could yield a similar current-induced magnetic flux magnitude within the core. That flux would add to or subtract from the flux due to the permanent magnet, depending on the position and orientation of the magnet.
 

MrChips

Joined Oct 2, 2009
19,280
So what you guys are saying, is that a magnetic material acts as a carrier of the magnetic field. Even though it doesn't create it itself, nor does it add more energy to it (how could it?)

So the iron rod acts as a "funnel" or "concentrator" to what's already there?
Actually, it is more than that. The type of material changes the magnetic flux created given by the formula

B = μ I N / L

where
μ = magnetic permeability
I = current
N = number of turns
L = length of solenoid

The key factor is μ which is a property of the core material used in the solenoid.

If we use the permeability of vacuum (or air) as a reference, i.e. μ0 = 1
The relative permeability of iron can be 100 - 5000 times higher.

 

MrChips

Joined Oct 2, 2009
19,280
Magnet vs iron core

To clarify a few things,

1) in physics terms, the coil alone with air or vacuum core produces a magnetic field H. Adding an iron core produces a magnetic flux B given by the formula:

B = μ H

where
μ = magnetic permeability
H = magnetic field
B = magnetic flux

2) The magnet core is already saturated. Adding an additional magnetic field H will not increase the magnetic flux B.

3) When the solenoid is energized, the air gap is reduced and the magnetic flux increases substantially. With a magnet core, you will require substantial effort to release the solenoid.
 

Tonyr1084

Joined Sep 24, 2015
3,584
Using a magnet would turn the solenoid into a speaker. With polarity going one way the magnet will be pulled into the core while the reverse side of the sine wave would want to expel the magnet, similar to the way a speaker works. You'd be able to use such a solenoid ONLY as a DC supplied solenoid, and if you get the polarity wrong then the solenoid would not engage.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,467
Take this solenoid, for instance. Its datasheet shows that it has an incorporated magnet at its bottom, which is shown in 'Detail "A"' ... my question is, why? Why does it need a magnet at all?

upload_2018-4-15_14-41-13.png
 

Tonyr1084

Joined Sep 24, 2015
3,584
Why does it need a magnet at all?
I can't answer that. For whatever reason it was engineered that way - it benefits something. Perhaps, and this is just a guess, perhaps it reduces the amount of current the device requires to hold the solenoid in place. OR maybe it prevents the solenoid from falling out too fast. It might be some sort of hysteresis designed in. But the one thing I noted is that this particular solenoid is DC powered. It absolutely won't work with AC. When the sine wave swaps polarity the solenoid will want to eject the core. Then we all may have to abandon ship.
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
I have used these they are in fact magnetic latching, they are similar to the latching relay of the same type, they have memory, so they retain state even when power is removed.
They require an alternate polarity on the coil to un-latch.
Max.
 
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