Magnetic field of a 4 pole magnet?

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Assume this is the General Magnetics forum for a minute. I have the rotor out  of a gizmo made out of alnico. It looks like a + and the poles are NSNS around. If it was 2 bar magnets I guess it would be NNSS? There is a hole in the center for a shaft. So it is really like 4 bar magnets in a diamond shape, right? I'm sitting here with a file and a piece of something magnetic and some filings on a piece of paper trying to see the field pattern, but its just a clump. Its part of a 'brushless tachometer'. Anyone ever heard of one of those?

 

Imagecraft compiler user

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We use that type of "magnet wheel" in our encoder apps.  (Generally "10 pole")  But I guess I never thought of it in N/S terms.  Hmmm--it might be that the outside is N-N-N-N with gaps in between of "not North".

 

I guess it is indeed N-S-N-S.  See the "Targets" section of this Web site..

http://www.phoenixamerica.com/Fi...

http://www.phoenixamerica.com/pr...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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See also from SparkFun

http://dlnmh9ip6v2uc.cloudfront....

 

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Bob--It has been a few years since we did the app.  Our main tech is pretty sure it was 8 or 10 pole, with all the N to the diameter and all the S to the center.  So with the reed-switch backup pair, we read N-notN-N-notN.  Same with the primary pair of Hall sensors.

 

That said, some of the references above seem to say (or at least imply) N-S-N-S.  That wouldn't work well with reed switches if they were close together.  Maybe on a 4-pole.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Here's a pitcher of my gizmo. About 1.5" diam. A little mag attracts and repels the poles alternately. I have a MAC gauss meter. Reads about 40G. Alnico magnet.

 

Attachment(s): 

Imagecraft compiler user

Last Edited: Thu. Dec 18, 2014 - 09:43 PM
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We use one of these but we make it out of four magnetic monpoles. Just need to be careful to arrange them in the NSNS pattern.

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Bob, this is a picture of a pitcher.

 

 

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ignoramus wrote:

We use one of these but we make it out of four magnetic monpoles. Just need to be careful to arrange them in the NSNS pattern.

Have you been buying your magnetic monopoles from monopoles r us? It turns out that they've been manufacturing their bar magnets from magnetic monopoles, and that their individual monopoles are more expensive than if you buy one of these bar magnets, then chop it in half to get two monopoles.

 

- S

 

Last Edited: Fri. Dec 19, 2014 - 02:40 AM
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YukYuk. Believe me, I have googled for "brushless tachometer", and info on this booger is scarce as hen's teeth. Its used in a military gizmo, so its almost secret. I was really hoping one of you guys with deep experience had bumped into something like this. We actually got one from the guys that asked us to build a bunch of them The boss says "Here. See if you can make his work". I have a copy of a xerox of a pdf that says it puts out .55V per rad/sec. I measured 225 mH and 1K ohms. I'm trying to calc what the relative permeability of the toroidally wound core is so I can get a quote on a custom wound toroid. The necropsy reveals that the wire diam is abt 2.5 mils. AWG 42. 1600 ohms per 1000 ft. Anybody have a toroid winding outfit that they like?

Hi Larry. I employ the vernacular in an attempt to bond with the common folks. Sometimes they offer up some uncommon wisdom. Just aks em.

 

Imagecraft compiler user

Last Edited: Fri. Dec 19, 2014 - 04:45 AM
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bobgardner wrote:
Just aks em.
Ain't no denyin' that they can learn us a heap here Billy Bob.wink

 

It's topics like this that pique my interest.  Nice one.

"I may make you feel but I can't make you think" - Jethro Tull - Thick As A Brick

"void transmigratus(void) {transmigratus();} // recursio infinitus" - larryvc

"It's much more practical to rely on the processing powers of the real debugger, i.e. the one between the keyboard and chair." - JW wek3

"When you arise in the morning think of what a privilege it is to be alive: to breathe, to think, to enjoy, to love." -  Marcus Aurelius

Last Edited: Fri. Dec 19, 2014 - 05:20 AM
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Monpoles R us are a bunch of bastards.. they have perfected the method of hiving off the magnetic monpoles without upsetting their humours ( see http://www.merriam-webster.com/d... ) thus maintaing balance of the fluxes. They now charge an arm and a leg for their monopoles. Those who can not afford to pay for amonopole get the "sweepings off the floor " leftovers.

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I figure they expect it to spin past a coil to sense the rpm. I think I'd try putting some hall effect sensor and watch for the Norse and the Souths to go past.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Are these Scandinavian, and you really meant "the Norse and the Swedes..." ?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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" norse and swede " a turn-ip of phrase

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theusch wrote:

Are these Scandinavian, and you really meant "the Norse and the Swedes..." ?

 

LOL, I love this website!

 

 

JC

Click Link: Get Free Stock: Retire early! PM for strategy

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The brown ring is a plastic encapsulant around a rectangular cross section ring of soft iron (?) with awg 42 wire wound toroidially. I think it puts out two hi and lo humps per rev. Spec for amplitude is .55V per rad/sec. I guess this is just a simple reverse engineering Florida Copy of the original item.

 

Imagecraft compiler user

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There exist thin foils to display th field of magnets:

http://www.supermagnete.de/physi...

 

There seem to be "radial amgnetized" disks here:

 

http://www.google.de/imgres?imgu...

Last Edited: Sat. Dec 20, 2014 - 08:25 AM
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Thanks for those links. They have new search terms to aid my search.

Imagecraft compiler user

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There exist thin foils to display th field of magnets:

 Hmmm--back when I was your age, there was a fluid that you "painted" on e.g. magnetic tapes, and you could "see" the bits with a magnifier.  Magna-Vue?  Magna-See? 

 

MagnaView:  http://unitednuclear.com/index.p...

MagnaView Fluid is a colloidal suspension of microscopic, individual magnets - not metal particles - microscopic magnets, in a liquid carrier.  These tiny magnetic particles have an average size of about 10 nanometers, and are coated with a stabilizing dispersing agent which prevents the particles from sticking together even when a strong magnetic field is applied. In the absence of a magnetic field, the magnetic polarity of the particles are randomly distributed, and the fluid has no net magnetization.

However, when a magnetic field is applied to MagnaView Fluid, the magnetic polarities of the particles orient along the field lines almost instantly and the fluid itself conforms to the magnetic field.

Since MagnaView Liquid responds immediately to changes in an applied magnetic field, when the field is removed, the magnetic particles once again randomize quickly.

 

MagnaView liquid is ideal for demonstrating the effects of magnetic fields and is also very useful in experimentation with magnetic fluid flow control.

...

 

 

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Can't relate directly to your magnet, Bob, but here's how we do it:

 

a) when we just want shaft speed, an aluminium disk with an even number of holes drilled through it close to the edge (for maximum speed) and neowhateverium magnets a couple of mm across and perhaps 5mm long stuck in each hole. These move past an iron-cored coil about the same diameter as the magnets and with a couple of thousand turns of 0.2mm wire. They're arranged alternate polarity around the edge of the disc and the coil axis is parallel to the disc axis - though that's for packaging rather than any deep philosophical need. This gives a sine wave pulse at one cycle per magnet pair. If we turn the magnets around, there's virtually no output as there's no significant flux change.

 

b) when we want speed and direction, we remove most of the magnets, leaving just two pairs diametrically opposite, both arranged in the same sequence. This gives either a N-S flux change in one direction or a S-N in the other, so we end up with a pulse twice per revolution whose polarity positive or negative gives direction.

 

In both cases, the height of the signal is roughly proportional to speed until things get saturated; we work at low speeds (down to one or two per second) with a normal speed of around 100 pulses per second.

 

If your magnet is n/s/n/s as you imply, it's just for speed.

 

Neil

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I would use a n/e/s/w magnet.

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steve17 wrote:
I would use a n/e/s/w magnet.
Good thing I wasn't drinking coffee...

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