Relative Magnet Strength Test

[TightWind]

I dont have a gauss meter, but I do have a digital scale. I bought some magnets from another source, not Gobrushless, Not GoBrushless.

I ordered N50's thinking I would upgrade the magnets in the stock motor.
I had a spare can, so I put the new magnets in the spare can. I noticed what I thought was weaker cogging when placed on the stator/shaft and turned. So, I used the remaining "N50's" and N45's to do a comparison.

I built a small frame of scrap balsa sticks and attached a pushrod for sticking a magnet. I placed the frame over top of scale which also had spare magnet taped to the center. With the scale zeroed and the magnets like poles facing each other I compared the "weight" reading from the repelling force of N45 vs. N50. This test was done using all the 3 spare N45 clumped together and all the 3 spare N50's clumped together - equal numbers of each type. This test was done a couple of weeks ago, so the only numbers I still remember is that the N50 read 22% less force on the scale then the stock N45. I repeated the test several times so when I calculated the percent difference I was pretty confident that the N50's read 22% less. The 3 magnets on the scale were N45's from the spare kit.

I was disappointed when I got this result and I complained to the place I bought them. A new set was sent promptly. So I did the test again when the replacements came in. The new N50's tested equal strength to the stock N45's and 22% stronger than the first N50's.

What is going on here? Did I get N50's in the kit and N45's from vendor X? I did not mix up magnets as they were marked in different sharpie colors as they were placed in motor cans,so the spares were marked.

Anyone?

[TightWind]

alright, I'll ask it this way. Should N45 have less relative attraction/repulsion than N50 for the same distance?

Moderator?

[LBMiller5]

The answer is yes,

The "N" Number of the magnet is directly proportional to the field strength of the flux field. A N45 magnet has 90% of the strength of a N50 magnet. Likewise a N48 magnet has 96% the field strength of a N50. A N40 magnet has 80% of the field strength of a N50. The number is the Maximum Energy Product. It is the point on the Demagnetization Curve where the product of B (Induction) and H (Magnetizing Force) is a maximum, and the required volume of magnet material required to project a given energy into its surroundings is a minimum. It is measured in Mega Gauss Oersteds, MGOe.

Your supplier could have mixed them up, or the weak ones could have been overheated at one point or another and lost some of their strength. These magnets start becoming de-magnetized at 80 degrees C, or 172 degrees F. If the magnets were shipped in a container that was at the top of the stack of an open deck container ship on a very hot sunny day, or if a box of them were left on a loading dock in the direct sun all day, it is possible for the temperature to get hot enough to partially de-magnetize the magnets. (Have you ever felt the hood of a black car on a hot sunny day?)

Here is a like to a great website that explains a lot about magnets:

http://www.magnetsales.com/Magnet_Design.htm

Hope that helps!

Lucien

[ScubaSteve]

I realized two days ago that I hadn't updated our website. All our stock GBv kits include N50s, not N45s.

I posted a long thread on ezone about magnets a while back that i'll try to find and paste here... You really have to be careful where you get your magnets. Rules of thumb are:

1) Don't listen to anyone claiming to have anything rated at higher than N50. The technology doesn't exist to the extent that we can afford them for R/C use. :wink:

2) Do not buy into the hype that certain vendors (I won't mention names) try to encourage by throwing around the word "licensed"... "We have licensed N45 magnets blah blah". Licensed or not, it have absolutely NOTHING to do with magnet strength - in fact it really has nothing to do with the magnet itself. If anyone's interested i'll go into detail but it's not really that interesting.

3) These magnets aren't the cheapest. We all have a pretty good idea of how much they cost regardless of the slight variation in price from vendor to vendor, so if you see someone selling NdFeB magnets for as much as you'd pay for the same type of magnets on your fridge, something's probably amiss... as they say, you get what you pay for.

I'm not saying GB is the only place to get magnets - there are plenty of reputable places all over the world. I'm just saying to be careful 'cause too many people buy into hype or don't use the best judgement. Ok off my soap box now. :roll: :wink:

FYI my source for magnet this and that is none of than George @ Engineered Concepts, from whom we buy 100% of our magnets. http://www.engconcepts.net

*EDIT - you're right Paul, it's .net :oops:

[paulvi]

Steve is that link right takes us to a future home of page.

[Q]

5 seconds of googling gave me http://www.engconcepts.net/

".NET", Steve, ".NET" !!!!!!!

[juki48]

thats funny, I've been buying odd sized magnets from george for a while now. he is great! he even gives you a fridge magnet that will hold someting up other than itself. what a concept :!:

[Q]

You want a fridge magnet that will stick your mother in law to the fridge?
Open up a hard drive and you've two of those. Soak them in acetone untill they come off the metal plate they are glued to. Totally impossible to get off a flat metal surface. You have to slide them to the side!

[juki48]

I know. I have two sets of them sitting on my desk here at work. I've been playing with them instead of working. :roll:

[paulvi]

5 seconds of googling gave me http://www.engconcepts.net/

".NET", Steve, ".NET" !!!!!!!

WACK!! the sound of my open palm hitting my forehead..

Thanks Q

I like those hard drive magnets I use 4 to erase 8mm Tapes.. They require reformating when done but works better that the RS deguaser

[DadHav]

Hi Guys, I know there's a string on this somewhere, but I thought I might mention it here. If you haven't made a gauss meter from a CDROM Hall Effect sensor yet, maybe this might give you a few ideas. The picture shows mine. I used a Radio Shack AA batery holder. This gave enough room for one battery a swith and gluing the aluminum tube in place. The sensor I believe came from a Sony CDROM drive. The sensor was so tiny that I actually installed it inside the 3/16 diameter tube. I did this by leaving it on a portion of the circuit board, soldering the leads, insulating the chip with black CA and then installing it with black CA. I rounded the bottom of the circuit board so that the sensor wouldn't move away from the magnet as much as if it were tipped. Meaning if you tip the probe. The divice itself for some rason doesn' require a millevolt meter. I've been using it on the regualar scale. It measures about .6 volt sittiing on an N50 magnet. The probe starts picking up the magnet from about 4 inches away but seams to be pretty linear even at the close range of the last inch. I use the probe from both sides either laying the board on the magnet or laying the tube against the magnet from the opposite side of the sensor. I'm thinking about adding a small trim pot to adjust the sensativity. Hope this helps with some ideas.
John
I wish these forums had a spel checker.

[bz1mcr]

Demagnetization at temperatures approaching 80 degrees C was already mentioned as a potential factor. There are three other reasons the strength of magnets is not always what we might think it should be. Allowing these high performance magnets to bang or snap together, can weaken the magnetic force and could even break them physically. Dropping magnets on a concrete floor can also have the same result. So magnets that are heated or subjected to impact can be weakened. The other factor is in the ratings and manufacturing variation themselves. In a good factory, on a typical day the N values are + or - no more than 2 numbers, but even with that, a magnet rated N48 could actually be stronger than some rated N50. Variation at this level would not explain how a magnet rated N45 could be stronger than a magnet rated N50, but they could be very close N45+7=N47 and N50-2=N48. With out expensive equipment most of us would think them the same. The final factor is size variation. Your force test is measuring the result of the materials magnetic strength and the amount of material. On a 2 mm nominal dimension the typical tolerance is - or + 0.1 mm. Force would change with volume which is affected by the tolerance of all three dimensions. Differences that seem small can add up to effect the result.