lab electromagnet from scratch

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Rich Feldman
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Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: lab electromagnet from scratch

Post by Rich Feldman » Sat Nov 02, 2019 9:23 pm

First real measurement of the radial flux density, B, indicated 0.60 teslas. That's much lower than I'd guessed at the beginning. It's roughly consistent with last night's initial SWAG, based on a copper pipe's physical parameters and rate of descent, judged by eye. I still don't know the sign, which could be checked with a magnetic compass at a safe distance.

Today's measurement is based on the lifting force from current in a voice coil. Flux measurement by integration of induced voltage will be done next.
The lifting force factor, in newtons per ampere, is identical to a loudspeaker parameter called Bl, in tesla-meters. Product of flux density B and the length of wire in the magnet gap. Ain't the SI system nice here? Some presentations found on the Internet suggest that 20 T-m is an ordinary value in powerful woofer drivers. Anybody here got experience with loudspeaker design or reverse-engineering?

My hobby lab style, as some friends know, often seeks quick gratification from materials and tools on hand. Buying new stuff, or having to clean up and find lost stuff, is no fun.
Today's "voice coil" started with a tube wound from 1-inch-wide paper, and some glue, using copper pipe as a mandrel. It has 12 turns of 26 AWG magnet wire, from a 5-lb spool that I _did_ find under cobwebs and dust.
It's driven by an adjustable DC power supply, whose digital current meter has a resolution of 0.01 ampere.
The resisting force is a stack of nickels (USA $0.05 coins) which are taken to be 5 gram weights.
Picture also shows that the ferrite ring magnets are far from concentric with the steel pole parts. I drew an eccentric circle on top plate directly above the inner edge of the ferrite parts. Is that just from sloppy assembly, or is it on purpose? Could be to leave more space for some other part in original Sunfire powered subwoofer box. Like the eccentric jet engine inlets on 737's, which are to increase the ground clearance.

For N_nickels = 0 to 4, I recorded the current at which voice coil began to rise from its support.
To get the most out of the few significant digits, I let Excel figure a straight-line fit:
nickels.JPG (17.21 KiB) Viewed 1191 times
The slope works out to 1.26 N/A, which we take as the Bl value.
The average coil diameter, times pi, times the number of turns, gives us wire length l = 2.107 meters. So we infer that B = about 0.60 teslas, around the middle of the gap, at the radius of this coil.

The flux density in this magnet will be about 27% greater at the inner pole face than at the outer pole face, simply because of the radial geometry. But the force factor, in N/A, doesn't depend on the radial position of any turn in a voice coil. A turn near the outside is in a weaker B field, but has a proportionately greater length of wire, so contributes the same amount toward total "thrust". It crosses the same total amount of magnetic flux (in webers or maxwells) per millimeter of axial displacement.

This same post just went up on another forum for hobbyists, but no more than that.

[edit]Got the sign without finding a compass, or applying right or left hand rule. The magnet assembly _is_ the compass.
Suspended with axis horizontal, it turns so the annular plate (outside pole) faces magnetic south.
The inner pole is connected to end plate which faces magnetic north.
Will be fun to model this in FEMM, with enough room on the outside to see and measure the far-field dipole moment.
All models are wrong; some models are useful. -- George Box

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Rich Feldman
Posts: 1314
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: lab electromagnet from scratch

Post by Rich Feldman » Sat Nov 09, 2019 8:27 am

Permanent magnet measurement exercise is complete. Ought to be good practice for using fluxmeter to learn about flux leakage in Big Three Inch electromagnet project.

I told the details step by step on another forum, as they unfolded:
That's different from the old forum, which has been almost abandoned for about a year. :-(
After _their_ change of software or hosting service left people unable to see old pictures or post new ones.

Here let's start with a FEMM analysis of my subwoofer magnet, whose center pole is hollow. My first use of permanent magnet material in simulation: stuff called Ceramic 5 in the material library under Hard Magnetic Materials, with no adjustment to its default magnetization strength. The FEMM exercise was actually done AFTER my first flux vs Z sweep in the lab.
Now back to the lab, whose RFL model 916 fluxmeter was presented years ago in this very thread, with this picture:
flux2.PNG (447.98 KiB) Viewed 1084 times
Today it's connected to the sense coil pictured about a week ago, wound at the end of a black plastic pipe that fits into loudspeaker magnet gap.

Here is a representative data point collecting procedure.
1. Set range buttons to 1 x 10^3 kilomaxwell turns.
2. Put sense coil holder all the way in. Now the coil surrounds the magnet center pole near its root.
3. Push the Reset button (of voltage integrator connected to the LED panel meter).
4. Pull sense coil all the way out (far enough for flux through it to be practically zero).
5. Read instrument digital display: 1.523. That indicates 1.523 million maxwell turns. Same as 0.01523 volt-seconds.
6. Divide by the number of turns in green-wire sense coil, 5. Result is 304.6 kilomaxwells (3.046 milliwebers).
That's the total amount of flux in center pole at its root. Average density in the steel there is 20,000 gausses (2.0 teslas).
This place is a bottleneck, because of saturation, so the radial flux in main gap can't be much more than 1/2 tesla.

Next part: repeat the reset, move, read sequence for different initial z positions.
Sense coil motion is constrained by a paper sleeve, that fits over center pole and a clear plastic extension:
The flux values F(z) decline from 3.046 mWb (304.6 kilomaxwells) near root of center pole, to practically zero 14 cm above that place.
Delta F between two z values represents flux that passed out of the center pole and outward through air between those z values.
So dF, divided by the cylinder section area 2 pi r dz, gives us the radial flux density B(z).
flux_c2.JPG (30.14 KiB) Viewed 1084 times
The quantitative match between measurement and un-adjusted simulation is amazingly close. Random good luck? Close encounter of the third kind?
All models are wrong; some models are useful. -- George Box

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