Has anybody remagnetized a rotary on-off mag base before? I was thinking about a large capacitor and some turns of wire on EI transformer core with the Is removed and the indicator base across the center and one side leg.
How are these magnetized at the factory?
Can you get the magnet out of the base?
If so, find a really powerful magnet and let it clamp to the base magnet. Then lightly tap the base magnet with a hammer. May need to tap several times. Must be very old mag base or very cheap magnet.
Paul
Probably your best bet is to replace the old alnico magnet with a new rare earth magnet. This is because if the alnico magnet is removed from the magnetic circuit it loses a lot of its magnetism. The alnico magnets in old and/or inexpensive mag bases were/are magnetized after assembly. This takes a LOT of current. I have seen the setups for doing this and they are not trivial. Eric
The magnets in my cheap magnetic bases are black ceramic magnets. I did once re-magnetise a screwdriver by putting it into a coil of wire and connecting it a DC power supply as I didn't have any permanent magnets handy.
I save the magnetising assembly at a magnet factory in Sheffield, UK, a long time ago. Which leads me to voice agreement with Eric. It fitted on top of a pallet, took some huge current, dust was flying and their were major creaking sounds as enormous forces resulted. Rich S
I've built 1000A test stations for devices such as electric locomotive controllers. The construction is considerably different from normal electronics, requiring copper buss bars, crimpers for 4/0 welding cable lugs, mechanically strong and heat resistant insulators etc. All conductors must be well supported to resist the magnetic attraction between them and to the steel chassis. Common test equipment won't measure the high currents or microOhm resistances involved.
My personal test gear collection includes second-hand lab grade and new import hobby grade shunts like this
Reminds me of the old mag cells. A building was a row of pots up one side and down the other electrically in series, running at about
30,000 amps. The workers commonly stored their tools by just slapping them on a buss bar above a cell. There was a massive tool drop every time they took a cell building down.
What's a mag cell/building?
I have some axial field servo motors. They have a magnetizing loop (#14 wire)-that they sharge with a capacitoe to top up the feild after they are assembled because without the small gap the magnets cannot hold a full field (apparently they quite quickly drop to about half strength if you dissassemble the motor) They hit that coil with close to 1000 amps for a millisecond or so to do the deed.
How do they keep the LC circuit from oscillating? If it's a clamp diode I'm curious which one they chose to handle 1000A. The energy it converts to heat is 0.5CV^2.
Some components such as diodes and breakers have a one-time surge rating tht's far above their normal operating current but AFAIK they aren't normlly tested for it in production, as it may degrade the part.
Electolytic production of magnesium metal from magnesium chloride, which was extracted from seawater. There were about 30 pots in one building. Each pot was a heavy steel pot, refractory lined, with about 10 10" diameter graphite anodes descending through the insulated lid into the bath within an inch or so of conical steel cathodes. Voltage drop was a couple volts, liquid magnesium metal and chlorine gas were produced.
It took a special breed to work in the cell buildings, but they did get a lot of breaks to avoid heat exhaustion. My cousins that grew up like I did in the rice fields worked there during a strike, said it wasn't much hotter than shoveling levees in a rice field, and a lot easier.
Pete Keillor
Any details on these magnetizers?
I had some blade magnets made for a guitar pickup, cut with a diamond from neutral (but permanent) ferrite into slabs 120x22x4mm and magnetised across the 22mm dimension in one of these machines.
It was a small machine, about 1.6m long, 1.2m high, with a C-shaped armature (gap upwards) about 40cm broad with a massive winding like a small oil drum across the bottom. The ferrites were spaced up with soft magnetic spacer blocks and clamped in to remove most of the air gaps.
The operator hit the switch and the lights dimmed, the concrete slab floor noticably thrummed, for about 30-40 seconds, and the magnets were cooked. I believe it was being fed from a rectified three-phase feed, which is 50Hz here.
Does that give you some idea?
Clifford Heath.
Do you know what sort of magnets these motors had? From what I've seen, Alnico magnets are the ones that lose their fields the easiest. This is why even the decent ones ships with keeper plates.
I suspect it's usually Alnico in these mag bases, had to send one back to Mitutoyo once, it just lost it's field and wasn't stored in the on position. They just swapped it for a new one.
There's little chance I'll locate the same sized rare earth to retrofit into the base itself. I don't want to mangle the faceplate to tear into this one either. I have a supply of large electrolytic caps and some mega SCRs. Don't mind trying something out if the magnetizing currents and times aren't something too insane.
I knew an old man (25+ years ago) in Mt. Dora Florida who restored 'Huff and Puff' engines. His prize possessions were a photo of him selling an engine to 'Grandpaw Jones' (and an uncashed check) who was part of the Grand Ole Opry'.
He also rebuild Magnetos and sold magnet rechargers that he built in a 10' x 20' rental warehouse. He used a large electric hacksaw to cut a large bundle of soft iron rods into the cores, and wound the coils on his lathe. There are businesses that recharge magnetos that may be able to do what the OP needs.
On 3/23/2020 12:47 AM, Cydrome Leader wrote: ... I have a supply of large electrolytic caps and some mega
I once tried to re-magnetize an indicator base & don't remember any details, just that it involved really large current and didn't work very well.
I do remember what I thought was a neat trick for switching. The switch was 2 copper leafs, separated by an air gap. To turn it on you hit it with a hammer! Very fast and very low resistance as the oxide is displaced and you have a large area bare copper-copper contact.
- discharge magnetizer circuits shown may be modeled as a series combination of a capacitor, a resistor, and an inductance. The electrical resistance must include the resistance of the source as well as that of the fixture (especially including the ESR, equivalent series resistance, of the capacitors), and also includes components from eddy
- current conduction in surrounding structures, in the magnet itself, from skin effect in the conductors, etc. In addition, the resistance may increase during the period of the pulse (by perhaps 30%) due to heating in the fixture (the resistance of copper and most other metals increases with temperature). The inductance of a fixture containing steel pole material is dramatically affected by whether the fixture is below or above magnetic saturation (the inductance dropping greatly at currents above saturation). Other effects may be of importance too, such as the retention of energy by the electrolyte of the capacitors, the absorption of energy by the magnet, and other nonlinearities. Nonetheless, in many cases the overall system behavior of the magnetizer and fixture is modeled to sufficient accuracy by assuming constant values for the resistance, inductance, and capacitance. Even where the assumption of const ant values of these parameters is not justified for final design, the linear analysis may provide a good first approximation and a check on the calculations. Where the linear approach using fixed values is not accurate enough, however, a computer simulat ion including all nonlinear effects may be used. The method is described in detail in the bound notes (reference 7).
- energy magnets often cause forces which could pull apart copper conductors in a single pulse, if they are not strengthened by other means. These forces are also more than strong enough to bend, crush, or extrude out epoxy potting plastics. (4) The thermal requirements must be met. the near
- i nstantaneous temperature rise in the windings during the pulse occurs too quickly for much of the heat to escape across even a single thin layer of electrical insulation. If this rise is too great, the insulation will fail, on a single pulse. A thermal t ime constant exists for this effect, and a thermal mass, which is often significantly different from that of the fixture as a whole (that is, the time constant is shorter than that of the fixture, and the mass is less). Both must be taken into account. A n extreme example is a large C frame fixture, used for automatically
An SCR switch blocks the LC oscillation that would immediately demagnetize the magnet. You still need a flyback diode to suppress the reverse voltage spike.
The actual SCR is a thin silicon wafer slice between the anode and cathode contact disks. The specified clamping pressure ensures that enough of the area of the wafer disk will be in electrical contact to distribute the current and the heat it produces across the wafer rather than allowing it to concentrate in a few spots and burn out the SCR. Driving the gate harder than the minimum helps rapidly spread the initial area(s) of conduction and even out the instantaneous heating.
The clamps GE sent us for our test stations consisted of two U channels connected by bolts. The dimples accept alignment pins that center the anode and cathode contact disks while you are assembling the awkward sandwich of SCR, contacts, insulation and clamps. We used a simple sheetmetal gauge that aligned two points when the channels had bent far enough to produce the specified clamping pressure.
Nice article. Rail gun power supplies have the same problems.
Search that subject at your own risk.
There's a two-part magnetization process for alnico, you have to observe the built-in (at ceramic-firing time) polarity when you apply current. It WILL magnetize one direction, not the other.
Usually, metalworkers have access to high DC current sources from Lincoln, Miller, Hobart, etc. and that's what it'll take.
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