For some time now I have been using what was a 160watt armature growler to remagnetise flywheel magnets. It was powered from the 240volt mains supply using a bridge rectifier & momentary switch. Worked quite well but without the inductive reactance of an AC supply, the current through the growler was high enough to shorten it's life...
I've now secured what was a 240/230 to 100volt transformer. Dad has ground the top of the iron core out so we can use this new unit for manetising, however the 240volt winding carries 13amps AC & I wasn't able to gauge the DC current, needless to say it was too high, dimming the lights in the shed!
My question is what should I hit it with. Conventional wisdom is 36VDC from some batteries. I had wondered about using a mig welder for a DC supply. I have a choice of windings on the transformer, the primary being •062" multi tap between 230, 240 & 250 volt or the heavier •095" 100 volt secondary side... or perhaps both in series if I could work out the polarity they create.
What do you think?
Anthony, There are several good older posts on this subject. What I recall is that it takes about 5000 amp-turns to remagnatize a Ford magnet, and that a coils magnetic flux strength transferred to the iron cores is greater closest to the iron core.
From all this it seems that a coil of say, 100 turns would need a power source capable producing 50 amps DC, and that a longer coil is better than a shorter coil with the same number of turns.
In your situation, you have two coils (primary and secondary) with an unknown number of turns, but with a turns ratios of about 2.3,2.4 and 2.5 to 1. It would seem to me that if the resistance in the two coils is close to the same ratio, you should have the same magnetic flux produced in each at a given voltage, although the current would differ in each coil (ohms law). I would connect each coil, individually, to test if each coil lifts the same weight and to determine the polarity of each coil. You will eventually want to wire the coils, in parallel, so that you have a N and a S at opposing pole pieces.
Looks like it was originally intended to power Japanese equipment given the 100V secondary.
Seeing as the secondary current was rated at 15A, the current in the 240V primary would have been only 6.25A at full power. I'd therefore be using the 100V winding to apply the DC. Besides, it will have lower resistance than the 240V winding which means you won't need as much DC voltage to get the same flux. Presumably the DC resistance is a fraction of an ohm or not much more. 12-24-36V are all worth trying. The key to it is to only momentarily apply power before it has time to get hot enough to ruin the winding insulation. It only needs a fraction of a second to do the magnetising.
I'd try a variac (variable auto-transformer)and a bridge rectifier w/a filter capacitor to power it. That way you could ramp the voltage up and monitor the DC voltage and current. Connect to the 250V terminal to give the most ampere turns. Connecting the primary and secondary windings in series if you can figure out polarity would give you the most ampere turns.
Then you would know what you need for a power supply.
I was also wondering if a bridge rectifier was needed at all, or if in fact half wave rectification would be preferable.
Old boss of mine swore the best magnetiser he ever used was one that had a fusible link. This unit was energized until the fuse blew & the subsequent collapse of magnetic field was thought to do the trick.
I wondered if a capacitor discharge unit would be even better...?
Either way I'll do some more experimenting shortly.
Hmm... Same theory used to recharge magnets in the car... several momentary flashes from DC source. The The half cycle field collapse produced from a half wave rectifier might show some interesting results.
There is considerable trial and error in
re-magnetizing T magnets. I made two coil units the first was sadly inadequate and the second one is greatly improved. I laminated some iron components and bent them to match the angle of the magnet and wrapped as much wire as I could. There is a wire size to number of coils ratio, the more the coils the better. In my case I used three 12 volt batteries(36 volts). The test is to pick up a one inch square X two inch long block of steel. The front drive gear in the transmission is equivalent in weight. Momentarily while the power is on tap the magnet with a hard wood block to aid in additional alignment. Nick Tesla