I have seen some very interesting “rigs” for recharging flywheel magnets, on this forum, form the elegant to the extreme. Everything from home made, hand wound, hand held gizmos to 300 pound boat anchors. All have one thing in common – they require from ‘one’ to ‘many’ storage batteries to work. The logistics of the power source can be difficult at best.
I have built (wound) several attempts at making a good magnet charger. My goal was the Holy Grail of magnet charging – “The Three Pound Lift”. I have used magneto coil rings with three 12 volt batteries and two battery chargers in series, I have made several hand held electromagnets and I have recharged magnets in the car. None of these methods achieved “The Three Pound Lift”.
I know that in order to make a good electromagnet I would need a “soft iron” core or pole piece. So where does one get the mythical “soft iron”? McMaster Carr – No, MSC – Noo, Local Metal Supplier – Nooo, Local Scrap Yard – Nooo, … you get the idea.
I know that the Model T ignition coil uses “Swedish Iron“ for the core so I know that this stuff exists. This got me to thinking and to recall basic transformer construction. A transformer uses a laminated soft iron core to transfer power from the primary to the secondary much like the ignition coil. So where can I get a transformer core to use for a magnet recharger?
I looked across my work bench, and there it was – My Growler !!! A growler, (used to test starter and generator armatures for shorts) is nothing more that a soft iron core with a big fat coil wound around it. It normally runs on 120 volts AC which produces an alternating magnetic field between the open part of the pole piece. So, if I put DC into the growler it will produce a fixed magnetic field , right?
My first test was connecting it to a 12 volt battery. I dug out a magnet from the junk box that had virtually no charge on it. I hit it with 12 volts. Not much change. So I decided to go all of the way. I put a full wave bridge rectifier (a four diode module) in series with the 120 volt AC line. The output of the rectifier is about 180 volts DC which I applied to the growler.
I hooked it up and put the magnet across the pole piece. I hit it with a half second pulse – big loud hum – but no fire flame or smoke. I hit it again – same results. I removed the magnet and put it on my test weight (a cast iron piston, wrist pin and added weights to bring it up to three pounds). Not only did it pick it up but I had to shake it pretty hard to get it to release the test weight. Success !!!
So, how does DC effect the growler? The maximum AC current that my growler draws is about 1.5 amps AC. It draws about 8.0 amps on DC. Because the duration of the DC current is very short, it is very unlikely that the growler will be damaged from this type of operation. There is certainly a risk of causing the growler coil to open and render the growler useless but I thing this risk is small.
WARNING – THE SETUP IN THE PHOTOGRAPH IS NOT SAFE. IT WAS ONLY A QUICK AND DIRTY PROFF OF CONCEPT EXPERIMENT. SEVERAL REFINEMENTS ARE NEEDED BEFORE IT CAN BE USED SAFELY.
Growlers are cheap at swap meets and on tBay. When done properly, they can make a safe, cheap, powerful magnet charger.
As always, your comments are invited.
Excellent Bob! Your growler sees 120 Hz positive sine waves - pulses - better than DC for recharging the magnets. Good job and thanks for sharing.
Brilliant bloody idea Bob..! I'm going home to find my growler;)
Seth, set me straight here. Using a garden variety bridge rectifier means that your 120V 60Hz supply is converted to 120Hz with a fairly rough wave form yes?
I was aware that switching/fashing the magnetiser was a very good way of improving it's effect. Would a half wave rectifier (single diode) be any better do you think? I was assuming not.
Asside from that, we use 240V 50Hz in Australia so while the growler windings would be lighter and use less current to achive the same magnetic saturation, would the increase in voltage help do you think?
Half wave rectification may give the winding more time to go from excursion to excursion. Or, the dead time between pulses may cause a back EMF that could cause the magnetic field to reverse. I would suspect thet even with 240 V and 50 Hz, the overall power would be nearly the same. These are great questions for someome to do a study with.
Here's what I made from information contained in a book I bought a number of years ago. A friend wired it and it has a rectifier (I think) as Bob mentioned.
With your 240V (rms) mains, remember that your peak to peak voltage will be around 680V so you'll need to double the peak inverse voltage (PIV) rating for your diode or bridge. For downunder, I'd use an 800PIV diode or bridge. Bob's arrangement should be comfortable with a 400PIV diode or bridge.
These voltages are lethal, and no isolation transformer has been connected between the mains and your charging equipment so UL listing would be out of the question here!
Bob's might be safest to leave the switch on the growler on and simply plug in and unplug that arrangement. Steve's looks safer to me with that non-metallic pushbutton.
In either case, stay away from the magnet when charging!
Your setup is what I have in mind for a mature setup of the Growler magnet charger. A clean safe "packaging" of the controls for the growler. That looks like the unit described in a book from Lindsay Publications - part of my inspiration as well. The growler arangement needs a safety fuse, control pushbutton, and a kickback supression diode to absorb the energy of a colapsing magnetic field. I would envision a box that you can simply plug the growler into and it would be controled by the box. The beauty of using as growler is that you don't have to machine any parts or wind a coil or buy expensive copper magnet wire. None-the-less, I really like your setup.
Not being of an electrical background, I never thought of using my growler to power up a magnet charger and you're right on about machine shop time and purchasing the correct wire. All of the above electrical components are in the little black box as you stated. Does a heck of a job !
Nice! With a good growler, a non-metallic double switch box, a switch, an outlet, and a non-metallic switch/outlet plate plus a couple of diodes and a fuse & power cord and you are in business.
Thanks for all the posts guys - a new project when time permits. In the mean time I can have my alt/gen/starter man round me up a good growler!
Bob, what's the unit on the bench, behind the bridge?
It looks sorta like it could be a magnet recharger.
Also, being that the magnet faces are not always cut square and then, when trying to pick up a weight w/a recharged magnet the faces don't make good contact w/the weight.
I've found that by adding small shims between the weight and the part of the magnet face that doesn't contact the weight will make a big difference in how much the magnet can hold.
You get the prize! I knew someone would ask me about that. It is a 45 pound de-magnetizer (ment to work on 120 VAC) that I run with a 12 volt battery to recharge magnets (a tBay find). It also produces great results. As far as the ends of the magnets not being square, you are very right. I usually touch the magnete against a belt sander. This gives a better contact surface which makes a better magnetic conductor and a stronger charge.
I understand there could be a significant voltage induce into the growler windings as the field colapses across it. How would I install a diode to supress this? What value should it be?
Is it a better idea to install a high value resistor across the windings to knock the top off these peak voltages?
When I get home tonight (it's the workday here) I will sketch up a complete drawing with part numbers and send it. Email me so I have your address. Also in the next week or two, I will build the "refined" version and post pictures.