Leonard CROW
Master
Magnet
Warning -- it is rumored that this magnet is "unhealthy", causing unpleasant mental/physical sensations
Leonard Crow : Design, Construction & Operating Principles Electromagnets for Attracting Copper, Aluminum & Other Non-Ferrous Metals
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The Magnet’s Secret:
Since the electromagnet’s windings are powered by AC, an alternately increasing and decreasing magnetic field is set up in its center core. When this varying field passes through a set of copper washers fastened to the end of the core, a large current is induced in them. The washers, then, act essentially as a transformer secondary.
The induced current sets up a strong, varying magnetic field in the washers. And the direction of this field is such that the washers and the core repel each other. If the washers were not anchored in place, they would spring out of their mounting as soon as the current was turned on.
The point is, though, that the varying field in the washers will induce, in turn, a large current in any metal object (ferrous or not) brought near them. This current, of course, sets up a magnetic field in the object. And the direction of the field will always be such that the part of the object in contact with the outside face of the set of washers will move the opposite magnetic polarity fro that face. Therefore, the object will be attracted.
Building Magnet Cores:
To start, first cut a 3-1/2" section from a 2" diameter mailing tube. Make a frame for the inner core of the magnet as described in Detail "A". In forming the frame the three wooden discs should fit snug inside the tube. The four ¼" diameter wooden dowels will pass through the discs by holes drilled then glued in to place to hold the assembly together. The core material will later fill the ¾" diameter hole drilled into the centers of the discs.
Now slide the frame into the mailing tube spaced ½" from one end and flush on the other end and glue in this position. Three copper washers will fill the ½" space later on (See Detail B Side View).
Set the closed end of the tube down on a table top and proceed to pack the core with lamination approximately ¼" wide and 3-1/2" long. Laminations may be removed from an old transformer or made from 18 to 22 gauge soft iron wire cut squarely on one end 3-1/2" long. The pieces should make a smooth surface when packed together.
Next, slide a 3-1/2" long, 3" diameter mailing tube over the completed assembly centered as near as possible. Fill the space radially around the inner core assembly with laminations like previously used wide enough to fit snug and 3-1/2" long (See Detail B End View)
Winding the Coil:
For this step you need a jig similar to the one in Detail C. This wooden cylinder with two fitted end-pieces is 3-1/2" long and 1/16" larger in diameter than the mailing tube’s diameter. The end pieces are sawed so that slots are formed to allow temporary tie wires (see next paragraph), and a metal crank handle runs through the center of the cylinder.
The jig is now ready. Center 12" pieces of hookup wire in each slot, pressing them flat. The coil will be wound over this and the wires will serve to temporarily hold the coil together when removed from the jig. Next, frill a hole in a block of wood clamped in a vice. This will hold the jig as you crank the other end.
The coil is made up of 600 turns of #14 cotton or enamel-covered magnet wire tapped at the 350th turn. Around 9 pounds will be needed. Insert the rod end of your jig into the block of wood, stick the first 6" if the end of your supply wire through a saw-slot, then start winding the wire in layers onto the cylinder.
When the 350th turn is reached, tap on a 6" length of wire and brig it out through a saw-slot. The point of tapping can be varied as much as 10 turns in either direction so that the tap out is at the end of a layer on the same end a s when we started the coil. Now continue until the 600th turn is completed and bring the end of the wire out the same end through a saw-slot and cut leaving a 6" piece as before.
Final Assembly:
Using the wires previously inserted, tie the windings together. Disassemble the jig and remove the coil. Using ½" wide cotton or linen tape, wrap the coil from the inside to the outside in overlapping layers. Once completed, coat the inside of the coils and the outside of the 3" mailing tube assembly with glue. Make sure the leads and the space for the copper washers are on opposite ends, insert the cores into the coil, and allow the glue to dry.
During the drying time make a ½" thick wooden ring with the inside diameter [?] wide and the outside diameter flush with the outside of the coil. This is a spacer between the coil and the top (See Detail B Side View). Cut grooves in the spacer to allow the leads to pass through the center and glue onto the coil.
Now with the same outside diameter as the spacer, cut a ½" wooden top. Mount a lifting ring in the center, made from brass or copper. Drill another hole for a 6’ length of #14 stranded, 3-wire conductor for the power cable. Push one end through the hole and connect the coil leads to the cable leads.
At the free end of the power cord mark each of the leads as connected with "start of winding", "tap", "end of winding". This is important for later hook up. Next glue the cable in its hole so connections cannot be pulled apart by flexing. The wooden top can be secured to the spacer ring by brass wood screws. Next coat the whole magnet with black insulating varnish or enamel to help secure the cotton or linen coil wrappings and protect it from moisture.
The last part of construction is the forming and installation of copper washers in the space between the ends of the inner and outer magnet cores. Specifications of the washers are in Detail D.
These copper washers are secured with flathead brass wood screws driven into the inner core. Counter sink the screws into the outside washer, and fill any excess space between the washers and the inner coil with slices of wood or cardboard so the last washer is flush with the inner and outer cores.
Do not substitute any other metal in place of the copper washers. Heavy current induced by the washers require that they be made of extremely low resistance metal.
Electrical Hookup:
If the AC line is connected between terminals 1 and 2 of the magnet coil (See Schematic A), the current consumed will be around 20 amperes --- quite excessive for use around the house. Connecting terminals 1 and 3 (See Schematic B) results in a current flowing around 4.25 amperes, and the strength of the magnet is reduced proportionately. In both cases the current performs little useful work; this inductive circuit lags about 90* behind the voltage.
This lag can be partially offset by adding an 80-uf, phase-shifting capacitance as shown in the modified parallel-resonant circuit of Schematic C. The current drawn from the line is about 4 amperes, while the currents flowing between terminals 1 and 2 and terminals 3 and 4, respectively, are 18.5 amperes and 9 amperes. This hookup results in a more powerful magnet than the hookups in Schematics A or B.
The maximum magnetic pull is obtained with the series-resonant circuit illustrated in Schematic D. In this hookup, 17 amperes flow through the whole coil, allowing the coil to pick up and hold 6 or more half-dollar coins or an equal weight f other non-ferrous metal.
The 80-uf capacitance specified in Schematics C and D is built up by paralleling several smaller capacitors. These must be of the non-electrolytic type with ratings of at least 250 volts if connected as in Schematic C, or 600 volts if connected as in Schematic D. Units totaling less than 80-uf could be used, providing they have the proper voltage ratings, but the current flowing through the magnet winding would be reduced.
High voltage appears across the capacitors, and since they are apt to retain their charge after being disconnected from the line, this should be enclosed in a metal box. As another precaution, the capacitors should always be discharged with a tool having an insulated handle before any work is done on the circuit.
Because of the peculiarities of the magnetic field around the copper washers, the magnet will not attract pieces of non-ferrous metal wider than their outside diameter or nattower than their inside diameter.
A final word of caution: the washers carry considerable current through them and get quite hot. Connect the magnets only when necessary.
Materials:
1 Cardboard mailing tube,
3-1/2" long, approx. 2" diameter
1 Cardboard mailing tube,
3-1/2" long, approx. 3" diam.
4 Wooden dowels, 3-1/2" long,
¼" diam.
1 Roll ½" wide cotton or
linen tape
1 Heavy brass or copper hook
(for lifting ring)
1 Line plug
1 80 uf, 250 or 600 volt
capacitor bank
1 Wood or metal enclosure for
above
1 6’ length of #14 stranded
3-wire cable (for power cord)
1 6’ length of #14 stranded
2-wire cable (for line cord)
9 pounds of #14 cotton or
enamel covered magnet wire
Miscellaneous:
½" wood stock for
center core frame, magnet top and spacer ring
1/16" sheet copper for
washers
Old transformer
lamination or 18 to 22 gauge soft iron wire for cores
Parts for winding jig
Flat head brass wood
screws
Cement
Insulating varnish or
enamel
Schematics A, B, C, D:
Terminal 1 = Start of
Winding
Terminal 2 = Tap
Terminal 3 = End of winding
