Rotor construction

The magnet rotor is also a casting. There is also a procedure later for assembling the parts. First collect together the magnet plates, magnet blocks, stainless wire rope, etc. as described next.

Magnet plates

Each magnet rotor is built on a steel disk, 6mm thick. See diagram 32. Do not use aluminium or stainless steel for this disk! The disks have to be made of magnetic material. The disk has holes to mount it to the hub - in this manual the hub has four holes, each 1 0mm diameter, on a circle at 4 inches (1 02mm) PCD. If a different hub is chosen, then all the jigs and moulds must match this hub.

At the centre of the disk is a 65mm diameter hole. There should also be four holes drilled and tapped (threaded) for M1 0 rod between the magnet positions, at 220mm PCD. Screw four pieces of M1 0 rod, 20mm long, into these holes. These will bond to the resin and help to secure the casting onto the disk.

32. MAGNET ROTOR DISK

The magnet plates must be flat, not warped. It is not easy to cut the outer circle without warping the plate. A guillotine can cut steel plate into an octagon (see diagram 33), without warping the plate. This is an alternative way to make the rotor disk. First cut a square, draw a circle on it, and then cut off the corners at 45 degrees. The length of each edge is 11 6 mm.

The magnets will be placed on the corners of the octagon.

 

33. ALTERNATIVE SHAPE : AN OCTAGON

The central hole is made with a hole-saw or it can be cut out on a lathe.

Grind the steel disks until they are bright and clean, just before putting them in the mould for resin casting. Remove any grease with spirits.

Magnet blocks

There are 8 magnet blocks on each rotor. Each block has a north pole and a south pole (see diagram 34).

34. POLES ON THE MAGNET BLOCKS

N AND S POLES ATTRACT EACH OTHER

POLES WHICH ARE THE SAME REPEL EACH OTHER

Take care when handling the magnets. Magnets can damage floppy discs, music tapes, credit cards and other magnetic media. Separate them from each other by sliding them sideways. They attract each other with strong forces. Take care not to let them fly together - they may break. Never use a hammer to assemble the PMG. You may break a magnet or break the resin holding it.

The top faces of the magnet blocks on the disk must alternate N-S-N-S-... There is a method to check that you are doing this correctly, as follows. Each time a magnet block is placed, hold it so that it repels the one before (see diagram 35). Then place it without turning it over. When they are all in, check with another magnet: it will be attracted, repelled, attracted, repelled, by each magnet in the circle.