From: daestrom@twcny.rr.com   
      
   On 11/16/2011 18:32 PM, m II wrote:   
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   >   
   > Johny B Good wrote:   
   >   
   >>   A modern alternator removes the need for laminated magnetic core   
   >> materials from the bit that rotates, allowing for solid steel poles and   
   >> a co-axially wound field coil. A design change that permits   
   >> significantly higher rotational speeds compared to the armature of a   
   >> dynamo.   
   >   
   >   
   > I beg to disagree.   
   >   
   > It could be argued that laminated armatures are far stronger than one   
   > piece, cast versions. Each lamination has a hardened 'skin' on it that   
   > forms during the manufacturing process.   
   >   
   > They are certainly more vibration resistant than cast pieces. The reason   
   > they are cast is one of economics. It's cheaper than assembling laminations.   
   >   
   > As for the co-axially wound field, it would make no difference. The   
   > reason alternators can spin so high is the fact that it's the *field*   
   > spinning instead of the main windings. The rotor wire is a lot smaller   
   > than that of generator, resulting in a great reduction of centrifugal   
   > force.   
   >   
      
   Well I disagree with your disagree statement.   
      
   It isn't a question of how strong the steel is, it's a matter of how the   
   winding is placed.   
      
   In conventional DC generator, the windings are placed axially in slots   
   with insulation and wedging.  This wedging fails at high speeds,   
   allowing the winding to spill out of the slot.   
      
   Also the turns at each end of the rotor where the wire exits one slot,   
   travels part way around the rotor and enters another slot is subject to   
   failure.  In tiny machines, the wire is self-supported.  In larger   
   units, the end turns are banded with insulation and steel wire to hold   
   them inward against the centrifugal forces.  Spin it fast enough and   
   when that support fails, the end turns bow outward, contact the stator   
   iron and get destroyed.   
      
   Because the alternator winding is mounted in between the two iron rotor   
   pieces, and they each have about 14 steel teeth bent towards the   
   opposite piece, they provide a very strong case to hold the copper   
   windings inside.  The 'teeth' are about 3/8 inch by 1/2 inch square   
   steel bar (or larger).  When assembled, there is very little space   
   between them and the wire is crossing the gap at nearly right angles,   
   not lined along the gap like the slots in a DC generator.  (The steel   
   pieces are forged and heat treated, not cast, BTW)   
      
   DC generators are also limited in speed by the commutator construction.   
     All the bars are held in place with just a couple of bands, one on   
   each end (on small machines, sometimes steel wire wrapped over the bars   
   with insulation).  At high speeds these bands can stretch from the   
   extreme forces.  This lets the bars slip in relation to one another.   
   Even if the bars don't come out completely from under the banding, once   
   a bar slips the brushes get chopped off by the high bar like a buzz-saw   
   in a matter of seconds.   
      
   These problems are also eliminated with an alternator and replaced with   
   single piece slip rings that are made of one piece (again, much stronger   
   construction).   
      
   daestrom   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)