From: liz@poppyrecords.invalid.invalid   
      
   john larkin  wrote:   
      
   [...]   
   >   
   > Resistors are the best way to dump power.   
      
   I once had to make a set of replacement resistors for the starter unit   
   of a 5 kW blower motor that took quite a while to run up to speed   
   because of its high-inertia load.  I had some slate bars about 2" x 2.5"   
   and 18" long cut to order by a slate quarry; then hand-wound them with   
   resistance wire .   
      
   The original wire had broken into short pieces when the unit was   
   destroyed by arcing but there was one piece long enough for me to   
   calculate its resistance using a four-terminal measurement.  When I   
   contacted the wire manufacturers, they supplied the exact replacement   
   which was still in production despite the unit having been made in 1919.   
      
   The windings were interesting, with several tappings and one section   
   wound two-in-hand to handle a higher current.  The control gear was   
   connected to the air outlet of the blower and, as the motor speed   
   increased, a cast-iron chamber was pressurised.  This caused a leather   
   bellows inside the chamber to collapse at a rate controlled by lead   
   weights hung on a lavatory chain.  The belows pushed a rod which slid a   
   set of brushes over some brass contacts on an ebonite panel and   
   progressively cut out the resistors.   
      
   The motor was interesting too.  It was a repulsion-start, induction-run   
   motor, which is quite rare nowadays but was popular in 1919, before   
   reliable starting capacitors were available.  It needed a major rebuild,   
   including skimming the face commutator.  The shaft was so long it   
   overhung the tailstock end of my lathe, so we had to support it on   
   wooden 'V' block bearings and feed it with a constant supply of oil.   
      
   A very challenging and satisfying job.   
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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