XPost: rec.crafts.metalworking   
   From: nospam@null.void   
      
   On Sun, 17 Nov 2013 11:18:53 +0100, depucca wrote:   
      
   > In article  "Lloyd E.   
   > Sponenburgh"  wrote:   
   >>   
   >> Ignoramus26083  fired this volley   
   >> in news:If6dnStz8dp0vRXPnZ2dnUVZ_sCdnZ2d@giganews.com:   
   >>   
   >> > But can this motor somehow excite itself to generate electricity?   
   >>   
   >> Short answer, "No."  Not unless it was a permanent-magnet motor or had   
   >> a wound armature fed through slip rings or a commutator.  There has to   
   >> be something to excite the fields initially.  Common AC motors use   
   >> reverse- EMF in the armature to repel the fields, but the armature gets   
   >> its excitation from the field windings.   
   >>   
   >> So-called "universal" motors can act as generators, so long as they   
   >> either retain some residual magnetism in their fields, or have a DC   
   >> supply to provide excitation until they can supply their own through a   
   >> rectifier.   
   >>   
   >> Lloyd   
   >   
   > Better answer, depends on the motor design.   
   >   
   > Technically any motor can act as a generator, but efficiency will likely   
   > not be optimal.   
   >   
   > Locomotives use their traction motors as generators to energize dynamic   
   > braking systems for example.  If the motors and resistors were 100%   
   > efficient, they could stop a train without using brakes, but that isn't   
   > the case.  They do generate quite a bit of energy though, which is   
   > dissipated as heat.   
   >   
   > As you say above, something must excite the fields.   
      
   Pretty much any standard 3 phase induction motor can be used as a   
   generator, provided that it has a load with a leading power factor   
   (normally the controller will connect as much capacitance as required for   
   this) or is paralleled with the utility line and driven above synchronous   
   speed.  Control is a bit tricky when not line connected, but a search on   
   induction motor generator should lead you to information on how to do   
   it.  A wound rotor or permanent magnets are not required, unless you want   
   to be able to generate power without using a complex controller.   
      
   You can find a fairly complete mathematical analysis in the chapter on   
   induction generators in "Principles of Alternating-Current Machinery" by   
   Lawrence, 4th edition, 1953.   
      
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