From: BobEngelhardt@comcast.net   
      
   I need a sanity check - I think I understand, but there's doubt.   
      
   If I rectify AC with a bridge and use the full-wave output to power a DC   
   motor, it's not the same as using "pure" DC.  There is an AC component   
   to the full wave.  (The motor is a brushed PM if it matters.)   
      
   If I take the Fourier series of the 1/2-sinusoid and consider each   
   component separately & then superimpose them, I should get the behavior   
   of the motor on the full-wave source.  The Fourier series consists of a   
   DC component and the even harmonics of 120Hz.  The DC will simply drive   
   the motor as would a battery.  The AC, however, will not have a net   
   affect on motor's output: for its positive 1/2 cycle it will contribute   
   to the output and on the negative 1/2 it will oppose it.  So the   
   superimposed result is that the useful motor output is due to the DC   
   component only and the AC components only produce a modulation (240,   
   480, ... Hz "buzz") on the output.   
      
   I long ago lost any ability to do the Fourier calculation, but somewhere   
   on the web (source lost), I found that the DC component (a0) is 88% of   
   the RMS AC input to the bridge.  (If it's not too much trouble, could   
   someone confirm this?)   
      
   Now here's the problem: reality contradicts theory (I hate when that   
   happens!).  The theory is that if I apply 20v AC, for example, to a   
   bridge & use the output to drive a DC motor, that motor will run at 88%   
   of the speed which it would if it was driven a regulated DC source of   
   20v.  (DC motor speed is linearly proportional to voltage.)   
      
   In a test, it doesn't - it actually runs faster on the rectified AC than   
   on DC!!!  That's impossible!  What's wrong - my understanding of the   
   theory, or my test?  Or both? Or ...?   
      
   Thanks, Bob   
      
   --- SoupGate-Win32 v1.05   
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