On Thursday, 18 February 2016 14:33:47 UTC+1, David Lesher  wrote:   
      
   > c) From what I have read, historically, railroads used low freq   
   > AC because they used series motors.  They had to, to get speed   
   > control; our everyday induction motors are speed-locked to the   
   > line frequency. [+/- some percentage of slip, but that's a few    
   > percent..]   
   >    
   > d) With a series motor, inductive reactance limits the current   
   > they would draw. X[lr] is a direct function of frequency; the   
   > lower the freq, the less it is. At DC, it is zero.   
      
   Just to expand slightly on this, an issue with using series motors with mains   
   frequency supply is that the inductance of the windings can cause flashover at   
   the commutator, which using a lower frequency resolves.   
      
   While modern traction uses VVVF (variable voltage variable frequency) control   
   units on 3 phase AC traction motors, there was the intermediate step of   
   control using a tap changer on the main transformer and rectifier on the low   
   voltage side to feed DC    
   traction motors.  This was the natural solution, but did not become   
   commonplace until the 1960s, which allowed the UK, France and a number of   
   other countieres to adopt mains frequency 25 kV AC, principally because until   
   the development of high enough    
   capacity solid state semiconductor power rectifiers, there was no means of   
   rectifying AC to DC on board a locomotive that was either small enough, light   
   weight enough, or robust enough (in terms of surviving the vibrations and dirt   
   of a railway    
   locomotive environement) to be practical.  Some mercury arc rectifier   
   locomotives were used, but the reliability left a good deal to be desired.   
      
   Robin   
      
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