Liquid rheostats

[NW Mailing List]

Gordon:
 
While your description of the function of the liquid rheostats is correct,  
those not familiar with wound-rotor induction motors could come away with 
the  impression that the rheostats were like variable series resistors in the 
direct  feed to the motors (like the old accelerating resistors in 
streetcars).   Unfortunately, it's a bit more complicated.
 
The best railfan-oriented reference is probably Mason Cooper's "Norfolk and 
 Western Electrics."  For those who want a short-form answer, think of it 
as  controlling the ability of a motor (three-phase induction motor) that 
would  prefer to run just a little slower than the synchronous speed for the 
frequency  (25 Hz) of the supply (the difference being called "electrical 
slip") to run a  LOT slower, during the time the train was accelerating.  When 
the rheostats  were fully at their "zero resistance" point, you had the 14 or 
28 mph running  speeds (depending on the number of poles the motors were 
connected for - fewer  poles gave the higher speed) which were where the 
motors were operating with the  lowest electrical slip, i.e. the closest to the 
synchronous speed.
 
Given how many lines it took to give the "short-form" answer, time to  
recommend you go read Mason's book for more, or contact me off line 
_drperiepa at aol.com_ (mailto:drperiepa at aol.com) .
 
Dave Phelps
 
 
In a message dated 5/3/2014 10:48:14 P.M. Eastern Daylight Time,  
nw-mailing-list at nwhs.org writes:

There was an earlier inquiry about the liquid that was  used in the 
rheostats of the N&W or VGN split phase electric locomotives,  but I don't remember 
the thread or that there was any reply to the  inquiry.
 
I just came across a Feb. 8, 1926, N&W Motive Power  memo that includes the 
following:

Rheostat consists of Soda Ash Solution, 10 lbs. to 300  gallons of water.  
A portion of solution is continually passed through  cooling tower to reduce 
temperature.

Of course, the liquid rheostats were used to  control the current flowing 
to the traction motors.  Considering the date  of the memo, the description 
of the solution likely applied to both the  N&W Class LC-1 and LC-2 electric 
locomotives.   The VGN  Class EL-3a electrics also used liquid rheostats, 
and because the soda ash  solution had apparently been successful in the N&W 
electrics and because  the VGN electrics were so similar electrically to the 
N&W electrics, it is  likely that the VGN EL-3a electrics used the same soda 
ash  solution.
 
The above-cited N&W memo also included the  following interesting tidbit:

Locomotive requires about 115 seconds to  accelerate from standing to 14 
M.P.H. with 1600 ton trailing load on 2%  grade.

Gordon  Hamilton


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