Liquid rheostats

NW Mailing List nw-mailing-list at nwhs.org
Sat May 3 23:02:51 EDT 2014


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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