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THE COAL TRADE BULLETIN. 37
THREE-WIRE SYSTEM OF ELECTRICAL DISTRIBUTION APPLIED TO COAL MINES.
By (ieorge R. Wood, Consulting Electrical Engineer for New River & Pocahontas Consolidated Ooal Co.
The contract for an electrical mining plant of
more than ordinary interest to operators was
closed in Pittsburgh recently. The New River &
Pocahontas Consolidated Coal Co., an offshoot of
the well known Berwind-White interests, recently
purchased from Col. W. P. Rend of Chicago a large
body of New River field coal, with four well-developed
operations, known as the Rend mines Nos.
1 to 4, located near Thurmond, W. Va., on the
Chesapeake & Ohio R. R. An electric power
plant is now operating in connection with these
mines, but it was desired to largely increase the
present output and probably to open an additional
mine or mines in a lower vein. The present electrical
power is supplied for mining machines,
haulage locomotives, fan and pump motors at 275
volts pressure, a most satisfactory potential for
mining work, in view of its safety to mine workmen,
as we'.l as the lower repair costs of mining
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Diagram Showing Mithod of Connection of Two Cenerators in Service and Connections
of the Single Three-Wire Machine.
machinery. Many mining plants, particularly
among the older operations in the Pittsburgh district,
are operated at 550 volts pressure, which is
that in general use on street railway work. This
is on account of the great saving in transmission,
the copper costs at 250 volts being four times as
great, at same percentage of loss in line, as with
500 volts.
In deciding on the pressure to be employed in
the operation referred to, an additional factor was
the large amount of 250-volt apparatus now in use,
including 17 mining machines and six locomotives.
This apparatus would have been expensive to rewind
for 500 volts, and the output of the mines
would probably have suffered in the meantime.
At the same time, the extent of the field would
ultimately make the operation at 250 volts an
extremely expensive one, running the estimated
copper cost over $50,000.
Under these conditions, the three-wire system of
distribution was favorably considered by the company's
consulting electrical engineer and the contract
awarded for three 300-K. W. 575-volt 3-wire
generators, with complete switchboard, generators
to be driven by cross-compound engines at 150
r. p. m.
The three-wire system is in use to a limited extent
on street railway systems, and in many lighting
installations. As ordinarily used, two generators
of the working voltage are connected in
series, with a third or neutral wire taken off between
the two generators. Motors or lights, as
the case may be, are connected between the neutral
and either outside wire, care being taken to balance
the loads on the two sides as nearly as possible.
With generators in se­
ries, the potential between
outside leads is double that
of the single machine, and
since with balanced loads
there is no current in the neutral,
this lead may be of
small capacity. With balanced
loads, therefore, the
system affords, in the case
under consideration, 550-volt
transmission to 275-volt apparatus.
In practice, of
course, the neutral wire must
be sufficiently large to carry
considerable momentarily unbalanced
load, but even In
tnis case the cost of copper
amounts to not much more
than one-third the amount on
straight 275-volt distribution.
A comparatively recent development renders unnecessary
the operation of two generators in
series, thus simplifying the system and reducing
cost of operation, particularly at light loads, and
the plant described will be equipped with standard
generators operating at 575 volts, with a neutral
wire derived from balancing coils, connected to
proper points in the armature winding. The accompanying
sketch shows the method of connection
of two generators in series, also the diagram of
connections of the single three-wire machine. The
balancing coils are external and entirely independent,
being connected to generator through slip
rings at end of commutator. With brushes raised
from slip rings, the generator is in every respect
a standard railway type machine.