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THE COAL TRADE BULLETIN. 51
MINE DUTY CONTROLLERS*
By Harrison P. Reed
The application of the electric motor to bituminous
coal mines has developed so rapidly that
there is no section of the industry in which the
motor has not been installed. Not only is the
application universal, but the use of electricity
in coal mines is steadily increasing, principally
due to the increased cost of producing steam at
the collieries.
The design of satisfactory motor and control
equipment for coal mines is a considerable problem
because of the severe operating conditions
encountered. The class of attendants, usually
employed by the mines to maintain electric motors
and their accessories, is unskilled, principally
because of the location of the mines and
the conditions of work. Many changes in help
take place for the same reasons, and it is, therefore,
difficult to train the attendants and make
experts of them. These conditions make it essential
that rugged and reliable electrical equipment
be installed.
As the use of electricity in the mines has increased,
the electric motor for operating mining
machinery has been developed much more rapidly
than suitable controlling devices. Only within
the past few years have those concerned realized
that the selection of a proper controller is as
important as the selection of a suitable motor.
Too often is the controller given secondary consideration
with the result that a good motor may
appear to disadvantage, simply because it is not
operated with the right control equipment.
At coal mines, motors are used for haulage,
hoisting, ventilating, pumping, coal-cutting, tipple
or breaker power, drilling, washing, machine shop
and blacksmith shop. In the design of electrical
equipment for mines, it is necessary to carefully
consider the following: explosive dust and gases,
continuity of operation, voltage fluctuations and
dampness. It is, of course, advisable to install
the control apparatus in a locality which is unaffected
by sparks or short circuits tending to
some derangement of the apparatus. The design,
however, must be made so as to minimize
leaks to ground, short circuits, etc., which may
cause ignition of explosives, mine
CASES OR OF COAL DUST
with disastrous results. In gaseous mines,
arc producers such as circuit breakers, switches
and sliding contacts of rheostats must be properly
protected, either by breaking the arcs in oil.
or by providing explosion-proof cases.
*Paper presented at the 293d Meeting ol ihe American Institute
ol Electrical Engineers Pittsburgh Pa.. April 9-10,
1914. under the auspices of the Committee on Use of Electricity
in Mines, and printed with permission.
On account of the unskilled attendants in the
mines, the electrical equipment receives less than
the usual amount of intelligent attention, while
on account of the conditions of operation, it
should receive more. Much is therefore left to
the designing engineer to solve, but on the other
hand, much could be done to improve conditions
by the employment of a supervising electrical
engineer at an attractive salary, whose duty it
would be to see that better attention is paid to
the care and maintenance of the electrical apparatus.
From a safety standpoint, and it is the writer's
understanding that we are to follow the slogan
"Safety First," the mine ventilating fan is the
most important power-driven machine used within
the mining district. Absolute continuity of
service is necessary during mining operations to
remove poisonous and explosive gases and coal
dust from the underground workings. Mine fans
are not usually installed within the mines, but
at the mouths of the shafts. Mine fan service
is, perhaps, the hardest duty possible for a commutator
type motor. It runs continually at a
constant load, and the mere fact that the load is
constant prevents the motor from running light
and allowing the brushes to polish the commutator.
It is still a debatable question as to whether
constant or variable speed mine fan motors should
be used. Some claim that the fans should be arranged
to operate at a peripheral speed of maximum
efficiency. This means constant speed; but
nearly everyone agrees that variable speed is
practically necessary on the mine fan except for
tunnels, subways and old workings. Opinions
differ as to whether it is better to use mechanical
or electrical means to vary the
SPEED 01' THE FAN.
Mechanical speed changing devices, such as
Reeves variable speed drive are used, but the
tendency is towards obtaining the speed changes
electrically, especially when this can be done
economically. Speed control of a direct-current
motor is easily and economically accomplished by
shunt field regulation. The speed control of an
alternating current motor is quite a different
problem. A straight slip-ring induction motor
with control by resistance in circuit with the
secondary is perhaps the most familiar type in
this country, but it is open to serious objection
because of the rheostatic losses with resultant
low efficiency. The commutating-pole slip ring
motor is used but very little on account of the
complexity of construction, which arises from