1296

THE COLLIERY GUARDIAN.

December 20, 1918.

A good head of oil above the contacts is the best
preventive against such occurrences, but as this
depends largely on the attention given, it is neces-
sary to provide a further safeguard against the danger
of injury to persons should the switch casting give
way—a not uncommon occurrence where switches are
almost air-tight and ignition takes place.

It was at one time the practice to bring only two
phases to the reversing switch, and carrying the other
straight to the motor. This saved a considerable
amount of repairs, but, owing to the motor always
being “live” when the isolating switch was closed,
several accidents occurred, due to the fact that in-
experienced persons were under the erroneous impres-
sion that the motor was “ dead ” when the reversing
switch was in the “off” position. The regulations
now provide that “ Every motor shall be controlled
by switchgear for starting and stopping, so arranged
as to cut off all pressure from the motor and from
all apparatus in connection therewith, and so placed
as to be easily worked by the person appointed to
work the motor.”

The first part of this provision certainly applies to
the reversing switch, in that it refers to “starting
and stopping,” but the remaining portion seems to
apply to the isolating switch, as the reverser does
not cut off all pressure from the apparatus, one side
of the reverser being still alive. The meaning is not
clear, but in any case the general practice now is
to interrupt all three phases in the reversing switch.
Two trifurcating boxes should be provided, one to
receive the cable from the isolating switch, and the
other for the cable leading to the motor.

Controller.

There are two types of controller in general use,
i.e., vertical and horizontal. The former type presents
some difficulties underground when oil immersed,
owing to the limited head room usually provided in
the haulage room. A controller, being of considerable
length, requires a very high lift in order to get it
quite clear of the oil and into a convenient position

Fig. 3.

Supply

B

Fig. 4.

rfrra
min
mm

Stop

KrtiraPd:

1

Fig. 5.



for repairs. The horizontal type is much more conve-
nient underground—or, indeed, in any other position.
The gear, when fitted to the cover, is brought clear
of the oil, and therefore little time is wasted in getting
at a defective contact during the working shift.
When the controller is of considerable size, a small
block and rtackle is required, a more convenient
device, however, being a pulley slung from the roof
and carrying a chain, one end of which is fastened
to the controller, whilst the other is attached to a
balance weight, so that but little effort is required to
lift the cover for repairs. The controller contacts
and fingers should be easily renewable. The drum
contacts, of course, present no difficulty, as new con-
tacts can be cut from a copper tube having the same
radius as the drum contact has to the centre of the
controller drum. However, in the case of finger con-
tacts the design should not be such as to necessitate
the scrapping of a complete finger when worn out,
this being expensive when the controller is working
on heavy service. The principle on which a large
quantity of finger contacts are now being made is
represented in fig. 3, where, it will be noticed, a copper
contact block is provided, which can be readily
removed. With controllers thus equipped it is a good
plan to have a few dozen spare blocks, so that when
repairs are carried out, instead of waiting to file up
the burnt contacts, they can be replaced by the spares,
the burnt ones being dressed up for future use as
time permits.

There is one point which requires special atten-
tion in connection with horizontal. controllers. The
practice in controller work generally is to mount all
the fingers on a piece of prepared wood as indicated
in fig. 3, or on some other insulator. The position
of the fingers in the horizontal controller often affords
a convenient lodgment for carbon and copper dust,
as w’ell as coal dust, more particularly between the
different sets of fingers—a situation where consider-
able E.M.F. exists when running at reduced speed.
These spaces should be kept free from foreign matter,
because when once arcing commences and chars the
face of the insulator on which the fingers are fastened
it is very difficult to stop.

Regarding the means by which the controller and
reversing switch are operated, it may be said that
most drivers prefer some sort of lever arrangement to
that requiring the operation of the wheel. In the first
place a wheel control takes both hands, and there is
not the same comfort in working as is afforded by a

lever arrangement. - The type of movement which finds
favour is that shown at A and B in fig. 4. This only
requires the use of one hand, and so leaves the other
free to operate a hand brake, where one is provided,
on the motor shaft.

Resistances.

There are three methods of arranging resistance for
the rotor circuit—(1) air-cooled metallic, (2) oil-cooled
metallic, (3) water resistance.

Air-Cooled Resistances.—Of metallic resistances this
is the more simple type, and consists of cast-iron grids.
There is however, one serious drawback in using these
underground, viz., the possibility of the grids be-
coming broken and causing arcing, apart from the
risk of igniting any coal dust which may collect bn
the grids. Assuming a haulage is in a perfectly safe
position as regards gas—and this hard to decide—
arcing due to a broken grid would not be objection-
able, but since the future of electricity underground
depends largely on the absence of any apparatus liable
to cause sparking, it is in the interest of those con-
cerned to keep out of a pit anything which might
tend to shake confidence and which might possibly
form the basis of enquiry in case of an explosion, even
though it had really nothing to do with the case at all.

Oil-Cooled, Resistances.—The strong feature of this
type of resistance is the impossibility of open spark-
ing, provided the oil level is maintained, and, in addi-
tion, much less space is required for a given output.
These resistances should be of ample size in order to
prevent abnormal temperature rises of the oil. In
any case provision must be made for a circulating
system, including radiators and pump, the latter being
driven from the shaft of the motor and being of a
design which will pump in the same direction whether
running backwards or forwards. Oil-cooled resist-
ances should be removed occasionally and the tanks
cleaned, an operation which becomes necessary owing
to the carbonising of the oil. This is particularly
so when the resistances are not of sufficient
capacity, and as a result become excessively hot

during times of heavy load. The resistances, too,
require changing every few months in order to clean
the grids, which become dirty and carbonised. If
spare units are provided there is no difficulty in having
to clean the resistances, as this can be done at any
time so long as the spares are available when a further
change becomes necessary. Considerably more work
is involved in maintaining a haulage with oil-cooled
resistances, but for the prevention of open sparking
this system leaves little to be desired.

As regards oil, this should be of high flash point,
not below about 400 degs. Fahr. Precaution must be
taken to ensure that no attempt is made to run the
haulage without oil in the resistance tanks. If this
is attempted when the resistances are in an oily state,
dense fumes will be-given off, which, if ignited, might
wreck the tanks. No hard-and-fast rules can be laid
down as to the changing of oil or resistances, this
clearly depending upon circumstances.

Liquid Resistance.

Liquid resistances, usually referred to as liquid
controllers, since they do the work of resistance and
controller, have been extensively used for haulage
plants.

A general idea of the arrangement is shown in fig. 5.
The controller consists of an arrangement whereby the
immersion of the suspended resistance plates—which
are enclosed in a suitable tank, and gradually
immersed in the liquid (ordinary soda solution)—is
varied by means of a weir connected to the controller
lever. The system comprises the weir tank and a tank
for cooling the liquid, which is circulated by means
of a small pump kept running continuously. The
speed with which the weir tank fills depends upon
the output of the pump, and therefore the apparatus
is fool-proof, inasmuch as the full load cannot be
placed suddenly on the motor, but is controlled by th '
rising liquid.

There appears, however, to be one difficulty with this
type as a result of this automatic control of accelera-
tion. In some underground districts in mines it
becomes necessary, owing to certain conditions, to
make what is called a “ shunt,” the journey being
quickly accelerated and its momentum relied upon to
carry it a certain distance. This obviously cannot be
done to advantage, even if at all, with a liquid con-
troller, and this point should receive consideration
when deciding on haulage gear and the liquid type
is contemplated. One great advantage is the low
maintenance cost, there being no controller contacts

or resistances to keep in order. Once the strength of
the liquid is properly adjusted, all that is required is
to make up for evaporation losses by adding clean
water.

NORTH STAFFORDSHIRE INSTITUTE
OF MINING AND MECHANICAL
ENGINEERS.

A general meeting of members of the North Stafford-
shire Institute of Mining Engineers was held at the
Mining School, Stoke-on-Trent, on Monday, De-
cember 9, the President, Prof. Sir John Cadman,
'D.Sc., in the chair.

The Training of Students in Coal Mining.

Prof. F. W. Hardwick’s paper on “ The Training of
Students in Coal Mining, with Special Reference to th?
Scheme of the Engineering Training Organisation ”
(Colliery Guardian, December 6, 1918, p. 1187), was
read by the lion. sec. (Mr. T. Yates).

Discussion.

The President, in opening the discussion, said
that the mining profession was rather different from
the ordinary engineering branches. It was of neces-
sity very largely hereditary. It was, moreover, rather
a close corporation, which was quite wrong, for there
should be opportunity for everyone who had a liking
and the ability for the profession of mining to enter
into it. His experience, however, was that a good
many boys from school who had ambitions in the
direction of coal mining, after their first trip down
a mine, swore they would have nothing more to do
with it. Thus, one had to be brought up in the en-
vironment of mining, in order to know the charms and
attractions of the profession. This training of students
in coal mining was by no means a simple problem, as,
indeed, was indicated by the great diversity of views
expressed in the various papers referred to by Prof.
Hardwick. Almost every mining professor or lecturer
at some time or other felt it incumbent upon him to
outline and justify a particular curriculum upon which
he was working, and he not infrequently found assist-
ance and stimulation by presenting his views in the
form of a paper, so securing a discussion which enabled
him to crystallise his ideas and to make such altera-
tions as public opinion, expressed by this means, might
suggest. Generally speaking, there appeared to exist
much confusion as to the best lines upon which mining
students should be educated—confusion which, on
analysis, did not in reality exist. That confusion
arose because the education of students in coal mining
was looked at as a whole, instead of being divided
into the various grades in which the personnel of a
colliery staff must inevitably be graded. In his
opinion, these divisions might be made up as follows :
(1) The mass of underground workers aspiring to
become underground officials; (2) managers and under-
managers ; (3) general managers and mining engineers.
There should be ample opportunity, by scholarship
and learning, for a free passage from the minor grade
to the major grade, but on general lines the training
was very different. The training in No. 1 grade com-
menced in the elementary schools and passed to the
night schools and County Council evening and Satur-
day classes, from which courses there should be little
or no difficulty for average candidates to qualify for
firemen, overmen, under-managers, and, possibly,
managers. The second grade required fundamentally
different treatment, for it should cover much more
application to the underlying sciences. In this case
the higher grade schools and the higher courses of
the County Council and the technical schools formed
the track through which the greater portion of these
students passed. In the third grade the public schools
and universities, or the technical colleges, formed the
channel through which these candidates were destined
to float. The elementary training of those entering
the university or technical college was not sufficiently
recognised. More attention should be given in the
schools to the preparation for matriculation, and
classes in the minor grades to which he had referred
should be constructed to lead to matriculation. To
examine a candidate for a university scholarship on
his proficiency in the art of mining, as a qualification
to enter upon a university course, was unsound. The
candidate’s educational framework should be tested,
and not his knowledge of the practice of his ultimate
profession. It was possible for anyone responsible for
the educational curriculum to substantiate his views
for that particular grade in which he was working,
and when an attempt was made to discuss the three
grades as one, confusion inevitably resulted. In all
such discussions there was a tendency to be blinded
by the present, instead of boldly probing into the
future. The young mind was the one that had to be
educated, and it was to that end that the educational
gun should be pointed. It was, perhaps, disquieting
that they could not see whether they had hit the
target. Posterity alone could decide. Mining, as he
had already pointed out, was largely an hereditary
profession, and few people entered it whose fathers
were not already in it, or in some way associated with
it. The Engineering Training Organisation was doing
good work in focussing attention upon this important
problem, and Prof. Hardwick was to be congratulated
upon bringing the matter before the institute.

Mr. E. B. Wain said it did not appear that Prof.
Hardwick had taken note of the future of education
as it would be decided by the new Education Act. The
question of matriculation and all the preliminary
training of prospective mining engineers and colliery
managers might, with very great advantage, be dealt
with by means of the tuition that would be set up
under the new Act. In the past the training of those
seeking to qualify as colliery officials and colliery
managers had been largely governed by the hard and
fast rules laid down by the Coal Mines Regulation
Act. Take the question of practical experience, which
appeared to most of them the one essential factor in
the education of a colliery manager. The man might