1004

THE COLLIERY GUARDIAN.

May 31, 1918.

blackdamp instead, thus pointing to either a slow
effusion or generation.

It was originally intended to carry the trenches much
deeper and have them inclined upwards to the collector ;
whilst at A, which was the more important of the two
places, numerous bores were also to have been put down
in all directions from the bottom.

Unfortunately, water level was found only 26 in.
from the surface, either on account of the natural con-
ditions or the continuous rain which lasted during the
whole of the three days the work was in progress This
circumstance modified the arrangements.

The barometer remained steady throughout the work,
and the solubility of methane in the water of the sample
tubes may be neglected.

e

E

D

Fi<5.7.

Shale Darnp

56*

Clay

The source of the methane may be:

(1)	From outcrops which undoubtedly exist in the

neighbourhood.

(2)	From old gob.

(3)	From the heating of carbonaceous matter in the

dumped bind by a boiler flue.

All these seem to the author to be possible causes of
the slow effusion proved to exist, but whether one or all
of them are concerned is impossible to say without fur-
ther experiment.

Scottish Oil Distribution.—The Scottish Oil Agency
Company has been registered with a capital of £100,000, in
£1 shares, with the object of forming a central selling
agency for the disposal and distribution of the production
of the Scottish oil companies. Mr. Spence, of the
Pumpherston Oil Company, has been appointed manager,
and Mr. J. A. S. Miller secretary.

National Federation of Colliery Enginemen and Boiler-
men.—The 45th annual conference of the National Federa-
tion of Colliery Enginemen and Boilermen was held at
Workington, last week.* The president, Mr. S. W.
Howarth, of Mansfield, occupied the chair, and delegates
were present from Scotland, Northumberland, Cumber-
land, South and West Yorkshire, Derbyshire, Nottingham-
shire, and Somersetshire. The chairman, in the course
of his address, dealt complimentarily with the activities
of the past year. Referring to the question of “ our
linking up with the Miners’ Federation of Great Britain,”
he said he had always been favourable to it, but not
in the sense which a good few of the Miners’ Association
meant. He would not like them to lose or sink their
identity as colliery enginemen and boiler firemen. The
interests and well-being of the Miners’ Federation and
theirs were identical. The secretary, in his report, alluded
to the executive’s efforts to get the 1911 Act relating to
visual signs and sanitary arrangements put into force.
The reason given for the delay was that all manufactories
in the country were engaged on munitions, and no time
could be spared for other than Government purposes.
The question of an eight-hour day for all colliery engine-
men and boilermen had been again and again pressed
before the various heads of Government Departments.
Resolutions were passed in favour of expediting private
members’ Bills in Parliament, the maintenance of the
identity and independence of all associations in this
Federation, the taking of drastic action to reduce colliery
enginemen’s and boilermen’s working hours, joining the
Miners’ Federation of Great Britain on the same terms
and conditions as are now in operation in Northumber-
land, income tax rebates for Sunday work, etc., the com-
pulsory employment (by Act of Parliament) of controllers
on every winding engine, irrespective of the depth of the
shaft, and the enforcement of the Coal Mines Regulation
Act concerning signalling indicators and the sanitary
arrangements of winding engine houses. Mr. P. Hearne
(Radstock) was elected president, Mr. R. Shirkie (Glas-
gow) secretary, and Mr. G. Annable (Ilkeston) treasurer.
The following were constituted the executive com-
mittee : Messrs. J. A. Gleen (Scotland), T. A. Casey
(Sheffield), D. Bramley (Castleford), J. Humphrey (Gos-
forth). Auditors : Messrs. T. Weatherby (Bedlington)
and W. Fletcher (Newstead).

SOME PRACTICAL CONSIDERATIONS
ON ELECTRICAL ENGINEERING.*

By Chris. Jones, M.I.E.E.

It has for some ears past been recognised that
electrical power is the motive power par excellence
for mining work, and that an efficient and reliable
supply of electricity is indispensable to the prosperity
of these industries, and with the progress of time th?
successful working of our mineral fields will more than
ever be dependent on transmitted power.

Periodicity of Supply.

The question of A standard frequency is of great
importance, especially so in view of the linking-up
proposals of various supply systems. While frequency
changers are successfully in use at collieries in this
country, the writer thinks that they are best avoided,
as they introduce high expenditures and extra plant
for which the supply of power will be dependent upon.
Manufacturers have to some extent adopted a
frequency of 50 and 25 with the object of reducing
the demands for machines of different charateristics
which they are called upon to supply, the result of
which should be better and cheaper apparatus with
quicker deliveries. The three-phase 50 period system
has proved sufficiently flexible to be used for all
colliery purposes, the great advantage being that with
this periodicity, speeds up to 3,000 r.p.m. can be
obtained as a maximum of 1,500 r.p.m. for 25 periods.
The increased speed range not only results in economy
in working and lower cost, but allows more latitude
in design. The increased speed is of great advantage
for centrifugal pumps, which are very prominent on
large scale in the South Wales coal field. With motors
of moderate power suitable for single reduction
haulage gears, it is found that with 50 periods the
cost is high, the power factor poor, and the space
occupied large, compared with 25 period motors, so
that 25 period has its advantage for such gears, but
the tendency to-day is to introduce double reduction
gear or tandem sets, owing to the width occupied by
single reduction, so that the effect of squeeze may
be minimised. The 25 period supply is, however,
adopted in some districts and is to be commended,
if speeds above 1,500 r.p.m. are not required owing
to higher starting torque, maximum torque, and
higher power factor of the motors, the latter especially
at low loads, and reference will be made to this
factor later on.

Power Factor.

The question of power factor has not received the
attention it merits. There is no doubt that many
alterations in existing lay-outs can be made which
will have the effect of largely improving present power
factor conditions. The disadvantages of low power
factor make their presence felt in higher costs of
power supply. Any alteration in tariff which would
place a premium on high power factor would
undoubtedly result in an all-round improvement,
because there are great possibilities for improvement
which are dormant or latent under existing charges.
There are but few power supply companies that will
offer any inducement to improve the power factor,
the cost being the same, whether it is 0-5 or 0'9 p.f.
A municipal authority has recently appreciated
the advantage by charging £4 per annum on k.v.a.
basis, but the units are metered on kw. hr.
cables of a larger size than is necessary for the same
voltage drop with unity power factor. This is of great
importance, especially when long feeders are con-
sidered. Cables are liable to be overloaded, due to
the low power factor of the system. A case in point
is one where an underground cable gave trouble, due
to excessive heating, which caused decentralisation of
cores. The power factor of the underground system
was 0'55, whereas the cables installed were based upon
0 9 p.f. The current in this case was in the neigh-
bourhood of double what it ought to have been had
the power factor been kept high. The lower the
power factor the higher the current for a given useful
load, and consequently the greater the C2R losses in the
alternators, feeders and mains, the increased losses
being inversely proportional to the square of the power
factor. If the power factor be increased from 0'8 to
unity, the C2R losses are reduced to 64 per cent, of
its former value. A low power factor is due to the
magnetising currents of power transformers, especially
when lightly loaded. This is of importance, and is
increasingly so the larger the plant under considera-
tion. The installation of transformers of liberal
rating, together with the variable loads obtained in
works and colliery practice, causes the average idle
current to be somewhat excessive under such condi-
tions. Low power factor is also due to induction
motors, which is the most prolific of all sources.

It has been the practice of some makers to offer
liberal rating motors in order to withstand the per-
missible temperature rise; this, of course, being at
the expense of low power factor. Again, another
factor is that, in a great many cases, motors of
different sizes are standardised, such as 10, 20, 30, 50,
70 and 100 h.p., and this practice is to be recommended
in collieries having a large number of motors, in view
of keeping down the number of spare parts, etc. An
alteration in the amount of air gap in an induction
motor does not sensibly alter the cost, and produces
more effect on the power factor than any slight changes
in the design; but an important practical feature is
introduced, due to the greater risk of breakdown
with small clearance, and it is essential to bear in
mind and to recognise that the induction motor is
essentially a high speed machine. The slower the
speed the higher the cost, combined with lower effi-
ciency and power factor. A case, for example, is
where a 250 h.p. motor which had a power factor of
0'9 when running at 500 r.p.m. at full load. Another
motor when run at 200 r.p.m. of same output the cost
is 40 per cent, higher, and the full efficiency about
4 per cent, lower, and power factor 0'78. It is an
advantage to employ, if possible, a few large motors

* Abstract of a paper read before the South Wales Insti-
tute of Engineers, May 24, 1918.

in preference to a large number of small ones, as the
latter have always lower power factor than the former.
It is advisable to compromise between high efficiency
and power factor and reliability, as motors are so
much like a link in a chain, so much being dependent
upon them (for instance, coal conveyor and coal cutter
motors). The author finds that coal cutter motors
give a reasonable power factor, considering the size
of air gap and its reliability. In cases where a motor
will operate on a fluctuating load, and run light for
long periods, the selection of the highest speed and
lowest horse-power is of special importance, in view of
the large current taken at reduced loads. It is essen-
tial that the apparent efficiency of motors be as high
as possible in order that the plant be profitably loaded
to the highest possible extent. Use may be made of
phase advancers, such as that of Prof. Kapp’s patent
vibrator. This is in use successfully on large motors,
and there should be a good field for its application
where there are a number of motors and where there
is a d.c. supply available. This latter requirement is
perhaps the chief drawback for its use inbye in mines.
It is generally understood that the minimum d.c.
supply volts is 65 volts, but the voltage for exciting
the field magnets can be anything one likes—it is only
a question of selecting a stout enough wire for the
field coils. The power of d.c. supply required is about
J per cent, of the power of the motor. The advancer
may be used as a slip regulator as has been done by
the American Westinghouse Company. Use may also
be made of rotary converters and synchronous motors
for the improvement of power factor. The latter may
be used as rotary condensers in the same manner as
spare alternators, which can be dismantled from the
prime mover. The transmitting of electrical energy
on lines indicated by the Coal Conservation Committee
will have to be carried under the most economical and
reliable manner, and one way to accomplish this end
is to transfer power at high power factor.

Earth and Insulated Neutral.

The modern practice is to earth the neutral point
of alternators on all systems, and this is best done
through resistances, as it provides a means of protec-
tion to the plant and is an insurance against serious
interruption of supply. The earthing of the neutral
point prevents the plant from being subjected to the
delta voltage to earth for any length of time, and
enables most faults to be removed without interruption
to the system, and also minimises static disturbances
and consequent insulation troubles. The inserting of
a suitable resistance in the earth connection limits
the current which may flow on a fault to earth, and
thus reduces the effect of disturbances on the system.
The frequent breaking down of insulation at points far
apart when working on insulated system has been cor-
rected by earthing the neutral. In a recent case a
system was allowed to work with a fault on one side;
this resulted in a short developing causing serious
damage to a fuse distribution gear, with the con-
sequent stoppage of coal drawing. Experience has
proved that almost every fault commences as a fault
between one phase and earth, and the majority of such
faults are isolated by protective gear before the arc
has spread to other phases; and in view of increased
use of overhead lines, which must be efficiently pro-
tected against lightning discharges, earthing the
neutral will enable lightning arresters to discharge
without distress, as if the neutral is insulated a fault
between one phase and earth increases the pressure
on the other two phases, causing the arresters on the
latter phases to discharge continuously until a second
fault develops. The earthing resistance should be
designed to pass sufficient current to operate with
certainty any automatic protective device on the
machines or feeders; it is essential that the resistance
be so rated to carry the maximum current continuously
for two minutes without being destroyed. The claim
of advocates of the insulated system, that this system
may be worked with one phase earthed, is void, in view
of the automatic isolation of faulty feeder, in the event
of a fault occurring, as called for in the H.O. Rules.
The claim also that two overload trip coils only are
required is also uncalled for under certain conditions.
With earthed neutral and protective leakage trip gear
two overload coils are only required. Where auto-
matic generator switches are used, it is now becoming
the practice to earth several running machines, so that
during the existence of an arcing fault on the system,
the serious risks which accompany the breaking of
the earth connection may be avoided.

Reactance.

It is necessary to consider the question of installing
reactance on systems of present-day magnitude in
order to limit the mechanical stress imposed on every
winding in the main circuit by the current flowing
through a fault between phases, and this is best
secured by inserting the reactance at a point where
all such fault currents must pass, which is at the
generator terminals.

Cables.

The types commonly used are vulcanised rubber,
paper sheathed with lead or vulcanised bitumen, vul-
canised bitumen throughout. Vulcanised rubber is
rarely considered for shaft and roadway cables on
account of its high price. The writer has used this
type extensively for interconnections between motors,
switch and control gear, etc. In some cases a sheath-
ing of vulcanised bitumen has been applied over the
rubber insulation, owing to the deleterious chemical
action in positions where the cables were installed.
This is also to be highly recommended over the armour
of other type of cables in special cases, as met with
in some mines. Paper cable has been extensively used
in mines for shaft and roadways with success. For
overhead suspension the writer has used this type,
but without the lead sheath. It is, however, claimed
that a paper cable has great advantages over bitumen
in that it possesses greater dielectric strength. This
is scarcely relevant, since, although the dielectric
strength of paper is higher than that of bitumen, the
latter is quite suitable for all usual colliery working
pressures. The writer has used vulcanised bitumen