776

The colliery guardian.

April 20, 1917.

factor in no sense justified a suggestion of bad
management on the part of the consumer; but he was
equally sure that, in many cases, great improvements
could be effected. The procedure to effect this could
only be determined by consideration of actual condi-
tions, and it was only possible to indicate in general
terms one or two lines along which investigation was
likely to be profitable. Since the day load require-
ments usually exceeded those of the night, the pro-
blem might almost be narrowed down to the question
of increasing the night load.

Pumping.

Owing either to insufficient lodge capacity or to the
pumps being too small, electric pumps were often run
for part of the day shift. In the case of the small
typical installation referred to when discussing maxi-
mum demand meters, if the reason for running the
pump on day shift was too small a lodge room, it was
clear that the value of enlarging the standage was
£200 per annum. Again, by no means all colliery
pumps were electrically operated, and even in cases
where the existing engines were reasonably economical
an examination might show that the general reduc-
tion in cost would justify the necessary capital outlay.
Another possible source of night load was for coal-
cutting, either direct or by motor-driven compressors.
Curves had been plotted to illustrate graphically the
relation between load factor and the total cost per
unit, and also another series showing the actual load
factors obtained during 1916 at various collieries.

Mr. Corlett, by means of a series of curves, showed
how the total cost per unit had risen during the last
three years, and also that the effect was more marked
on low than on high load factors.

On the question of linking up, he remarked that if
a group of collieries could be linked up and metered
at a common supply point, and the electricity pur-
chased in bulk, savings might be expected: (a) From
reduced rates owing to a larger demand; (6) from
diversity factor. The resmtant maximum demand
from two collieries metered at one point could not pos-
sibly be more than the sum of the two separate
demands, and would almost certainly be appreciably
less, to an extent more than sufficient to warrant the
expenditure on transmission lines.

Conclusion.

In conclusion, he ventured to suggest that attention
to the following points would fully repay the trouble
involved: —

(a)	Consider in advance the extent and incidence of
initial and future loads.

(b)	Whether current should be purchased or
generated.

(c)	If purchased, study contract provisions with
reference to working, conditions, and endeavour to
improve load factor.

In such strenuous times as the present, no apology
was felt to be necessary for putting forward a plea for
economy. Any reduction in colliery costs or demand
on the supply company’s machinery tended to the
common good.

On the motion of the President, a vote of thanss
was accorded Mr. Corlett for his paper.

DISCUSSION.

Explaining a series of curves which he exhibited,
Mr. Corlett said he had plotted the costs per unit of
two different supply agreements, both of which were
on the maximum demand principle, but having no
other points of similarity between them. One was
for a maximum demand on a pure sliding scale; any
power required up to 100 kw. was charged at one rate
per kilowatt; then between 100 and 200 kw. all the
power required was charged at a lower rate; and simi-
larly all along the line up to 1,000 kw. The second
curve was on the principle of a semi-sliding scale, the
first block of kilowatts up to 200 being charged at one
rate per kilowatt; up to 400 at a lower rate; and all
above 400 at a still lower figure. He had taken figures
for 1917, 1916, and 1915. The agreements were the
same in each case, the only difference being due to the
increased cost of coal. For each of those years he had
taken the. total cost, in fractions of Id. per unit of
current, with the load factor at 25, 50, 75, and 100 per
cent, respectively, the last being an ideal load factor,
which was seldom attained in colliery work. Some
interesting facts were brought out by the curves.
Taking the 25 per cent, load factor in 1917 on the
sliding scale, the cost per unit right through, from
100 to 1,000 kw., was quite appreciably higher than
the semi-sliding scale. At 1,000 kw. they practically
coincided within a small fraction of Id. With a 50 per
cent, load factor, the two scales almost met at
1,000 kw. With 25 per cent, load factor, at 100 kw.
demand, the cost on the semi-sliding scale was 0’84, as
compared with 1-17 on the other. In 1916, when the
price of coal was not so high as last year, the average
came out at 0*82, but, owing to the difficult incidence
of the coal variation clause, the pure sliding scale beat
the semi-sliding scale at 920 kw. on a 25 per cent, load
factor; at 750 on a 50 per cent, load factor; at 680 on
a 75 per cent, load factor; and at about 560 on a 100
per cent, load factor. In 1915 the results were rather
more marked. Under the same conditions, the meet-
ing points were: 750 kw. on a 25 per cent, load factor,
640 kw. on a 50 per cent, load factor, 430 kw. on a 75
per cent, load factor, and 300 kw. on a 100 per cent,
load factor. If coal ever came back to the rates paid
in 1915, there were considerable advantages in the
semi-sliding scale. The supplies were in no way com-
petitive, being separated by some hundreds of miles,
and were on different bases, but he wished to empha-
sise the very marked reduction obtained, in the total
cost per unit, by improving the load factor. Taking a
comparatively small load of 300 kw. at current prices
on a 25 per cent, load factor on a pure sliding scale,

it worked out at 0-825, at 50 per cent, load factor
0*64, at 75 per cent, load factor 0-545, and 100 per cent.
0-52. A 50 per cent, load factor was by no means an
unusual or difficult load factor to obtain, provided
the conditions were suitable; and the cost was reduced
from 0-825 down to 0*52 by merely adding a night
load. Another curve showed several load factors which
had been actually obtained. One was a load factor of
a fan circuit, and the actual load, levelled out over
the whole year, came out at 94 per cent. In one
month the load factor dropped down to 80, but he
could not explain why it should be so, unless the
gentleman who read the meters came about two days
late; 94 per cent, was 6 short of the ideal, and two
days in a month would probably represent about the
same percentage — even one day would appreciably
alter the load factor for that particular month. It
might be asked why, in the case of a fan job, a 100
per cent, load factor was not obtained? The reply
was that the best regulated fans occasionally stopped
in the course of the year, and there were also little
variations in the speed of the fan due to varying
periodicity at the supply station. No supply station
that he had heard of had been able to keep an abso-
lutely uniform speed, and he thought that a 94 per
cent, load factor was by no means a bad one. At the
same colliery, for all other motors exclusive of the
fan, the curve varied slightly between 20 and 30 per
cent, until certain additions were made to the load,
whereupon the load factor went up to 40 per cent. At
another colliery, with a load which consisted of a
comparatively small fan, which, of course, ran continu-
ously, and a number of intermittent haulages, main
and tail haulers, which ran during the day, and some
centrifugal pumps, which ran for a part of the night
shift only, they got a load factor which practically
kept between 50 and 60 throughout the year. In
another instance, where it was almost entirely an
intermittent haulage day shift load, the average was
about 25 per cent. Arrangements were now under
consideration at that particular place for putting a
considerable amount of pumping load on to the night
shift at the colliery, which, he imagined, would bring
the load factor much higher; but 25 per cent, was very
low if the desired result was cheap current. Under
favourable conditions, 50 per cent, could be obtained,
and was being obtained, in quite a number of cases,
and, as he had pointed out, it brought the cost down.

The President remarked that electrical engineers
had thought that electricity had not received the recog-
nition in mining that it was fairly entitled to. Of
course, it was natural, and easy to understand, that
in the early years of the introduction of electrical
appliances, mining men should look askance at, and
be somewhat chary about, the introduction into their
mines of a power which seemed to bring additional
dangers, of which there were already quite sufficient.
With the machinery which was supplied nowadays,
however, and with proper maintenance, a good many
of the objections had been overcome. Personally, he
thought that all mining men who adopted electricity on
the most approved lines would have no reason to
regret it. At the colliery he was connected with they
ceased to use steam underground 10 or 12 years ago;
they did away with compressed air, and adopted elec-
tricity. Since then, everything had been done elec-
trically except the winding in the shaft, and he need
only say that he had had no cause at all to regret
the change.

Mr. Drummond Paton remarked that there was
hardly anything to criticise in Mr. Corlett’s paper, but
he regretted the question of linking up had not been
dealt with more fully, because he was particularly
interested in that very important subject. The indi-
vidual colliery companies had the source of supply in
their own hands, and he had always advocated that
they should be the first economists in the consumption
of coal, and supply their own power. In the Rhine
Valley the collieries within a radius of 15 miles had a
common centre for all the waste coal, and materials
for which it was difficult to get a market were cen-
tralised at one particular place. That was a point
colliery people in this country should consider, and
he had suggested that the electrical engineers, as
representing one industrial community, should com-
bine forces, for the purpose of economy, with those
representing the mining community. He was quite
sure that on the question of linking up there was a
field which might be considered by that society.

Mr. Jackson observed that one method which had
been adopted of selling current was a kilowatt charge
of from' £5 to 25s., according to the load factor, and
there was no doubt that, if the load factor were
increased, they could decrease the price per unit. He
thought it would be much eheap er for collieries which
were anywhere near a generating station to buy,
rather than make, current. There was something to
be said in regard to Mr. Paton’s point about link-
ing up. If the collieries within a certain radius could
accumulate their inferior stuff, and provide a large
generating plant in which the material could be pro-
perly handled, beneficial results would follow. It was
interesting to see how the coal was handled at a
modern generating station; a spade never need be
used, and they could deal with coal containing 10,
12, or 15 per cent, of ash. If the colliery companies
therefore could send their inferior fuel to one centre,
the power could be generated much cheaper than at
the generating station.

Mr. Stanley Greenhalgh said it would be inter-
esting if Mr. Corlett would give definite illustrations
of how the total costs were worked out. Taking
300 kw. at a 50 per cent, load factor, how was the
total arrived at?

The President remarked that, although the paper
dealt mainly with instances where it was contemplated

the supply would be taken from a central generating
station, still there were numbers of collieries so situated
that the supply could not be obtained in that way,
and where, if the intention was to introduce electric
power, the question of generating would have to be
considered. In those cases two main questions arose:
should the current be alternating or direct, and what
should the voltage be? He should like to hear whether
Mr. Corlett agreed that, under the most, approved
practice at the present time, three-phase alternating
current was the best for mining ; what voltage he would
recommend as the best in a single colliery generating
its own power, with no transmission—i.e., the power
generated at the surface to be sent immediately under-
ground for use, say, within two miles—and also what
he would advise in a case where there was some trans-
mission, the power being generated at one colliery for
the purpose of supplying a group within a two or three
miles radius, and then to go Underground.

Mr. Willis asked whether the author had con-
sidered the question of using a synchronous motor
generator (for converting from alternating current to
direct current), which would give a direct current quite
as suitable, and, in some respects, more suitable for
certain work in connection with collieries ? He
should also like to know whether Mr. Corlett had con-
sidered the question of using a synchronous motor
generator for materially improving the power factor
of the colliery, and, if the power factor was improved
in this way, whether the power company should
acknowledge that improvement by giving a reduction
in the price of the current, because it would enable
•them to use, in the first place, less copper in their
transmission line, or, if the transmission line was
already laid before the improvement of the power
factor, it would give them an opportunity of selling a
certain amount of energy which had been set free?

Mr. Thompson asked if the author could give some
idea of the relative safety of the two systems men-
tioned by the president? It was a remarkable thing
that all the officials at the Home Office who had to
do with collieries, and looked after the supply of
electricity for them, seemed to be wedded to the
three-phase system. They did not help colliery people
at all in those matters. They had a lot of details
which might be circulated with advantage, but these
were very carefully kept on one side. In the Regula-
tions, the only current which was apparently con-
sidered worthy of consideration was the three-phase,
or alternating current of some kind or another. He
did not think there was much to be said as regards
cost in the matter of the two systems, unless it was a
question of long distance transmission, but it would
be very interesting to know the relative safety factors,
because one gathered the Home Office people were on
the wrong tack altogether in that respect.

Mr. Corlett said the question of the. electric supply
to collieries was a big subject, and the reason he did
not discuss at all fully the linking up question was
because it was rather difficult to do so on purely
general lines. What he had tried to do was to tabu-
late some of the more important considerations affect-
ing the choice as to whether the current should be
purchased or generated. The point raised by the
president was by no means easy to answer. No indi-
cation was given as to the extent of the plant, which,
however, had a very distinct bearing on the question;
but, briefly, he might say that, for an ordinary colliery
supply of reasonable dimensions, he found that the
most suitable voltage was 3,000. If the size of the
plant justified it, he would generate at 3,000, and use
3,000 volts on motors of 50 horse-power and upwards,
transforming down to 550 for the smaller motors. If
the plant was of much smaller capacity, and no large
motors were required, or were not likely to be
required, he would generate at about 550. For a
transmission of two miles, the question whether they
should generate at 550 and transform up, or generate
at a higher voltage, say, 3,000, really depended on the
ratio of the two loads. If it was desired to use
300 kw. locally and 50 kw. some distance away, and
the 300 kw. was all made up of relatively small units,
the proper thing to do would be to generate at 550,
and step up for the small outlying places. It was
really a question of setting the two sets of transform-
ing costs one against the other. In reply to Mr.
Willis as to the use of motor generators for conver-
sion from alternating current to direct current, of
course, that was one of the obvious methods of doing
it, and it had the advantage of effecting an improve-
ment in the power factor, but, so far, he had not come
across a supply company which put any cash value
upon that improvement; they were not prepared to
make any reduction in the bill on account of any-
thing done in that direction. Mr. Thompson had
raised a very big and contentious subject, the relative
safety of the alternating current and direct current
machinery; more particularly, he supposed in reference
to underground work. His own opinion was very
strongly that the alternating current plant had a
higher factor of safety than the direct current plant.

Lightning Flash in a Mine.

Mr. G. J. Ridyard mentioned a curious incident in
connection with a flash in the workings of a mine at
Tyldesley, Lancashire, probably caused by a lightning
discharge conducted from the surface. He said that
the Nelson pit, Tyldesley, owned by Messrs. William
Ramsden and Sons Limited, was sunk to the Arley
mine, whence a rise tunnel was driven (from a point
about 15 yds. east of the shaft) in a south-easterly
direction. Vertically above the tunnel was a road
driven in the Haigh Yard seam from the mouthing in
the shaft, and along that road passed an endless
haulage rope, Jin. in diameter, from a surface haul-
ing engine, and a knocker wire, also from the surface,
both of which passed the top of the tunnel in the