326

THE COLLIERY GUARDIAN

February 15, 1918.

full-load current to start up, no fuses which would
stand without blowing would be of any protection to
it against overload.

There does not appear to be any reason why fuses
should not be provided on the starting as well as the
running side of a star-delta switch; and fig. 6 shows
how this could be arranged. The fuses on the starting
side would be heavier than those on the running side;
but the point is that, should anything be wrong, some
protection is provided.

It is obvious that fuses cannot be introduced into
the line ends of the motor windings when connected in
star, but there is nothing to prevent their being placed
in the ends forming the neutral point, and it is here
that the fuses are shown in fig. 6.

In connection with switching-on motors—especially
underground—without protection, some points may be
noted.

Although the starting periods are very short, this is
not sufficient reason to justify the cutting out of fuses
or other protective devices during such periods.

It should be nofed that the majority of electrical
breakdowns do not occur during the normal operation
of apparatus, but usually when it is switched into ser-
vice. Whether the breakdowns occur actually at the
moment of switching current on, or at the moment

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the apparatus was last switched out of circuit, is a
debatable point, but the possibility exists in either
case.

If a short-circuit occurs in the windings or leads of
a squirrel-cage motor of moderate size at the moment
of switching it into service unprotected, it is a serious
matter; and on some power circuits which have to be
heavily protected by heavy fuses or circuit breakers at
the sub-stations, a motor and its sw’itchgear may be
absolutely ruined in a few moments. There also
appears to be an element of danger on an earthed
neutral system, should a motor winding break down
to earth, especially near the line end, as there would be
an abnormal rush of current to the protecting cover-
ing of the cable, where this is used as the connection to
the earthing system at the surface of the mine.

Taking a case where a motor is installed in such a
way that its connection to earth depends upon the
armour of the supply cable: the armouring of a cable
may be broken at several points along its length by
joint boxes, etc., where the armour connections may or
may not be good. These, together with the compara-
tively high resistance of the armour wires, are suffi-
cient, during abnormal rushes of current, to raise the
potential of the motor to such an extent that anyone
coming in contact with it might receive a fatal shock,
under certain conditions which most colliery electri-
cians will recognise.

Again, one often hears reference made to what are
known as surges. These are conditions which can be
better imagined than explained, so little being actu-
ally known of the subject; but that some such condi-
tions exist, under heavy rushes of current through an
electrical system, is sufficient for most of those who
have to deal with facts rather than theories.

It is well known that very often when a short-circuit
occurs on one part of a system, breakdowns of insula-
tion take place far away from the actual seat of the
trouble, and therefore it is to the interest of an elec-
trical installation to have all apparatus, above and
below ground, protected by suitable gear, under all
conditions of starting and operation.

Auto-Transformer or Compensator.

The ideal form of starter for squirrel-cage induction
motors is to be found in the auto-transformer. The
starting torque and current taken by a motor vary as
the square of the voltage. As the type of motor under
consideration is usually started up light, there is no
necessity for a large starting torque, and therefore a
comparatively low voltage will bring a machine up to
speed, and the function of the starter in this case is
to supply the low voltage necessary.

An auto-transformer has only one winding, which is
connected across the mains, and therefore has the full
line voltage impressed upon it.

This winding serves as both primary and secondary,
the latter being formed by tapping off at some inter-
mediate point along the winding. This tapping is one
secondary lead, whilst either end of the winding may
be used as the other secondary terminal—in which case
it is common to both primary and secondary. The
position of the tapping relative to the ends of the coil
determines the electro-motive force of the secondary
circuit. That part of the winding thus dosed by the
secondary’ circuit carries both the primary and secon-
dary current, as will be seen by fig. 7, which repre-
sents a simple auto-transformer for a single-phase line,
and shows the direction and values of the different
currents flowing in the two circuits.

An auto-starter, as they are often called, is usually
provided with a number of tappings whereby different
secondary pressures may be obtained to suit various
conditions of starting.

The current which a motor will take on a given
tapping can be easily calculated. Suppose a 40 horse-
power 500-volt squirrel-cage motor is to be started
from a 40 per cent, tapping, i.e., 200 volts. The full-
load current of such a motor would be about 45

Fig. 9.

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amperes. If this machine were switched straight on
to the line, it would take about five times full-load
current, i.e., 225 amperes. The impedance between
the motor terminals would be = 2-2 ohms.

225

Now, when switching the motor on to the 40 per

cent, tap, i.e., 200 volts, the current will be



amperes approximately, while the current taken
from the mains will be X 91 = 36-3 amperes.

500

These values are shown in fig. 7, the single-
phase transformer being selected for simplicity; but
the principle is the same as in a three-phase auto-
transformer.

It will be observed that no difficulty presents itself
when fuses are placed in the main circuit for protect-
ing the motor from overload, as in this case the start-
ing current in the mains does not reach full-load cur-
rent by about 8’7 amperes.

Figs. 8 and 9 show the switching diagrams of two
typical auto-transformers, one with two transformer
windings, and the other with three; but there does not
appear to be any greater advantage in starting with
three windings than with two, except that, in the Case
of large motor starters, it avoids unbalancing the
system.

General Conclusions.

From the comparisons already made, it will be clear
that, all things considered, the auto-transformer
method of starting squirrel-cage induction motors is
by far the best. However, for small motors on the
surface, star-delta starting switches are quite satisfac-
tory, always provided that both starting and running
fuses are placed in the circuit.

Star-delta switches should be avoided underground,
owing to the heavy currents drawn from the mains in
starting, and the necessity of employing fuses as a
means of protection. It would be difficult and expen-
sive to design a circuit breaker which would give pro-
tection on both starting and running sides of the star-
delta switch.

In connection with employing auto-transformers
underground, there are a few considerations which
might be noticed.

The oil-immersed type should invariably be chosen,
and preference given to one which provides an ample
head of oil above the switch contacts.

As small air space as possible should be left in the
containing tank when the oil is at the proper level,
and an efficient ventilating and flameproof arrange-
ment should be provided to afford a ready means of

escape to any gases which might be formed by the
normal or abnormal working of the apparatus. Auto-
transformers, as well as star-delta switches, are now
usually fitted with a catch arrangement which pre-
vents the switch being placed in the running position
before having first been placed in the starting position.
The arrangement is so designed that only by a rapid
movement can the switch handle be brought from the
starting to the running side. Anything approaching
a slow movement causes the switch to catch in the
“ off ” position, in which event it must again be placed
in the starting side before an attempt is made to bring
it quickly over to the running position. This is neces-
sary, owing to the motor circuit being momentarily
interrupted while the switch is being changed over.

As regards protection, there seems to be a general
practice of making the starting switches on auto-
transformers act as the circuit breakers as well. These
switches are necessarily placed near the transformer
windings, in order to reduce the lengths of leads going
to the various terminals; and in case of an explosion of
gas within the tank due to a damaged transformer coil,
it seems just possible that the switch may be so
damaged as to be unable to open.

It is therefore advisable in underground work to
use a form of switch and auto-transformer pillar which
arranges for the circuit breaker and starter to be in
separate tanks.

In connection with starting, makers usually specify
the number of times a machine may be started per
hour by a given starter. It is also sometimes specified
that if a machine takes one minute to reach full speed
(which is termed the starting period), the motor shall
not again be started until 12 or 14 times that period
has elapsed.

It is necessary that such rules should be adhered to,
as considerable heat is generated in the transformer
coils during starting, which heat must be given time
to dissipate in the oil in which the coils are immersed.

THE OCCURRENCE OF COKING COALS
IN SCOTLAND.*

By Robert W. Dron.

During the past year the Scottish Advisory Com-
mittee on Iron and Steel Production undertook very
extensive investigations as to coking coal in Scotland,
and the results of these investigations have been made
available to the public. The changing conditions
regarding the supply of iron ore, and of fuel for smelt-
ing it, have been emphasised by war conditions. It is
obvious that provision of adequate supplies is of vital
importance for the future of the coal and iron industry
in Scotland. Two important changes had occurred,
during the last century: Firstly, the practical exhaus-
tion of the native Blackband ironstone ; and, secondly,
the establishment of plant to recover the by-products
from the coal used in smelting. The question of fuel
supply is of even more importance than the supply of
ore. In this connection, the outstanding fact is that
the available quantity of suitable splint coal is
becoming exhausted. Hard coals of secondary
quality are no doubt still available, but many of these
are unsuitable at present on account of the high per-
centage of sulphur and phosphorus. Whether the
furnaces can be adapted to the use of the available
coal, or to the use of coke, is a question that the
metallurgist must consider in the light of future possi-
bilities as to the supply of coal and coke. Scottish
furnaces burning splint coal have a capacity of only
300 tons per week for each furnace, whereas in England
(with the use of coke) the capacity is 1,200 tons per
week, and in America a production of 3,500 tons per
week has been attained. There is at present a con-
siderable production of high-class metallurgical coke
from the Kilsyth and Bannockburn coal fields in Scot-
land, and outside this area there are supplies of semi-
coking coal from which suitable coke for blast furnaces
could be made.

With regard to the geological conditions which may
have operated to affect the coking properties of the
coal seams, the author has arrived at the following
conclusions: —

(1)	That throughout an area along the north and
north-west of the Scottish Central coal field certain
of the coal seams of the carboniferous limestone
series and of the lower coal seams of the coal
measures have acquired more or less of the coking
quality.

(2)	That in many cases this property is enhanced
by the action of intrusive whin stone.

(3)	That outside of the above area there is little
chance of finding coals which can be treated com-
mercially as coke-making subjects.

Reverting now to the position of the blast furnaces,
the problem is: What are the prospects of finding suit-
able coke to replace the splint coal as that becomes
exhausted? The district from which the best metal-
lurgical coke is at present being produced is practi-
cally fully developed, and no great extension of output
can be looked for. Doubtless some of the supplies from
existing coke ovens may be diverted from their present
markets, but it does not seem likely that a solution
on these lines will prove useful or practicable.

A virgin area of coking coal may be found at work-
able depths underlying part of the Central Valley coal
measures. The proving and development of this area
will probably proceed by stages, and be spread over a
long number of years. It is a question for the future,
and does not appear to offer much hope for immediate
purposes.

The greatest possibilities for the near future lie in
the further investigation of the semi-coking coals
having between 11 and 16 for a caking index. Can a
satisfactory metallurgical coke be obtained from such
coals? Would the yield of by-products and power gas
be sufficient to make it a paying proposition?

* From a paper read before the Mining Institute of Scot-
land on February 9.