April 12, 1918.

THE COLLIERY GUARDIAN.

743

the resistance of the fields of two generators, so that
if they were both connected to the same supply
voltage, the current through their field would not be
the same. This may also occur when identical
machines are running together, owing to one being
warm and the other just started up. In such cases
the variation in resistance is taken care of by the
introduction of resistance into the field of the machine
receiving too much exciting current.

Over Excitation.

Most collieries suffer more or less through the effects
of low steam. This is not because insufficient coal is
available to ensure a constant steam pressure, but
it is often thought to be more economical to sell all
the coal that is likely to burn, and to retain for the

Fig. 9.

Fig. 10.

colliery boilers a grade of fuel which most central
stations would consign to the refuse heap.

All generators are designed to produce their rated
voltage with a given exciting current when running
at their normal speed. If the speed is reduced while
the voltage is maintained constant, the exciting cur-
rent must be increased. Where an automatic voltage
regulator is employed, a reduction in speed and
consequently in generator voltage immediately causes
the regulator to increase the voltage of the exciter
in order to obtain a larger field current to maintain
the voltage of the generator at that for which the
regulator is adjusted. The consequence is that, when
the steam pressure is low and the speed of the
generator below normal, the exciting current is often
abnormally large, overloading the exciter and also the
winding of the generator field. A generator field
has been known to break down completely through
overheating caused by over excitation due to low
steam pressure and reduced speed.

Limitation of Short-circuit Current.

A turbo generator, at the moment of short circuit
will develop an abnormally large current, rising to
many times that of full load. It varies over such a
wide range that figures are difficult to give, as a
variety of considerations determine the number of
times full load current which a generator may reach
on short circuit.

For instance if we consider a turbo generator built
a few years ago, its short-circuit current for an
instant may reach, say, 20 times full load current.
The same generator as made to-day, with perhaps
identical windings, but provided with better ventila-
tion, heat-resisting insulation, and various other
improvements, may be rated as double the k.v.a.
compared with the one referred to, but the short-
circuit current will be the same in both. However,
the modern machine rated at double the capacity of.
the older one will have a momentary short-circuit
current of 10 times full load.

It is found that the short-circuit current falls at
the end of the first second to below 20 per cent, of
the maximum. It remains steady at this value, and
is termed the sustained short-circuit current.

When a short circuit takes place, the current is
only limited for the first instant by the reactance of
the machine, but the demagnetising effect of the
enormous current is such as practically to wipe out
the generator field, and immediately this comes into
play the current is reduced to a steady value. The
strains placed upon end windings of generators under
short-circuit conditions are very heavy, hence these
windings are usually found to be strongly braced so
that no movement can take place.

in uoniieucion wicli very large generators, provision
is oiren maue iu reduce tile cur rent m case o± a
snort circuit, co grouna oil u system running witn an
earciiea neucrai. me cun ent is nniiLeu oy connect-
ing wnac is Known as a current-nmiLiiig reactance in
series witn tne neutral oi cue geneiator. in some
cases it is necessary to peace current-iinnting
reactances m series mtn tne tnree generator pnase*
in oruer to limit tne current on slioi t circuit octween
nne*. it may be mentioned, tnac wnere tnese are
employed, a great saving is eiiected in tne breaking
capacity 01 switchgear.

me current-limiting reactances are designed to
oiler only a sinaii impedance to tne normal load
current oi tne genera cor, and they usually consist
oi comparatively lew turns oi wire oi large section,
in order that tne U2K losses in the cons shall be a
minimum under normal conditions.

Fig. 8 shows a generator provided with current-
limiting reactances on each phase as well as in the
neutral, it will be noticed tne switches are provided
tor short-circuiting the reactances if desired.

It will be noticed from fig. 8 that although
protection is provided for the generator against
damage due to outside breakdowns or short circuits,
it will be obvious that a breakdown on the generator
windings will have no effect on the circuit breaker,
and the generator coils might burn out in consequence
of grounding or short-circuiting between turns, before
the exciting current could be cut off.

A well-known firm supplies an arrangement, shown
in fig. 9, which provides protection in such cases. It
consists of six current transformers, three of which
are connected in the three leads forming the neutral
point, whilst the remainder are connected in the
terminal leads as they leave the generator. The
current transformers are so connected, that normally
no difference of potential exists between the connec-
tions supplying the tripping relay, but in case of
a fault developing in rhe generator, the relay is
energised and the circuit breaker may be made to
open or the generator field broken by a suitable
switch. The application of any or all of the
protections mentioned, depends entirely on local con-
ditions, such as size of plant, character of load, etc.

Vibration.

In conclusion, it may be said that when vibration
occurs in a generator running at normal speed, the
cause may be due to mechanical displacements or
want of electrical balance.

The rigid construction of rotors as manufactured at
the present time, seldom gives trouble due to
mechanical displacements, and this may also be said
with regard to electrical balance; but it is not impos-
sible for any machine to develop one or both.

Vibration due to electrical causes may be looked
for in the field coils, and before making any electrical
tests, and in order to prove the accuracy of the
assumption, the machine should be brought to normal
speed without the field being excited. If no vibration
occurs, the mechanical balancing may be taken as
being in order. If, however, vibration is set up when
the machine is excited, the trouble should be
immediately looked for in the field coils.

If some of the turns in the windings have become
short circuited, the poles will be unequally mag-
netised, and consequently the attraction between the
various poles and the stator core will be unequal,
hence the vibration. An ordinary fall-of-potential
test will show the resistance of the field coils, or,
better still, the direct resistance can be obtained by
means of a Bridge Megger. In any case the tests
should be made as shown in fig. 10, i.e., A—B, B—C,
C—D, and D—E. The faulty coil or coils will show
a lower resistance than the others.

A machine found to be developing this trouble
should be immediately shut down and the makers
advised. Such a failure might be detected in its
early stages by the switchboard attendant noticing
an increase in main field current for a given load,
this being checked against similar conditions of load
by referring to the station log book, which is ever
a reliable guide to the condition of the plant when
carefully kept.

It is announced that, in view of anticipated foreign
competition, the larger Swedish gas and coke works pro-
pose to establish an economic association for the purpose
of eliminating from the home market competition in the
trade in by-products of gas works.

U.S. Bunker Regulations.—The U.S. War Trade Board
announces that the rules governing licences for bunker
fuel and ships’ stores have been amended so that American
vessels not requisitioned by the Shipping Board are brought
within the regulations prescribed for neutral vessels in
paragraphs IV and V of the General Rules No. 1, which
were published on January 19 to go into effect February
1, 1918. Paragraphs IV and V, as amended, read thus:—
IV. No application for “bunkers” by any neutral
vessel or by any vessel of U.S. registry not requisitioned
by the United States Shipping Board shall be approved
unless the person or persons managing, owning, charter-
ing, or controlling such vessels shall have reported to
and filed, in duplicate, with the War Trade Board, the
names of all the vessels and masters, and any
changes that may from time to time have occurred
respecting said vessels and masters managed, owned,
chartered, or controlled by him or them. V. No appli-
cation for “ bunkers ” by any neutral ship or by any
vessel of U.S. registry not requisitioned by the United
States Shipping Board shall be approved unless the person
or persons owning, managing, chartering, or controlling
such vessel shall enter into an agreement, in a form to
be approved by the War Trade Board, agreeing to comply
with and be bound by each and all of the following regula-
tions. Failure to comply with any of these regulations in
the case of any one vessel may involve the refusal of
“ bunkers ” to all of the vessels of the particular person,
firm, or corporation managing, owning, chartering, or
controlling the vessel in question.

MINING AND EDUCATION IN
SCOTLAND.

In the course of a paper on “ The Education (Scotland)
Bill,” read before the Mining Institute of Scotland, on
the 6th inst.. Mr. William Jarvie (Bothwell) explained
that at a meeting of the Institution of Mining Engineers
held in Leeds on December 13, the Education Bill was
brought up for consideration and it was decided that
the matter should be remitted to the councils of the
federated institutes for discussion. In due course the
measure came before the council of the Mining Institute
of Scotland, and in view of the great importance of the
Bill and its far-reaching effects on the mining industry,
it was decreed that the subject ought to be discussed at
a general meeting of the institute with a view to formu-
lating recommendations for presentation, if necessary,
to the central council.

Effects of the Bill on the Mining Industry.

In considering the effects of the measure on the
mining industry it might be assumed that only very
elementary education was required for even the most
efficient general workmen. The officials were almost
entirely drawn from this class, and these, especially in
the higher grades, required a good technical education,
which ought to be founded on a sound general educa-
tion. It would appear that the effect of raising the
standard of education as proposed, would be to educate
the general workman to a higher degree than might be
actually necessary for his occupation in mining, but at
the same time it would give him a better opportunity
of rising to any official position. The effects of the
secondary and continuation classes as outlined in the
Bill, might in time prove highly beneficial to the mining
industry, but the chief concern meantime of those asso-
ciated with the industry was to consider and, if possible,
put forward suggestions as to how the necessary facilities
were to be given to students with the minimum disloca-
tion of the industry. The provisions of section 17, which
dealt with continuation classes, had a most important
bearing on the mining industry. With certain excep-
tions, attendance was compulsory, between the ages of
15 and 18 years of age, for at least 320 hours per annum.
A weekly attendance of 74 to 8 hours appeared to be in
view. The classes must be held between 8 a.m. and
7 p.m., unless with the approval of the Department.
The scheme was to provide general education, special
instruction for various employments, and physical
instruction. Employers of labour must afford any
facility necessary for attendance at continuation classes,
and the time required for attendance, including time
for travelling, if added to the working time, must not
exceed in the aggregate in any day or week the period
of employment permitted by the Act.

Minimising* Dislocation.

At first sight it would appear that considerable
dislocation of the mining industry would result from
the operation of section 17, but a doser examination
showed that the amount of this dislocation depended
largely on how the local educational authorities used
the powers conferred on them. He (Mr. Jarvie) under-
stood that Mr. Fisher and Mr. Munro had expressly
stated that their intention was to secure the desired
educational results with the least possible dislocation of
the industry. It would be most unfortunate if such an
important matter was left on an indefinite basis, and
they had therefore to consider whether any representa-
tions should be made by the institute. The question
arose, was it possible, as section 17 stood, to arrange
classes so as to avoid any dislocation of the mining
industry ? This involved many factors which required
separate examination before a definite opinion could
be given. The first was the number of affected
persons engaged in the industry. In one colliery
selected as representative he (Mr. Jarvie) found that the
number of such persons amounted to 1.5*7 per cent,
of the total number now employed. That estimate
included a small number under 15 years of age, which
would probably reduce the total affected to 15 per cent.
If one-fifth of these were absent from work during the
whole of each working day he estimated the output
would be reduced by 2*34 per cent, assuming their
output capacity to be 75 per cent, of the average person
employed. The loss of output would be greater if all
were absent on the same day, or if a proportion were
withdrawn for part of each working day, and might
possibly reach 5 per cent. The objection might be
raised that owing to the large number of adults on war
service the proportion of young persons affected was
relatively high, but in the colliery selected the proportion
had not varied from this cause by more than 1 per cent.

“ Part Time ” Difficulties.

Among the difficulties of withdrawing workers for a
part of each working day were:—(1) The dangers
arising from traffic on haulage roads; (2) the disorganisa-
tion of the work and the impracticability of substitution;
and (3) the time lost in winding persons during working
hours. Mr. Fisher had stated that, owing to the special
difficulties in the mining industry of providing facilities
for daily instruction, he would be prepared to accept
instead one whole day’s attendance at classes per week.
On these lines a reserve of 25 per cent, of young persons
would be necessary for substitution, in strict
proportion. Such a reserve was certainly not avail-
able. The matter was also complicated by the
difficulty of supplying vacancies in the different classes
of work with suitable labour, and a further problem
would arise, at the close of each session of classes, in
finding employment for this reserve during the holidays.
Another point was the capacity of the youth of the
mining industry to absorb and benefit by instruction
during classes held in time in addition to, but not
deducted from, ordinary working time. The writer was
personally of opinion that the classes should be held
after the ordinary working hours, and that no undue
strain would be put on the student, as it was a change
from manual to mental work. The matter was one
on which different opinions might legitimately be held,
but he thought that, before the educational authorities
adopt a course which would seriously affect many
of the industries of the country, and particularly