THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.

FRIDAY, OCTOBER 4, 1918.

No. 3014.

The Running and Maintenance of Induction' Motors at Collieries.

By L. POKES.	/

the machine cannot be communicated to the atmo-
sphere surrounding the motor. Such motors should
be of liberal rating for the work they have to do,
owing to the high temperature rises which take place
when severe overloads have to be carried.

In the earlier applications of the induction motor
drive to colliery plant, the results were not altogether
satisfactory. On the one hand, designers at that tim^
did not appreciate the onerous conditions under which
colliery electrical plant had to work, and on the other,
the colliery owner had not discovered the futility of
putting in cheap apparatus and of installing motors
which were far too small for the work expected of
them.

In recent years, however, things have completely
changed. The design of the induction motor of to-day
is such as to make it a most reliable machine, and
this, together with the fact that colliery owners at
last see the benefit of installing the very best appli-
ances and of ample capacity has placed the adapt-
ability of this class of apparatus to colliery conditions
beyond all doubt. Indeed, in many cases the intro-
duction of a modern induction motor drive has proved
more reliable in actual operation than the engine
which it. displaced.

It must not, however, be assumed that the induction
motor as manufactured to-day is a machine which may
be installed and then neglected indefinitely. On the
contrary, if satisfactory running is desired reasonable
attention must be given to those things which are
common to all classes of electrical machines, whether
installed at collieries or elsewhere. These may be
briefly summed up as follows : (1) Cleanliness; (2)
. freedom from excessive overloads; (3) protection from
damp; (4) periodical varnishing of windings; (5)
attention to bearings; (6) intelligent usage.

The present purpose' is not to deal with repairs to
induction motors after they have broken down, but
rather to give attention to those points which will
help to do away with the necessity for repairs. The
successful colliery electrician or engineer is not the
man who always manages to repair a breakdown, but
rather the man who by close attention prevents
breakdowns occurring.

Selection of Motors.

To those who have had considerable electrical expe-

• rience on the coal fields, the selection of the right .
type of machine for working under specific conditions
determines whether that machine is going to be a
success or otherwise. It cannot be too often empha-
sised that expert attention may be powerless to obtain
good results from a motor which has been indis-
criminately applied to unsuitable operations. The
number of different types of induction motors now
obtainable leaves no excuse for indiscriminate appli-
cation.

The conditions found more or less in and about all
collieries may be classified roughly as follows: —

(1)	Dry and Clean. — There are very few places
about a colliery which will comply with these con-
ditions, but the power station and fan house can
be considered as coming under this heading. There
are certain other auxiliary drives, such as some of
those employed for gas-producing plant, which also
may have these ideal conditions.

(2)	Dry and Dusty.—These may be taken as the
general conditions prevailing about the various
houses and engine rooms, both above and below
ground, where motors are called upon to operate.
Screens, engineering shops, haulage plant, crushing
plant and chaff cutting machinery, etc. Motors em-
ployed on these will invariably be working where dry
and dusty conditions prevail.

(3)	Damp and Clean.—This will usually only apply
to pumping plant underground which is installed in a
proper pump room.

(4)	Damp and Dusty.—These terms appear contra-
dictory, but nevertheless exist. Most washery plant
will come under this heading, as will also mortar mills
and saw mills and other auxiliary plant doing various
work about the surface of a mine.

(5)	High Temperature.—These conditions are un-
usual excepting in the workings of collieries. In most
cases the conditions do not allow of the installation
of any but a totally enclosed motor, and no selection

. is necessary.

The types of motors from which a selection may
be made to suit any of the conditions referred to are
as follows: (1) Open type; (2) protected type;
(3) enclosed ventilated type; (4) totally enclosed
type; (5) flame-proof type; (6) drip proof type.
The variety of types only goes to show how the various
conditions have been studied and apparatus designed
to meet the numerous requirements.

Conditions No. 1.—Whereas certain conditions make
a particular type of motor imperative, the ideal con-
ditions call for no special type from the point of
view of maintenance. The deciding factors are
mainly first cost and the necessity of installing a
machine, which has its windings and terminals properly
covered to protect persons from making contact with
them.

A motor Cannot be too easily got at for cleaning
under any circumstances, but the more “open” the

design, the greater facility there is for cleaning and
generally getting at the interior of the motor. It is
therefore permissible to adopt an open type motor in
a clean and dry situation, provided the motor is one
of considerable size and separate bearing pedestals are
supplied. Failing this, the protected type of motor
would be quite suitable.

Conditions No. 2.—The proper selection of the
correct type of motor under these conditions is perhaps
the most difficult, owing to the variety of applications,
each of which requires special consideration. Apart
also from the question of dirt and dust, it is more
necessary to provide adequate protection to persons
working near the motors than is necessary in specially
built houses similar to the power house or fan house,
where persons are employed solely for operating the
machinery.

The type of motor which is suitable for the great
majority of positions is the protected type, fitted
with ‘ end shield bearings. These machines may be
alternatively enclosed ventilated motors, which are
practically the same as the protected type, with the
addition of expanded metal covers over the openings
in the frame. These machines are usually a sound
mechanical job, and their electrical parts are well
protected.

A totally enclosed motor would in many cases be the
most desirable, but its limitations with reference to
heat dissipation are so narrow that even small motors
are much more expensive for a given size than any
other type, owing to the necessity of allowing a very
liberal design to enable the heat to be dissipated
within the machine itself. A totally enclosed motor
much over about 40 horse-power is prohibitive, and even
a 40 horse-power machine costs, roughly, double the
price of some of the other types for the same output.

Some people may be tempted to put in pipe venti-
lated motors, due partly to their low first cost. Before

1 B

.k._

V ___________

O Vi

I	V -	__

j	v- - V; 1- V* __

Il

I ir- v, +	+ ir3

Fig. I.	.	Fig. 2.	Fig. 3.

such a course is decided upon, careful consideration
should be given to the condition of the outside air
from which the motor will have to draw its supply
for cooling. It is obvious that such a supply should
be free from dust, and therefore it follows that as
a rule the pipe ventilated motor is unsuitable to work
anywhere in the vicinity of screens or washeries, etc.
It is a useful type, however, to adopt if the stone-
crushing plant is some little distance away from the
pit top.

In connection with large haulage motors under-
ground, located near the downcast shaft, the “ open
type ” motor should receive attention before a de-
cision is made. Where head room is limited and room
generally cramped, it is inadvisable to instal a motor
which has to be constantly dismantled in order to
clean it properly. The open type machine can be
cleaned without dismantling and the possibilities of
damage due to improper slinging or careless handling,
are reduced to a minimum.

Conditions No. 3.—Machines under these conditions
will invariably be those connected with pumping plant.
A certain amount of discretion is needed in this direc-
tion. Some pump rooms are all that could be desired
so far as their suitability for a motor is concerned,
whilst others may provide less favourable conditions.

As the pump room is usually in the “ intake ” air,
an ordinary “protected type” machine will be suit-
able, provided the pump room is free from water
dropping from the roof. Where, however, this cannot
be prevented, it would be better to instal a motor of
the “drip-proof” type.

Condition No. 4.—As a rule the machines suggested
for conditions No. 2 will be equally applicable in
this case. The heat generated by a motor is usually
sufficient to counteract any dampness which exists, so
long as the machine is worked regularly.

Condition No. 5.—As already stated, this condition
invariably applies to positions inbye, and totally en-
closed motors are compulsory. In addition they should
be flame-proof, so that any explosion of gas within

Protection of Slip Rings.

As a rule it will be found safer to have all slip-
ring motors enclosed in suitable casings, and under-
ground this casing must be of flame-proof con-
struction.

Electrical Considerations.

Stator.—The stator is that portion of an induction
motor which produces the rotating magnetic field.
The three-phase windings may be designed so that they
are connected in one of the following forms: (1) Star
connection; (2) delta connection.

Star Connection.—The star connection may be
found in two forms: (1) Series connected star, shown
in fig. 1; (2) series parallel star, shown in fig. 2.

When new motors are purchased, one of the first
things which should claim the attention of the colliery
electrician is to become conversant with the connec-
tions’ of the stator. %To do this may save a deal of
trouble if a breakdown should occur, especially should
it be come necessary to break any connections for
testing. The parallel star connection, however, will
usually only be found on large slow speed motors.

There has been from time to time a good deal of
discussion as to the advisability of earthing the
neutral point of the generating plant. It is not pro-
posed to enter upon this, but in so far as the earthing
or otherwise of the neutral point affects the possi-
bility of breakdown, this matter will be discussed.

Pressure Distribution in a Star-connected Stator with
Neutral Earthed.

A three-phas^ system with its neutral point earthed
has a constant difference of potential to ground from
any one of the line wires. This potential is equal to
the line voltage divided by the V3.

In the case of a' winding such as shown in fig. 1,
the distribution of' potential throughout the windings

will be apparent. , The dotted line surrounding the
figure represents the ground or the frame of the motor,
which will, of course, be grounded.

Taking the points a (v), b (-vj, c (v2) and d (v3) in A
phase, and their relation to the ground, and assuming
the pressure between the lines to be 3,000 volts = V,

V
then the voltage from a to earth is equal to =

1,734 volts = v, and the pressure to ground at b, c and d
will be as follows:—

b = 1,734 — 578 = v -	= 1,156 volts.

c = 1,734 — 578 4- 578 = v — vY 4-	= 578 volts.

d = 1,734 — 578 4- 578 4- 578 = v —	4- v2 4-	= 0.

From this it will be seen that the pressure to ground
gradually decreases as we leave point (a) and pro-
ceed along the winding towards the neutral point. It
is therefore evident that the possibility of breakdown
to earth is much greater at the line end of each of
the windings than at any other point.

It is an advantage to specify, when ordering three-
phase induction motors, that the three windings must
be separate and so arranged that the “star” connec- /
tion can be conveniently made at eitheY end of the
windings. It will be obvious that such an arrange-
ment might be useful when an insulation breakdown
occurred on one of the windings at a point near the
line. The changing of the star connection over, so as
to bring the faulty part near the “ star ” may in many
cases enable work to be continued until repairs can
be executed.

Neutral Insulated.

Motors connected to a supply with the neutral point
of the generators insulated are, in the normal working
of the system, subjected to less insulation stress than
when the neutral is earthed.

Since no point of the system is earthed, the poten-
tial to ground is not such that insulation breakdowns
readily occur, but the danger of such a system lies
in the possibility of one of the lines becoming earthed.
This will not of necessity open the circuit breakers
controlling the system, and consequently the wind-