742

THE COLLIERY GUARDIAN.

April 12, 1918.

a circuit breaker open or any unusual noise which
might indicate to the attendant that something was
wrong.

(3) The temperature of the air leaving some
machines is rather high, and the quantity often very
large, so that such a large volume of heated air being
delivered into the building makes the temperature
high, this being particularly the case where several
machines are at work.

It is always an advantage to keep the power house
temperature as low as possible, owing to the greater
range of temperature rise available where the initial
temperature is moderately low.

The general practice, therefore, is to discharge the
heated air through a suitable air duct to the outside
of the building.

Filtering the Air Supply.

The high resistance offered to the air current by
the restricted openings and ducts in a turbo
alternator allows a great deal of the dust carried

Fu. 4.

StaLoi

water is allowed to run. The particles of dust adhere
to the wet baffles, and also any water in suspension
is removed from the air before passing to the
generator.

The apparatus requires a pump in order to maintain
the circulation of water, the air being supplied by
a blower, or drawn by the generator supplied with
suitable fans on the rotor ends.

For small sets, the blanket filter meets the case
quite well, and if the blankets are systematically
cleaned, the method is quite successful.

The water filter requires a good deal of space,
which makes it difficult to apply this system to exist-
ing plant where room is limited, but on new plant,
exceeding, say, 1,000 k.v.a. to 2,000 k.v.a., its use is
recommended.

Arrangements should be made where possible for
taking the cooling air from either inside or outside
the building. In hot weather the air outside will
be much cooler, and where separate ventilation is
used, a smaller quantity of air is necessary. How-
ever, it is unwise to take the air supply permanently
from outside, as in very cold weather fine particles
pf snow or frost may be drawn in with the air, and
undergo condensation on coming in contact with the
hot windings of, the machine. Since condensation of
this kind might become dangerous, the air supply
should be taken from inside the building in very cold
weather.

Cleaning.

In spite of filters, either wet or dry, it is impossible
to prevent dust accumulating inside generating plant.
It gets there in various ways. Some dust is made by
the machine itself—probably from the insulation of
the windings. A little gets through the filter, and a
much larger quantity is drawn in through the
openings for rotor shaft in the end shields of the
generator. The idea that a filter removes all
necessity for cleaning generators is quite wrong, and
end shields should be regularly removed in order to
clean down and varnish end windings, and also to see
if oil or other matter is finding its way into the
machine.

Referring to the illustrations of the various methods
of ventilation, it will be noticed that many of the
ducts through which the ventilating air has to pass
are at right angles to the axis of the machine. It
is very difficult to get at these ducts for cleaning,
unless a machine is completely dismantled and the
rotor removed. This entails a good deal of work,
but it ought to be carried out once in three or four
years, depending on the amount of dust that is flying
about during the operation of the plant, and also
on the conditions noticed when the end shields of the
machine are removed.

The removal of the rotor is an operation requiring
great care throughout the whole operation, especially
in withdrawing it from inside the stator, as the
laminated core of the rotor and stator will not slide
one over the other without displacing the core plates
and probably driving some into the winding tubes.

J) isma n t ling.— -When dismantling becomes neces-
sary for cleaning purposes, the generator coupling
should be disengaged from the turbine, and the
bearing covers removed on either side of the

round the centre of the rotor by steel slings, under
which strips of hard wood, a few inches wide, should
be placed to prevent the slings pulling into the air
ducts of the rotor and damaging the plates.

While the machine is apart, all air ducts should
receive special attention, being thoroughly cleaned
by compressed air after all dust and dirt has been
loosened by a brush or other suitable means. It is
an advantage after all dirt and dust is removed to
spray one thin coat of varnish over the exposed tubes
and sides of the slots, as smooth surfaces do not hold
the dust nearly so readily as rough surfaces do.
Special attention should be given to the bottom half
of the stator, as the slots at this point become blocked
more quickly than at any other part.

The replacing of the rotor is simply carried out by
reversing the operations of removal. The iron sheet
is placed in the bottom of the stator before intro-
ducing the rotor, and when the machine is in place
and the rotor resting in its bearings, the sheet may
be removed by working it up to the top of the rotor
and withdrawing it. The insertion and withdrawal
of the iron sheet at the top part of the machine is
necessitated by the bearings, etc., being in the way
of introducing or removing it from the bottom.

Electrical Considerations.

Slvp Bings.—It will no doubt have been noticed
that one of the slip rings on a turbo alternator wears
much more quickly than the other. This wear is
often very rapid when the rings are made of soft
metal, although it still takes place even when steel
rings are used. The current which supplies the
excitation of the field is, of course, fed into the rotor
by means of one slip ring, while it leaves the rotor
by the other ring, and it will be found that the ring
conveying the current from the rotor is the one which
wears most. There appears to be some chemical
action, assisted by the passage of the current from
the ring to the brushes, which deposits copper (if the
rings are of that metal) upon the brushes, and this
in turn may play some part in the rapid wear of
the slip rings through the metallic coating impairing
the lubricating properties of the contact surfaces.
Slip rings are usually difficult to renew, owing to the
necessity for removing everything from the generator
shaft in order to get the old ones off and the new
ones into place. It is, therefore, an advantage to get
as much wear out of the slip rings as possible, and
it is a good plan to change over the leads of the
generator field periodically, so that the polarity of
the current is changed with reference to the respective
rings.

As a result of this practice, the rings both wear
equally and their combined life is nearly doubled.
The ring surfaces should be kept as true as possible,
and any uneven patches should be turned off imme-
diately they appear. Sparking at the slip rings is
very destructive, owing to the partial breaking of
the magnetic field current.

Splashes on the rings, which are often noticed when
generators are standing, are possibly due to rapid
changes in load, such as are caused by short circuits
on the line. These may induce heavy currents in the

into a machine to lodge in various places, and very
little actually gets blown right through. The present
practice is to filter the air in some form before it
passes into the generator, and thus a great deal of
dust is prevented from entering. There are two kinds
of filters, i.e., blanket filter and water filter or air
washer.

Blanket Filter.—This consists of suitable material
stretched over wooden frames and held in position in
such a way that all air going to the generator passes
through the covered frames, the dust being deposited
on the outside. The blankets have to be changed
periodically, and thoroughly cleaned. This is best
done by hanging them over a line and introducing
compressed air into each pocket so that the dust is
blown off the outside. The blankets should never be
beaten, as they would quickly become torn or broken.
The great danger with blanket filters is that of fire.
Should the blankets once become ignited while the
plant was running, hardly anything could save the
whole from being destroyed. There are, however, two
precautions which might be taken. A fire hydrant
might be installed in a convenient place, and a suit-
able hose kept constantly connected in case of fire.
Tn addition, a fire-proof door might be introduced
in the outlet conduit, so that it could be quickly
closed in case of fire and the fire confined within
the machine. In the lay-out of such plant the blanket
filters should be placed out of reach of accidental
ignition, and smoking should be strictly prohibited
anywhere in the vicinity.

Fireproof windings have from time to time been
suggested, but these have not met with much success.
The two essentials, i.e., mica, and asbestos, are in
themselves just what is required, but mica requires
some addition in the form of varnishes to hold it
together, and asbestos must have some kind of filling
in order to improve its insulating qualities. In both
cases these additions necessary to their use impair
the fireproof qualities, and the difficulty more or
less remains.

Air Washing.—This method has received a good
deal of attention in recent years, and in manv respects
it is much better than filtering by means of blankets.
The greatest advantage seems to lie in the coolness of
the air supplied to the generator as a result of passing
through the washer. The idea consists in passing the
air through fine sprays of water, thus adding to it
a considerable amount of moisture. Much of the
impurities contained in the air are removed bv the
water sprays and fall to the receptacle provided for
receiving the deposit, whilst any matter left in
suspension is removed by passing the air through a
series of baffle plates over which a small stream of

Fig. 6.	Fig. 7.	Fig. 8.

LX'i

C.rc.ui L.

generator. A large sheet of | in. iron should be
obtained, about 18 in. or 2 ft. wide, and long enough
to project slightly over each end of the rotor. The
sheet should be curved exactly to the shape of the
inside of the generator stator.

After removing the end shields, the sheet of iron
should be introduced between the top of the rotor
and the stator, and, while held at each end, should
be carefully worked down on one side of the rotor
until it lies on the bottom half of the stator and
under the rotor. After removing the holding-down
bolts of the generator and dismantling the exciter
field, a lift should be taken by slings fastened to the
stator. As the stator is lifted, the rotor comes to rest
on the iron sheet and the whole is lifted by the crane
into a suitable position for removing the rotor.

Having placed the whole machine on suitable pack-
ing, the crane should be fastened to one end of the
rotor shaft, while a strong trestle is placed under the
opposite end, so that a large pole may be used as a
lever to ease that end of the rotor upwards as the
crane lifts and travels slightly away from the stator,
pulling the rotor with it in easy stages.

The iron sheet either travels with the rotor or
the latter slides on it, which is immaterial, as in any
case the iron readily slides over the surface of either
without catching the laminations.

When the rotor is half way out, the end of the
shaft should be packed up and a final hitch taken



rotor circuit at the instant, with the result that a
momentary flash takes place between the brushes and
the ring. *

In connection with generator fields, it may be
mentioned that under no circumstances must fuses
be inserted, as their operation might be fatal to the
insulation of the rotor, and possibly to the switch-
board attendant as well, should he be near. A field
switch may be inserted in the exciter field, and this
should be provided with an auxiliary contact for
short circuiting the exciter field when the switch is
opened. The connections for the exciter and main
field of a generator are shown in fig. 6. The main
field rheostat as shown in circuit with the’generator
field is useful for two purposes.

In the case of fig. 6, where a generator is excited
by an exciter carried on the end of the rotor shaft,
it is sometimes found that, on very light loads, the
exciter is working at an unstable point on the
saturation curve of the exciter field, which makes the
voltage of both exciter and generator difficult to
regulate. By inserting a small amount of resistance
in series with the main field, the voltage of the
exciter has to be raised, which thus enables the
exciter to work in on a more stable point on its
saturation curve.

Another case in which a main field rheostat is
useful is when generators are separately excited
(fig. 7). There may be a slight difference between