THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.	FRIDAY, OCTOBER 25, 1918.	No. 3017.

REPAIRING INDUCTION MOTORS.

By L. FOKES.

It may be broadly stated that practically all induc-
tion motor breakdowns are due to electrical causes.
Mechanical failures are seldom met with unless accom-
panied by electrical breakdown, and therefore, whilst
the former deserved consideration, the latter is of
greater importance, because it is the ultimate result
of breakdowns generally—and as these remarks concern
electrical repairs in particular, they will be confined to
that part of the subject.

Electrical Breakdowns.

From fig. 1 it will be seen that electrical breakdowns,
as applied to the stator of an induction motor, fall
under two distinct headings, each being subdivided.
On the one hand, we have breakdowns of insulation due
to external causes, whilst on the other the failures are
due to internal weakness.

Excepting insulation failures due to abnormal core
temperatures, they are common to both stator and
rotor, though, as regards the latter, they are confined to
the starting period—there being no appreciable differ-
ence of potential between points in the rotor windings
and between those points and earth when the rotor is
running at normal speed. At the present time, when
labour is scarce, it is not always possible to obtain the
services of an expert winder for carrying out repairs to
induction motor windings, and therefore it is the object
of these remarks to give a few hints which will enable
the average colliery electrician, with a little care, to
carry out a winding repair with some measure of
success.

Cleanliness.

Too much emphasis cannot be placed upon the need
for absolute cleanliness when carrying out any form of
repairs to windings. It is often the case that a machine
situated in an awkward place underground cannot be
conveniently brought to the surface for repairs, and
must therefore be dealt with in position. This involves
the handling of winding materials under the worst
conditions, and unless proper attention is paid to

tion of the insulating tape. A rough coil collects dirt
much more readily and soon becomes coated, with the
result that the outer tape rapidly deteriorates and is
very difficult to keep clean.

Stator Windings.

Stator windings take one of two forms, z.e., they are
either hand-wound into suitable tubes, previously
inserted in the stator slots, or are wound and insulated
upon a special “former ” and afterwards inserted in the
slots and held in place by fibre wedges.

Repairing a Hand-wound Coil.

The section of such a winding is shown in fig. 2. An
extreme case will be assumed where a coil has been
damaged by the rotor coming down on the stator and
has driven some of the core plates into the tubes and
completely destroyed a coil.

The method of procedure should be as follows.:—The
outer insulation of the coil to be removed should be
carefully stripped, and the exact number of turns of
wire ascertained, in order to have the same number in
the new coil.

The complete coil may occupy two or more slots on
each side. A coil wound in six slots, i.e., three slots on
either side, would be referred to as having three slots
per coil. If the total wires of such a coil are counted,
their number divided by three will give the number of
wires per slot. It may, however, be more convenient to
ascertain directly from one slot the number of wires
which it contains.

Before removing the damaged coil the services of a
carpenter should be obtained, in order to make two wood
blocks, which must fit close up under the coil to be
rewound. The blocks must be shaped exactly to the
angle of the existing coils so that the new one will
occupy the same position by being shaped upon the
wood blocks, which are shown in fig. 2.

Fig. 2.

the wires must be placed in the slots evenly and without
crossing.

This is accomplished by what are called needles—
shown in fig. 4. The needles should be of the same size
as the wire to be wound, and made of steel or wood ;
but failing either of these, lengths of wire may be cut
from the old coil and used instead.

There should be exactly the same number of needles
as wires to be inserted, and they should be placed in
layers, as indicated in fig. 4—each layer being separated
from the next by a strip of pressphan, which not only
helps to fill the tube and prevents the movement of the
wires, but improves the insulation of the winding
between turns. When the end of the wire is entered
into the tube, a needle is withdrawn in front of it and
so clears the way.

Fig. 5 shows two tubes representing one on each side
of a coil, and indicates the rotation in which the
winding should be carried out. Starting at S (fig. 5),
the wire passes through to the opposite side of the
stator, and returns through No. 1 in the left-hand tube,
after which it enters No. 2 on the right, and so on until
No. 8 is reached (marked F). The end at F then starts
into No. 1 of the next pair of tubes, and the process
described is repeated.

One of the chief difficulties of winding, as experienced
by a novice, is the handling of the long length of wire,
which might be considerably over 100 ft. in length. To
try to pull it out straight each time the end is threaded
through a tube, is impracticable, since the wire would
be damaged by dragging about the floor, and some
other method must be used. An effort will be made to
explain how the difficulty may be overcome, and
winding made so that the winders—assuming there is
one at each side of a machine—may remain seated
throughout the winding operations.

Figs. 6 and 7 represent the same tubes at different
stages—the needles being omitted, in order to make the
figures more clear. It will be supposed that winding is
commenced in fig. 6, and that the length of wire has
been pulled through tube B, leaving a short end as
shown.

Fig. 4.

I

Stator
Breakdowns.

Winding.

Core

Wood,
Block.

Vf ooJ,

B lock

r-

External
Causes.

Internal
Causes.

Water.

Between
Phases.

Mechanical
Damage.

Overheating
Of Iron Core

Defective
Insulation.

Coal and
other Dust.

Oil and
Grease.

To
Earth.

Over
Loading.

Between
turns of

Windings. .

Fig. 1.

hffi

C.

Fig. 3.

wa
oio
oio

Fig* 5.

keeping the winding materials free from abrasion,
grease and moisture during the winding operations
little success can be hoped for, and many valuable hours
may be wasted on a winding which ultimately breaks
down. It is not sufficient to carry out a repair in a
rough and ready manner that will continually cause
anxiety lest it should give way.

Neatness.

Next to cleanliness in importance is neatness. This is
necessary for several reasons, i.e., (1) appearance;
(2) facilities for insulating; (3) keeping end windings
clear of end shields ; (4) presenting a smooth surface.

1.	Appearance need not be considered, as it will be
attained if the other requirements are satisfied.

2.	It will often be found that very little room is
allowed between the different end windings to allow of
insulating the coils, so that, when coils are carelessly
wound and take a great deal of room, it is very difficult
to insulate them.

3.	It should be borne in mind that, so far as insula-
tion is concerned, the weakest point is that part of the
winding outside the slots, i.e., the end windings. It is
therefore of great importance that the end shields of a
motor do not come into contact with the end windings,
as this has often been known to cause a breakdown to
earth.

In cases where the end shields are very close fitting,
extreme care is necessary when replacing a damaged
winding, or it may be found impossible to get the end
shield on again, owing to the new windings projecting
further than the original.

4.	Finally, it is necessary that a finished coil should
have a smooth surface. This depends not only upon
the actual shape of the coil but also upon the applica-

Having done this, the coil may be removed by the
quickest method, which is to saw through, or otherwise
sever, the complete coil on either side of the machine,
and then pull the short pieces from the tubes.

The removal of the tubes will be necessary if damaged
in any way, and this may be carried out by splitting
them down the centre where the tubes are exposed
between the teeth of the stator. All roughness inside
the iron slots should then be removed before inserting
new tubes.

On most high tension machines the tubes are usually
of mica, are very hard, and thus easily driven into
place. On certain low tension motors, however, some-
times the tubes are simply made up of a piece of
pressphan.

At first sight the replacing of the pressphan tubes
appears difficult, but fig. 3 shows how this can quite
easily be carried out. A piece of hard wood is procured,
and cut to fit the slot into which it is desired to insert a
tube. The section would appear as A in fig. 3, but
would be a little smaller at all points than the slot
to' which it has been shaped. A piece of pressphan
being then wrapped around the stick, as shown at B,
fig. 3, the two are inserted into the slot, and while the
pressphan is held with one hand the stick is carefully
withdrawn, and the tube remains as in C fig. 3. If the
tube is coated with varnish before insertion, it will hold
firmly in place after a short time.

It may be stated here that pressphan tubes are very
difficult to work with, owing to the great care necessary
in winding, to avoid damaging the end.

The tubes having been inserted, winding may be
commenced. As already emphasised, neatness is of the
greatest importance, and if this is to be obtained

Further, the end has been inserted into tube A on the
other side of the stator and pulled through just suffi-
cient to allow of the end being again entered at B as
shown. At this stage all the slack wire is at the other
side of the machine.

Having placed some clean brown paper on the floor,
the winder commences to pull the wire through tube A,
as shown by the arrow, and turns it in coils upon the
floor at his side, as shown in fig. 7. An examination of
the coil will show that a twist is placed in each turn of
wire laid down.

After all the wire has been pulled through, the coil
lying on the floor is carefully turned bodily over to the
left and placed upside down. This takes the whole of
the twists out of the coil, and also places the end to be
fed into tube B on the top of the coil instead of at the
bottom as shown in fig. 7.

The end just entered at B may be pulled through
while the loops lift from the coil on the floor and leave
the wire without any twist after it has all been pulled
through. Of course the same process is carried out on
each side of the stator as winding proceeds.

Shaping and Insulating.

The winding is shaped on the wood blocks already
referred to, these being wedged or tied in position.
They only determine the shape of the coil in so far as
the bottom layer is concerned, the remainder being
neatly arranged so as to preserve the same shape as
that of the wires in the slots.

The strips of pressphan between the layers of wires
in the tubes should be continued through the end
windings. If pressphan is found to be too bulky and
stiff, it may be replaced by Empire cloth, which is of