Tímarit Verkfræðingafélags Íslands - 01.06.1950, Blaðsíða 8

30
TÍMARIT V. F. I. 195 0
Fig 5.
Pollock Commutator before Machining.
tire length by the steel strips on each side of it, the
copper need be made only as deep as is necessary for
wear and anchoring. At the same time, the segments
are perfectly free from expand longitudinally without
hinderance and without giving rise to increased stresses.
While this type of commutator has advantages over
the vee-ring commutator for machines of all types, it is
particularly applicable to machines which regularly run
at high speeds such as exciters for steam turbo-altema-
tors, or which can attain high speed under runaway con-
ditions such as exciters for waterwheel alternators, also
to machines subjected to sudden shock load or quick re-
versal, which require to have a commutator of low in-
ertia combined with high mechanical strength.
CANTED FIELD WINDINGS.
A problem similar to that experienced in the design
of a commutator and again arising from the poor mech-
anical strength of copper has to be faced in designing
strip-on-edge field windings for high-speed salient-pole
A.C. machines.
If the copper is arranged in the conventional manner
as shown on the left hand side of Fig. 6, the centri-
fugal force L gives rise to two components P and M,
the former causing the copper to compress against the
pole shoe and the latter tending to make the coil ex-
pand and burst.
The force P gives rise to no difficulty, but the copper
must be supported against the force M by means of vee
blocks between the poles, as shown in Fig. 7, or some
similar device. This has the disadvantage that it serious-
ly impedes the flow of air along the sides of the coil,
which is the natural direction of air flow through the
machine, and thus reduces the cooling effect of the fins
generally provided for this purpose and formed by mak-
ing some of the copper strips wider than the others,
as shown.
As can be seen, the copper segments A are interlocked
with the steel retaining strips B, and these steel strips
are welded at intervals along their length to steel rings C.
The insulation D between segments and to earth con-
sists of mica reinforced with treated glass cloth. The
arrangement of the rings, which can be spaced as close-
ly as is necessary to resist the total centrifugal force,
is clearly illustrated in Fig. 5, which shows a Pollock
commutator after removal from the building jig and
prior to machining of the outer surface.
Since each copper segment is supported along its en-
Fig 6.
Diagrammatic Section
of Rotor Pole with
Normal and Canted
Field Windings.
L P
R
Fig 7.
Typical Salient-Pole Rotor with
Vee Blocks, Squirrel-Cage Damp-
ing Windings and Aerofoil Fans.