5 x
SePt- 14, 1948.
WITNESSES:
E. I. POLLARD
POLE TIP STRUCTURE FOR‘?ALIENT
POLE SYNCHRONOUS MACHINES
Filed Aug. 18, 1945
2,449,506
INVENTGR
fr?esff. POZ/df‘d.
ATTORNEY
Patented Sept. 14, 1948
2,449,506
‘UNITED STATES PATENT orrice
2,449,506
POLE TIP STRUCTURE FOR SALIENT POLE
SYNCHRONOUS MACHINES
Ernest I.‘ Pollard, Wilkinsburg, Pa., assignor to
Westinghouse Electric Corporation, East Pitts
burgh, Pa., a corporation of Pennsylvania
~ Application August 18, 1945, Serial No. 611,416
2v Claims.
(C1. 171—-252)
2
1
to be able to use breakers of minimum size. A
subtransient reactance greater than 13.2% is
sometimes specified, at the same time that a tran
sient reactance under 21% is desired, thus pre~
, My invention relates to salient-pole synchro
nous dynamo-electric machines, particularly spe
cial machines designed for use as ship’s-service
generators, such as are used for general ships’~
senting con?icting requirements which have
service power-requirements other than propul
SlOn.
.
heretofore been difficult or well-nigh impossible
of ful?llment.
The general and quantitative effects of satura~
'
, The general objectof my invention is to provide
a novel construction which reduces the saturated
tion, on the internal impedances or reactances of
transient reactance of a salient-pole synchronous
machine.
‘
.
10
synchronous machines, have been well known and
analyzed since Kilgore’s paper on “The effects of
saturation on machine reactances,” in A. I. E. E.,
May, 1935, page 545. As stated in this paper, an
average value of the saturated transient react-.
The saturated transient reactance is a measure
of the clip in the terminal voltage, resulting dur
ing the ?rst second, or a few seconds, after a
heavy load is suddenly thrown on a generator,
such as when a large turret-training motor, or a
ance of
large steering-gear motor, is started on a ships’
known designs, for transients involving currents
of from 1 to 3 times the rated current, at full
service polyphase electric-power system. Such a
voltage-dip is undesirable, because it results in
machines
of , previously
voltage, is 0.895 times the unsaturated transient
reactance ‘which is obtained at half-voltage.
However, the Kilgore paper was devoted exclu
lamp-?icker, and it may, if excessive, cause mo
tors to be tripped off, but principally it is unde_
sirable because it disturbs the performance of
radar equipment. A great deal of effort has been
devoted to reduction of voltage-dip, through im
provements in the designs of exciters, regulators,
sively to studying and explaining the discovered
phenomena, without examination of the possibili
ties as to what could be done to change these
phenomena.
generators, and ships’~service systems.
The most important generator-characteristics
which affect the magnitude and the duration of
the voltage-dip are the transient reactance x’d
and the open-circuit time-constant Tdo. Of these
two, :r'd alone practically determines the magni 30
tude of the dip, so that a low value of Tdo is not
of great importance.
salient-pole
,.
-
.
An important part of the transient reactance
of a salient-pole synchronous machine is the
?eld-leakage part, which is caused by leakage
flux in the space between adjacent poles. A sub
stantial part of the transient reactance due to
this leakage-?ux is due to the leakage between
adjacent pole-tips. It has been known that, for.
The magnitude of the
saturated conditions, such as for a short-circuit
voltage-dip, when starting a motor of a given size,
from a ships’-supply system energized by a syne
at rated voltage, the pole-tips tend to saturate,
thereby reducing the ?eld-leakage flux and the
chronous, generator, can be reduced by reducing
35 transient reactance.
the transient reactance :r'a of the generator, or
by resorting to a generator .of ‘a larger rating, or
by‘ reducing the operating-time of the voltage
‘
However, heretofore, the radial thickness 0
cross-sectional area of the overhanging pole-tip
portions of the salient poles of rotating-?eld
salient-pole generators has been controlled by
regulator which controls the generator, or by in-V
creasing the speed ofv responsev of the exciter 40 various factors, such as considerations of me-.
chanical strength, for withstanding the shearing
forces due to centrifugal action, tending to break
off the pole-tips; and the contours of the tips
reactance of the generator be low, a value under
have been dictated by the necessity for properly
21% being sometimes speci?ed.
The subtransient reactance :r’hr of the gener 45 distributing the airgap ?ux of the machine. The
ator is a measure of the magnitude of the current
dimensions of the pole-tip portions are, therefore,
fairly well ?xed, and frequently can not feasibly
that would have to be interrupted by the circuit
be changed.
>
breakers. This reactance is affected less by satu
My present invention relates to a novel form
ration, andv it is smaller than the unsaturated
which energizes the ?eld-windings of the genera
tor. It is desirable, therefore, that the transient
transient reactancabecause of the effect of the 50 of the pole-tip portion, in which one or more holes
or openings are provided, in the magnetic mate
damper-bars during the ?rst two or three cycles
rial thereof, for reducing the effective magnetic
of the transient. In large generators, ,a high
cross-section of the tip in an amount su?icient
value of the sub-transient reactance is desirable,
to produce an effectively increased transient-re
in order to keep down the current which must be
interrupted in the event of a short-circuit, so as 55
actance saturation, with respect to leakage-?ux,
3
2,449,506
4
flowing circumferentially within the tip and
the shank I3, The tops of these pole-tip portions
thence across the interpolar spaces between the
i6 merge into the central portion of the con-3
toured or curved pole-face portion I2, while the
bottoms or back sides of the pole-tip portions ‘l6
are ?at, to serve as abutments for retaining the
pole-tips, during transient conditions.
These
pole-tip hoies can be placed in such positions that
they do not impair the mechanical strength,
which is to say that the cross-section at the base
of the tip is not aiiccted, but the cross-section of’
the tip is reduced at points further out'alon‘g the
tip, thus materially increasing the tip-saturation
e?ects beyond the effects previously obtainable,
and considerably iI'iCI‘eaSiIlgthQ-ratiO of-th'e sub
transient reactance to the transient freactanceqf.
the machine.
,
When damper-bars are omittedfi'omtherpole
tips, the difference between the reciprocals of the
transient and subtransient reactances is not par
ticularly affected by poleatipsaturation, because
this d-i?eren'ce is determined by the effect of the
bars near the pole-center on the ?ux-distribution
in that portion, rather than by the flux-distribu
?eld-coil l5.
} I
The pole-face portion |2.also carries damper?
bars l1‘, which are disposed in suitable, slots l8
‘ iii the pole-race portion. ‘ In a machine in which
vit is desired towhave as high a subtransient re
actance. as possible, it hasbeen customary to pilt
these damper-bars I‘! only in “the central per
tionsof the pole-face portion [2, and not in the
pole-tip portions. [5, and it has also been
customary to put the damper-bar slots 18, par
ticularly theven‘d-slots of each group, at a con
'siderable dGIJthrdQWII from the airgap 6, as shown
in?the drawing. In other machines, however, the
damper-bars I‘! are disposed all along the pole
tio'nrin-the poieétips, as-will be subsequentlyex
faceportion l2, including the pole-tips l6; and
while'my presentinventionis particularly ap
piainedmore in detail. Hence, my invention pro
videsa~ means for considerably increasing the ra
plicable “to the high-subtrans-ient - machines‘, it ‘is
tic-of subtransient to transient reactances. Thus,
it isépossible ioramach-ine-to be produced, having
improved characteristics, without necessitating
an increase in the size of the machine to obtain
those‘characteristics. My invention is particu
neverthelessapplicable also'to machines having
theimore widely» distributed damper-bars H.
In- accordance-with my present» invention, I‘
provide each pole-tip portion IS with one-or
more openings 20 in- the magneticlmaterial there
of. These‘ holes“ or openings 29* are» provided‘ in
addition to any- damper-bar slots‘ or- holes it
crease.theQsubtransient reactance, the damper 30 which may be provided, and in addition-to any
barsare bunched-close to the center-line of the
rivet-holes (not shown) or any other openings
larly-effective in designs in which, in order-to in
pole.
With theio'regoing. and : other- objects in view,
my invention consists in the structure, combina
tions, parts, systems ‘and methods vhereinafter de
scribedand-claimed; and illustrated in the ac
companyingdrawing, in whichthe three ?gures
are. fragmentary diagrammatic end-views of. a
half, 'of- a- pole of -a- synchronous machine em
bodying my invention, witnthree-di?erent ?ux
distribution-conditions, Figure-1‘ depicting the
no~load ?ux-distribution, Fig. 2 ~ depicting a the
short-circuit?ux-distribution vduring the con
tinuance of the subtransient effect, and-Fig. 3
depicting ‘the. short-circuit?ux-distribution illus
trative of theatransient e?ect.
My invention is illustrated asbeinga-pplied to
a six-pole, rotating-?eld,~ salient-‘pole, three
phase?synchronous generator, which is, shown
only iratgmentarily in-the drawing, and which
comprise a‘stator-member 4, which is the-arma
ture-member of themachine, vand aerator-mem
ber 5, which is the l?eldjmember-of ‘the machine,
with an- airgap B-in between-“the two members.
The stationary armature-member 4 comprises a
laminated magnetizable, annularcore-member,
having winding-receiving slots 1 therein, with an
armature-winding 8~ carried by the slots, the
armature-winding being broken away from all
but-one of the slots, forelearness of illustration.
The rotating?eld-member 5;comprises a rotat
able shaftIIO to which are keyed six salient pole
pieces- I I, only one-half of one pole being shown
in-eachi-?gureiof the drawing. Each salient _po1e-.
piece H is made of laminated magnetizable.
material. It has apole-face portion l2~adjacent
tothe airgapB, this pole-face portion being suit-.
ably contoured for distributing the?ux, asis well
known: Each pole-piece alsohas ashank-por
tion l3, which is keyed to the shaft, as shown
at > M,‘ and which carries a ?eld-coil’ or winding
I5; T-he pole-face portion i2 includes two sub
stantially identical pole-tips l?goverhanging the
shank-portion in a circumferential, direction,
one pole-tip-iii being disposed ‘on either side of
such as vmight otherwise beprcvided; My-special
pole-tip openings "are of such-‘shape-and size
asto reduce the e?'ective magnetic ‘cross-section
of the tip- l5, in--an ‘amount which is sumcient
to produce an effectively-increased‘transicnt-re~
actance saturation, with respect “toleakage-‘?ux
which?ows-circurriferentially within the._tlp i6.
and ~ thence-across the interpolar Y spaces 2| be
40 tween the“ pole-tips > of adjacent poles.~ during
transient conditions.
Since thepurpose of the special pole-tip holes
20-’~is-'to-reduce/the-magnctic cross-section with
respect-to laterally‘ or circumferentially ?owing
?ux, it is not necessary‘topjut thesespecial open
ings-20* at;a~p_la'ce- where, they would critically
weaken ‘the pole-tip-Ik'or' impair its mechanical
strength. Indesigning the pole-piece, it is neces
sary-to design the pole-tips l6'so as to have an
' adequatejradial thickncssat ‘the bases of the tips,
where they~ join onto'the shank-portion H; as
the centrifugaliorces which tend'to shear or
break‘ off the pole-tips tend ‘to. cause breakagev at,
thee-bases ofythe tips, asindicated at 22 in Fig. 1;
It‘ is desirable, jandipossible, in carryingput my:
invention, toep'ut the special "saturati'onrproduc-.
lHg_~tiP-ODBITiHgS' 20ifar' enough‘ out onth'e tips,,,
away-from? the bases ofthe tips, so'as not to pro
duce new - shearage-sections '23-'24 which i are
su?i'cientl-y_short;radially, to. critically reduce the
mechanical'strengthj ofjth‘e tips” in resisting,
centrifugal force.
It will-‘be‘understoodjhat theipole-pieces are».
symmetrical with _ respectto the~ . centerline 2 5 Jo!
the polejasliscustomarir insynchronous genera
tors. ,
The purpose ,an'diie?iecii. ofw myspecial pole-tin
holes 120 1 will‘ best ‘,bemnder‘stood-by considering:
the progressive vt'lianges. in, the distribution. oflthe >.
useiul?uxv , and the ._ leakagrg-?uxhfter the »appli-:_
cation of ‘aw'short-circuit or other sudden, increase
in loading. ,_ While the, ‘general. basic ideas. either
progressively, shifting,‘ ?uxedistribution; during’
transients, ,has long .been- known: ‘r sinner-at . least:
‘as early as Kilgore’s 1935 paper, the possibility
5
andqadvantage v‘of obtaining increased pole-tip
6
the damper. mid?eld-windings is permitted ‘to
saturation without impairing the mechanical
change.
strength of the tips, has not been recognized and
exploited in designs where the radial, depth of
the tips is made as small as mechanical con
siderations will permit, or, in general, where a
which interlinks with the damper-winding I],
because this winding has a relatively small total
cross-sectional, area, as compared to the total
The ?rst change , occurs in the flux
crossfsectionaljarea of the ?eld-winding | 5. The
stored magnetic energy. of the flux which orig
reactanceof the machinewithout as greatly re
inally interlinked with the damper-‘winding I1
is thus transformed quite rapidly into heat in the
ducing the subtransient reactance thereof‘.
.
Fig.1 is for an open-circuit or ‘lightly-loaded 10 damper-bars, thus permitting the ?ux-inter
linkage in the damper-windings to decrease to'a
condition of the machine, and it shows the dis
tribution of the useful flux and the leakage-flux,
very low value during the subtransient-reactance
when the. ?eld-Winding I5 is normally excited.
period, lasting for anywhere from a cycle to
The useful flux is the ?ux which crosses the airseveral cycles of time, on the basis of the alter
hating-current frequency of the machine.
gap 6 and links the armature-winding 8, and then
passes eircumferentially around the stator-mem
As the. flux which interlinks with the damper
her 4 t0 the next pole, as indicated at 30 in Fig.
windings 11 decreases, the damper-windings be
determined effort is made to reduce the transient
1.
This is the flux which generates the rated
armature-voltage at the terminals of the ma
chine. The remaining ?ux is the leakage-flux, as
shown at 3| in Fig. 1.
come substantially ineffectual, which means that
the major portion of the flux is no longer crowded
through the high-reluctance leakage-path which
is laterally bounded by the damper-bars "and
denly short-circuited, the terminal-voltage be
the armature-conductors 8, as shown at II in
Fig. 2. On the contrary, practically all of the
comes substantially zero, and the internal voltage which is generated in the armature-windings
8 must ‘be consumed by the internal reactance
drop inside the machine. The initial value of
shown in Fig. 3, which spreads out freely across
all of the radial cross-section of the pole-tipsl?,
' When the terminals of the machine are sud
?ux of the machine becomes a leakage-?ux, as
in anew leakage-flux path of lower reluctance,
approaching the reluctance of the space between
the internal machine-reactance, under such con
the pole-tips l6 of. adjacent poles. The sub
ditions, is de?ned as the subtransient reactance
1:”d of the machine. Since the armature-resist 30 transient reactance-effects of Fig. 2 are over,
within a few cycles after‘ the application of ‘the
ance is low, compared with its reactance, the
power-factor of the short-circuit current is ap
short-circuit, and then the conditions are ap
proximately as depicted in Fig. 3.
proximately zero, and lagging with respect to
the induced voltage, so that the magnetomotive
Obviously, as is well known, if the dampen
force of the armature-reaction is at its maximum
bars I‘! are concentrated near the centerline25
of the pole, rather than being distributed over
value at the centerline 25 of each ?eld~pole, and
is demagnetizing with respect to the ?eld, and
the short-circuit current is a direct-axis cur
the whole airgap surface or pole-face 12 of the
pole,rthe subtransient effect of the damper-bars
is to prevent a smaller proportion of the total
rent. This demagnetizing armature-reaction at»
tempts to instantly reduce the flux in the ma 40 flux from shifting at the ?rst instant of short
circuit. Other conditions being the same, this
chine, but flux represents energy, and can not
be changed instantly. Consequently, induced
results in a higher value of the subtransient re
currents simultaneously appear, in the ?eld
windings l5 and in the damper-windings l1, op
posing any change inthe amount of flux which
links with either one of these windings.
actance it"s. An increase in the subtransient
reactance x"d is also obtained by burying the
damper-bars l1 deeper below the surface of the
' Fig. 2 is drawn for the ?rst instant after the
short-circuit, and it represents the distribution
of flux in the machine at this instant. The
amount of ?ux linking the ?eld-windings l5 and
the damper-‘windings H is the same as in Fig.
1 before the short-circuit, but most of the useful
?ux has been shifted, from the portion .30 of
the armature-core which is back of the armature
slots 7, to a leakage-path which is bounded later
ally by the damper-bars I? and the armature
Winding 8, respectively, said leakage-path causing
the ?ux to flow circumferentially through the
pole-tips l6, and across the interpolar spaces 2!,
as shown in Fig. 2. Since the reluctance of this
leakage~path is usually several times that of the I
normal ?ux-path across the airgap 6 and thence
through the stator~core 4, a relatively high short
circuit current, of several times the normal full
load current-value, must flow in the armature
Winding 8 in order to transfer the flux from the
armature-core 4 to the leakage-path of higher
reluctance, as shown at 2| in Fig. 2.
pole, or below ' the, pole-face I2, because the
deeper position of the damper-‘bars increases the
effective cross-section, and hence reduces the
reluctance, of the subtransient leakage~path 2!
of Fig. 2, because that path is composed of the
radial space between the damper-bars I7 and
‘the armature conductors 8. Both of these meth
ods have been employed to increase the sub
transient reactance 1!!"d of the larger ratings of
ships’-service generators, in order to reduce the
size of the circuit-breakers (not shown), which
are required to interrupt the short-circuit cur
rent.
After the damper-bars I 1 have become sub
stantially ineffectual, at the termination of the
subtransient period within a few cycles after the
{application of the short-circuit, the conditions
shown in Fig. 3 prevail, and most of the flux which
then remains in the machine is distributed across
a leakage-pathwh-ich is laterally bounded by the
?eld-coil l5 and the armature-coils 8, respectively,
this . leakage-flux
extending
circumferentially
from pole to pole, as indicated in Fig. 3. The ?ux
During the conditions depicted in Fig. 2, cur
in the machine now diminishes at a rate which is
rents are induced in boththe damper-windings 70 determined by the rapidity with which the stored v
I"! and the ?eld-windings i5, resulting in 12R
magnetic energy of the ?ux can be converted into
losses, in these windings, which dissipate, as heat,
heat by the FR losses in the ?eld and armature
the energy which was stored ‘in the magnetic
coils I5 and 8. Since the area of copper in the
?eld of the machine. As this stored magnetic
?eld-Coil I5 is much greater than the e?ective
energy is diminished, the linkage of flux with 75 area of the damper-bars H, the rate of decay of
2,449,506
8
7
the ‘flux is now much slower than during the sub
transient period, and the final decay of the ?ux
requires a much longer time, known as the tran
sient period, and usually lasting for a second or
several seconds, depending upon the design.
During this transient period, as depicted in Fig.
3, while the current which is induced in the ?eld
winding 15 is dying out, the distribution of the
flux remains substantially the same, as depicted
‘in Fig. 3, with the ?ux merely diminishing grad
ually in its magnitude. If the exciter-voltage
which is applied to the ?eld-windings [5 has not
leakage-flux path, thus ‘materially decreasing ‘the
transient reac‘tance. It will further be noted, ‘that
I have produced this ‘effect without as greatly ai
fecting the ‘subtransient reac-tance, and also with
out critically a?ect-ing the mechanical strength voi’
tho pole-tips.
I claim as my invention:
1. A salient-pole ‘synchronous dynamo-electric
machine comprising a stator-member and a. rotor
member, with an airgap in between, said stator
member being an armature-member having an
armature-winding, and said rotor-member being
been changed during the transient, the ?nal value
a salient-pole ?eld-member having a plurality of
of the ?e1d~current is the same as the direct-cur
rent value before the short-circuit was applied .to
the machine. The ?nal or sustained value of the
salient .pole- pieces made of magnetizable material,
each pole-piece having a pole-face portion adja
cent to the airgap, and having also a shank-por
armature-current is merely the current sufficient
tion, the pole-face portion including two substan
to prevent more ?ux-linkage than is required to
tially identical pole-tips overhanging the shank;
generate an armature-conductor voltage equiva
portion, one on either side thereof, damper-bars
lent to the Ix'a internal~impedance drop of the 20 in the central portion of the pole-face portion but
armature-winding itself.
not in the pole-tips, and a ?eld-winding on- the
The effectiveness of my special saturation-pro
shank-portion of each pole-piece; characterized
ducing pole-‘tip openings 20 will be apparent from
by each pole-tip having one or more openings in
the foregoing representation of the ?ux-distribu
the magnetic material thereof for reducing the
tion conditions, and particularly from a compari
e?ective magnetic cross-section of the tip in an
son of the flux-leakage path 2! of Fig. 2 with the
amount sufficient to produce an effectively in
much more spread-out ?ux-leakage path which
creased transierit-reactance saturation with re
spect to leakage-flux ?owing circumferentiaily
is shown in Fig. 3.
During the subtransierrt time, as shown in Fig.
within the tip and thence across the interpolar
2, most of the flux is crowded out toward ‘the ail‘ 150 spaces between the pole-tips during transient con
gap-edge of the pole-tip portions i6, by reason of
ditions; the pole-face portion, including the tips,
the damper-winding effect which causes most of
of each pole-member having a ?ux-distributing
the ?ux to continue to interlink the damper-‘bars
contour which is symmetrical with respect to the
l1, before beginning to leak circumferentiaily in
cen-terline of the pole.
2. A salient-pole synchronous dynamo-electric
the path indicated at 2i in Fig. 2. This effect of :-"
the damper-bars is even more noticeable, in ma
machine comprising a stator-member and a ro
chines in which the damper-bars I‘! are disposed
tor-member, with an airgap in between, said sta-‘
tor-member being an armature-member having
an armature-winding, and said rotor-member be
ing a salient-pole ?eld-member having a plurality
of salient pole-pieces made of magnetizable ma
terial, each pole-piece having a pole-face portion
adjacent ‘to the airgap, and having also a shank
portion, the pole-face portion including two sub
all along the pole-face portion I2, extending out
into the pole-tip portions I6, as well as over the
central portions of the pole-face !2. By reason 11
of the restricted cross-sectional area of the sub
transient leakage-flux, which has to pass through
the damper-windings before it can begin to leak
off in a circumferential direction, this subtran~
sien't leakage-flux path is always considerably sat
stantially identical pole-tips overhanging the
urated, so that the subtransient leakage-path in
herently has a considerable reluctance, due to sat~
uration, as well as due to the restriction of the
radial cross-section of the leakage-path.
In Fig. 3, however, the leakage-flux is subs‘tan- '
tially free to spread itself out uniformly over the
entire radial cross-section of the pole-‘tips, so that,
shank-portion, one on either side thereof, damper
bars carried by each pole-face portion, and a
?eld-winding on the shank-portion of each pole
without my special saturation-producing tip
openings 20, there is relatively little saturation of
the magnetic material of the pole-tips i=6, Hence,
in all previously known designs, the reluctance of
piece; characterized. by each pole-tip having one
or more saturation-increasing openings in the
magnetic material thereof for reducing the ef
fective magnetic cross-section of the tip in an
amount sufficient to produce an effectively in~
creased transient-reactance saturation with re~
' spect to leakage-?ux flowing circumferentially
within the tip and thence across the interpolar
the transient leakage-path which is shown in Fig.
spaces- between the pole-tips during transient con
3 has been considerably smaller than the subtran
ditions; said saturation-increasing pole-tip open
sient reluctance of Fig. 2, not only because of an
ings having no damper-bars therein; and the
increase in the effective cross-sectional area of 60 pole-face portion, including the tips, of each pole
the leakage-path, but also because of a consider
member having a flux-distributing contour which
ably reduced amount of saturation.
is symmetrical with respect to the centeriine of
the pole.
The effect of my invention, in deliberately intro
ducing the saturation-producing openings 20,
ERNEST I. POLLARD.
which are added, over and above any other open
ings which might be provided in the pole-tips 16,
REFERENCES CITED
will now be readily apparent. These saturation
producing pole-‘tip openings 20 are added for the
sole purpose of reducing the effective radial cross
The following references are of record in the
?le of this patent:
sectional area to circumferentially ?owing leak
age-?ux.
In this manner, I am enabled to ma
terially increase the reluctance of the transient
Number
1,495,969
UNITED STATES PATENTS
Name
Date
Alexanderson _____ __ May 27, 1924