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Oct. 15, 1946. ‘
Filed April 14, 1944
Patented Oct. 15, 1946
Lee v.A. Kilgorc, Wilkinsburg, Pa., assignor to
. . ,esti‘ng'house Electric Corporation, East Pitts
burgh, Pa., a corporation of Pennsylvania
Application April 14, 1944, Serial ‘No. 531,039
3 Claims.
(Cl. 172—_-274)
My invention relates to electrical damping
meansconnected to alternating-current dyna
mo-electric machines, for producing positive
reactance-branches l0 and H, one or said
branches, such as I 0, having an inductive re
damping in ‘the machine, over, a ‘predetermined
range of speed of the machine or over a predeter
mined range of frequency of its electric currents.
actance, While the other branch, I I, has a capaci
tive reactance, as indicated by the capacitor 12.
The aforesaid inductive and capacitive reactances
In this way, I provide a means for preventing the
are tuned to a frequency which is slightly higher
self-excited torsional vibration or oscillation
than the constant line-frequency of the terminal
which would otherwise occur, as a result of the
negative damping inherent in the machine, or I
may prevent oscillation as a result of governor
or voltage-regulator action, in a machine which
inherently has only weak positive damping. vMy
invention is applicable both to synchronous mo
tors and generators, and to wound-rotor parallel
connected induction motors having their sec
ondaries paralleled, as well as their primaries.
While at present the most important applica
tions oflmy invention are in connection with poly
lead 8 in which the tuned circuit is serially con~
nected, and at least one of the branches ID or II
of said tuned circuit includes resistance, as indi
cated at IS, in an amount suitable for materially
altering the slope of the damping-torque curve
of the machine.
The effect of this serially-connected tuned cir
cuit, having resistance, and tuned to a frequency
slightly higher than the frequency of the normal
currents traversing that terminal lead of the ma
chine, will be understood by reference to Fig. 3,
phase machines, the invention is not limited 20 which shows the damping torque of the machine,
plotted against the speed of the machine, or the
frequency of the electrical currents traversing
The general object of the invention is to pro
the primary winding of the machine, the scale of
vide a tuned, parallel-resonant circuit .in series
the abscissae being di?erent, according to
with some winding-circuit in which the frequency
is, substantially constant, which would normally
be the primary winding-circuit of the machine,
the tuned-circuit being tuned to .a frequency
slightly higher than the normal operating fre
quency, andincluding resistance in an amount
suitable for materially altering the slope of'the
damping-torque curve of the machine.
With the foregoing and other objects in view,
my invention consists in the systems, combina
tions, apparatus, parts, and methodshereinafter
whether speed or frequency as indicated.
The torques of an alternating-current machine
may be calculated by superimposing thecurrents
and ?uxes produced by the primary and sec
ondary voltages, considering each separately. In
doing this, the circuits must be assumed to be
closed through any externally connected imped
ance or system. .At constant speed, the secondary
voltage or excitation of a synchronous machine
produces a current which reacts with the syn
described and claimed, and illustratedin‘the sac
companying drawing, wherein Figure 1 isa vdia
chronously rotating flux of the primary winding,
to give the steady torque. If the rotor oscillates,
grammatic View of a synchronous polyp-hase
motor or generator with my ‘invention applied
thereto; Fig. 2 is a similar view of two wound
at a small frequency f0, and through a small
angle, about the average position of the rotor,
then currents will appear in the primary winding,
in addition to the current of the line-frequency
secondary polyphase induction motors, having
both their primary windings and their secondary
windings connected in parallel, with my electrical
damping means connected thereto; and Fig. 3 is
a ‘curve-diagram which will be referred to in the
explanation of the invention.
, ,
In Fig. ,1,‘ I have shown a polyphase synchro
nous machine, either a motor or a generator,
having a stator member 4 carrying the polyphase
primary windings, and a rotor member 5 carrying
the direct-current exciting windings ,6, which are
energized through slip rings 1.’ Connected in
series with each of the polyphase terminal leads
8 of the machine, or at least in series with some
of said leads, are parallel-resonant tuned circuits,
~ 1‘, said additional currents being included in the
frequency-range between <f+fo> and (‘f-f0).
These additional induced currents produce com
ponents of ‘damping-torque which may be re
garded as ‘resulting from the e?ect of the sec
ondary oscillations, considered as if they were the
primary of the machine, inducing these currents
of the frequency-range between (f-l-fo) and
(f-fu) in the primary winding, considering the
latter as if it were the secondary winding of an
induction motor operating at standstill.
It‘ the resistance of the primary-winding cir
cuit is constant, considering it now as if it were
each comprising a. plurality of parallel-connected 55 the secondary, then the torque, which will be a
damping-torque, will have a negative slope, or
ondary phases of the two machines being paral
leled together to a common secondary polyphase
terminal 28 which is connected to a secondary
energy-translating means 29 which may be a
resistance or a motor-generator set, or any other
means for absorbing energy from, or feeding
energy to, the secondary windings 26 of the two _
damping-torque just described is plotted against
the speed of the machine. The normal operating
speed of the machine is that which corresponds
to the line-frequency f, as indicated.
including slip rings 21, the corresponding sec
what is known as negative damping, as shown‘
by the curve l5-—-|B in Fig. 3. In this curve, the
At this
speed, the ?uxes of the primary and secondary
are rotating synchronously, with no frequency
difference between them. If, now, torque~varia
machines 2| and 22,
In Fig. 2, the tuned circuits are shown at 30',
tions or speed-oscillations are produced in the 10
but instead of being connected directly in the
machine, if these oscillations are of a low arnpliw
primary leads 3| and 32 of the respective motors
tude, as they certainly will be during the initial
2i and 22, they are connected through differen
stages of their development, then the frequency
connected current-transformers 4| and 42,
diiference J‘o will also be low, not exceeding. say,
15 connected in the corresponding phases of the pri
2 cycles, in a 60-cycle machine.
mary windings of the respective motors. In this
It will be observed, in Fig. 3, that the slope of
manner, no energy is absorbed in the damping
the torque-curve Iii-l6, at the normal operating
circuit 30 when the currents to the two motors
speed or frequency f, is a negative slope, pro
2! and 22 are balanced, as they normally would
ducing negative damping, because a momentary
be when the motors are connected to a common
increase in speed, as indicated at (f+;f0) results in 20 load, or shaft. However, due to the inherent neg
a smaller braking or damping torque, tending to
ative slope of the damping-torque characteristic
feed energy into the oscillation and increase the
of the motors, there would be a tendency for the
amplitude of the momentary increase in speed.
motors to exchange energy between each other,
The effect of my present invention is to intro
resulting in surges of current, in which case the
duce a variable resistance into one of the wind
ings of the machine, preferably the primary wind
ing because the frequency of the primary cur
rents is substantially ?xed, during normal, steady
state operation. The effect of the tuned circuit
is indicated by the dotted curve 11 in Fig. 3,
which, superimposed upon the constant-resist
ance damping-torque curves !5-—-l6, produces a
hump IS in said curve, thus reversing the slope
of the torque-curve IB—IB, so that it has a posi
tive slope l9, at the operating-frequency or speed 35
differentially connected current-transformers 4|
and 42 feed energy into the tuned circuit 30,
which is tuned to a frequency slightly higher than
the norma1 line-frequency, thus introducing a
variable-resistance effect which increases as the
oscillations become greater, thus introducing the
positive damping effect which is indicated at I9 in
Fig. 3.
I claim as my invention:
1. An alternating-current dynamo-electric ma
chine inherently subject to negative damping
I. This is brought about by reason of the fact
tending to cause self-excited oscillations through
that the currents circulating in the tuned circuit
a narrow frequency-range above and below an
Ill-ll (Fig. 1) are quite small, except when the
operating-frequency of a winding of the
frequency approaches the frequency to which the
in combination with electrical damp
tuned circuit is tuned, at which time the magni 40 machine,
ing-means comprising a damped tuned circuit.
tude of the circulating currents in the tuned
and terminal connections for serially connecting
circuit sharply increases, to a maximum at the
said tuned circuit to said winding, said tuned cir
frequency to which the circuit is tuned.
cuit comprising a plurality of parallel-connected
Hence, the resistance of the tuned circuit has
reactance-branches, one of said branches having
practically no effect upon the resistance of the
an inductive reactance, another of said branches
primary-winding circuit in which the tuned cir
having a capacitive reactance, said inductive and
cuit is serially connected, until the frequency of
capacitive reactances being tuned to a frequency
the primary-winding circuit approaches the fre
quency to which the tuned circuit is connected,
and as this tuned-circuit frequently is approached
more and more closely, the energy-losses in the
resistance of the tuned circuit increase steadily,
thus producing the effect of an increasing re
sistance, in the primary circuit, as the frequency
of the primary current increases toward the fre~
quency to which the tuned-circuit is tuned. In
equal to said average operating-frequency, plus
said narrow frequency-range above said average
operating-frequency, plus a small additional in
crement of frequency, and at least one of said
branches including resistance in an amount suit
able for materially altering the slope of the
damping-torque curve of the machine so as to
produce positive damping within said narrow fre
Fig. 3, the tuned-circuit frequency is indicated
at ft, and the relation between resistance and
quency-range above and below said average op
reactance, in the tuned circuit, is such as to cause
the resonance effect of the tuned circuit to be
spread out over a su?iciently broad base so as to
extend to frequencies lower than the minimum
tially constant-frequency alternating-current
dynamo-electric machines having parallel-con
expectable oscillation-frequency (f——-fo) in the
machine whose oscillations are to be damped.
For example, on a 60-cycle system, the resonant
circuit might be tuned to a frequency ft of, say,
63 cycles; or any other suitable value may be
chosen as the exigencies of any particular case
may require.
Fig. 2 shows the application of my invention
to the case of two polyphase induction motors
2| and 22, having primary stator members 23
which are energized in parallel from a common
supply-line 25, and having wound-rotor sec
ondary members 26, having secondary-terminals
2. Electrical damping-means for two substan
nected primary leads, said damping-means com
prising differentially connected current-trans
formers in corresponding primary leads of the
r two machines, and a damped tuned circuit ener
gized from said differentially connected current
transformers, said tuned circuit comprising a. plu
rality of parallel-connected reactanoe-branches,
one of said branches having an inductive react
ance, another of said branches having a capaci
tive reactance, said inductive and capacitive re
actances being tuned to a frequency slightly
higher than the constant line-frequency of the
primary leads, and at least one of said branches
including resistance in an amount suitable for
materially altering the slope of the vdamping
torque curve of the machine so as to produce
positive damping within a, narrow frequency
range above and below said constant line-fre
quency of the primary leads.
3. Two parallel-connected, Wound-secondary
induction-motors as de?ned in claim 2, char
acterized by a, common polyphase secondary
whereby the secondary windings of the two mo
tors are connected in parallel relation to each
other and are both connected to said common
energy-exchanging device.
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