Note: Descriptions are shown in the official language in which they were submitted.
~ 17GE-29~7
HIGH SENSITIVITY BRUSH ARCING MQNITOR
FOR A DYNAMOELECTRIC MACHINE
Background of the Invention
This invention relates to apparatus for monitoring
arcing of brushes in dynamoelectric machines.
Elec-trically conductive brushes are used in dynamo-
electric machines, such as -turbine-driven generators, to
conduct current to and from collector slip rings or
commutators mounted on the generator rotor. As a result
of brush wear, misalignment, slip ring imprefections, rotor
vibr-ations~ and so forth, arcing may occur be-tween brushes
and the rotating slip ring or commutator surface upon which
the brushes ride during operation. This brush arcing, even
if of low voltage potential, may cause de-terioration of
the slip ring surfaces and may, if undetected and cor-
rective action not quickly taken, lead to excessive arcing
and possible damage to slip rings and brush holder riggings,
forcing generator outages and expensive repairs. Thus,
early detection of brush arcing is of considerable importance~
To achieve early detection of brush arcing, instrumenta-
tion has, over time, been developed to monitor the composite
electrical waveform produced at a brush (or bank of brushes)
during generator operation and to respond to changes in the
waveform which are characteristic of brush arcing. Although
advances have been made which permit the extraction of
arcing information from the composi-te signal even in the
presence of high amplitude noise spikes and in the presence
of a certain amount of noise falling within the same frequency
band as does the arcing portion of the signal, difficulties
are still experienced at times in distinguishing the arcing
cornponent from other, background high frequency noise com-
ponents inherent in the brush signal. For example, it has
proven to be particularly difficult to separa-te the arcing
component from high frequency noise componen-ts which increase
~ 17GE-2987
in magnitude as the electrical load on the generator is
increased. Viewed somewhat differently, the signal to noise
ratio of the composite signal is so low under some generator
operating conditions that the arc indicative portion of the
signal cannot be confidently distinguished from the noise.
In U. S. Patent No. 4,163,227 to Sawada dated July
31, 1979 (which represents a significant advancement in
arc monitors over earlier systems using gating techniques)
signal processing circuitry is disclosed which provides an
enhanced signal to noise ratio by carefully conditioning
the composite brush signal to remove low Erequency com-
ponen-ts and large voltage spikes. Among the signal condition-
ing circuitry disclosed, a discriminator network is included
to limit the processed signal to components of one polarity.
The discriminator establishes a flow threshold level to
ensure that even the lowest arc signal components are passed
to the indicating and alarm sections of the monitor. A
problem arises with this scheme, however, in that the
discriminator's low threshold provides no barrier to the
type of noise, mentioned above, which increases with
generator load and which falls in a frequency band near
that of the arc signal.
Further, an arc monitor as described in the afore-
mentioned U.S. patent 4,163,227, has not been able to
provide high resolution of the arc signal. That is, such
prior art monitors have not been fully capable of dis-
tinguishing a few bursts of arcing from sustained arcing
over a relatively longer time period and have not had the
capability of determining the amount of arcing on a unit
time basis. This capabi]ity of particularly important
since is provides an indication of the condition of the
brushes.
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~ 17GE-2987
Accordingly~ it is the general object of the present
invention to provide improved circuitry for brush monitoriny
instrumentation of the type generally described above, to
thus overcome the shortcomings associated with such prior
art instrumentation, and to proviae a greater capability
for detecting low levels of brush arcing in a dynamoelectric
machine before severe permanent damage is caused.
A more particular object of the invention is to
provide brush arc monitoring circuitry which is able to
distinguish arcing components of a composite brush signal
from inherent noise components particularly of the type
which increase in amplitude and duration as load on the
monitored machine is increased.
A further object of the invention is to provide
sufficient resolution in brush monitoring apparatus so that
shorter, individual bursts of brush arcing can be dif-
ferentiated from more sustained periods of arcing thereby
permitting determinations of arcing on a unit time basis.
Still further objects and advantages of the invention
will be apparent to those of skill in the art from the
ensuring description of the p.rinciples and operation of
the invention and of a preferred embodiment thereof.
Summary of the Invention
Brush arc monitoring apparatus according to the
invention includes, in a preferred embodiment, signal
conditioning circuitry for receiving a composite brush
signal from monitored brushes and for moving from the
signal low frequency components and recurring high
voltage noise spikes; a stabilized variable threshold
discriminator operative in conjunctlon with a feedback
network includiny a relatively long time constant
integrator and buffer amplifier to provide a discriminator
17GE-2987
threshold which increases with increasing levels of back~
ground noise so that only the arc indica-tive portions of
the signal are passed, thus substantially increasing the
signal to noise ra'io; a non-linear high frequency
amplifier receiving the discriminated signal and providing
high gain for low level input signals and very low gain
for high level input signals; and a short time constant
first integrator for integrating the output signal from
the non-linear amplifier to provide a signal indicative
of brush arcing. Further, to provide an alarm upon the
occurrence of brush arcing, an alarm network is provided
which includes a comparator for continuously comparing
the arc indicative signal with a reference signal which
is automatically adjusted to compensate for changes in
the average level of the output Signal from the non-
linear high frequency amplifier. This compensation
provides for automatically quenching (i.e., resetting) the
alarm comparator. Such quenching is achieved by providing
a second integrator having a time constant somewhat longer
than the first integrator. The second integrator output
signal is summed with a fixed reference value to provide
the automatically adjustable reference. By appropriately
selecting the relative time constants of the firs-t and
second integrators, resolution of the alarm network is
narrowed to a unit time basis, thus quantifying the
measurement to provide an indication of the condition, or
quality, of the brushes.
Brief Description of the Drawings_
While the specification concludes with claims
particularly poin-ting out and distinctly claiming the
subject matter of the invention, the invention will be
better understood from the following description taken in
~ 2 17GE-29~7
connection with the accompanying drawings in which:
Fig. 1 is a block diagram of a preferred embodiment
of the invention;
Fig. 2 is a detailed circuit diagram of portions of
the preferred embodiment of Fig. 1;
Fig. 3 is a plot of the gain characteristics of the
nonlinear amplifier of Figs. 1 and 2;
Figs. 4a-4c are illustrations o a conditioned brush
signal comparing the effects of a fixed discriminator
threshold and a variable discriminator threshold, the
latter according to th~ present invention, on that signal;
and
Fig. 5 illustra-tes curves pertaining to the operation
of the comparator and alarm circuitry of Fig. 1.
Detailed Descriptio_ of the Invention
As shown in Fig. 1, a preferred embodiment of the arc
monitoring apparatus includes inputs 22 and 24 for receiving
composite signals from positive and negative brushes 26 and
28, each of which may comprise a polarity of brushes
connected in parallel, and which, during operation, bear in
a conventional manner against rotating collector slip rings
30 and 32 of the generator, the remainder of which is not
shown.
The composite brush signals have been described in
detailed in the above mentioned U.S. Patent No. 4,163,227
to Sawada dated July 31, 1979. Generally, in addition to
an inherent noise component, these signals include high
amplitude recurring voltage spikes from the generator
excitation system, low frequency components, and, iE
arcing is occurring at the brushes, a relatively low level,
high frequency component caused by the arcing. It is
the latter component of the brush composite signal which
~ ~L~ 17GE~2987
is sought to be detec-ted and which may be~ for present
purposes, viewed as the information content of the composite
signal. Thus, all other components of the composite brush
signal are resarded as noise whose presence hinders an
accurate detection of brush arcing. As has been pointed
out in the aforementioned Sawada et al U.S. patent, the
signal to noise ratio is very small in t'ne composite
signal.
In the block diagram of Fig. 1, the composite signals
received at inputs 22 and 24 fxom brushes 26 and 28 are
fed to a dual-channel filter network 34 which removes low
frequency components. The filtered signals then enter
clipping network 36 which also includes two signal channels
and wherein all signal components above a preselected
amplitude are attenuated down to that amplitude. ~ilter
network 34 and clipping network 36 are substantially
identical to the corresponding networks disclosed in the
above-referenced Sawada et al patent. As described there-
in, these networks 34 and 36 provide an enhancement of the
signal to noise ratio in the brush signal.
The conditioned signals from clipping network 36 pass
to a dual-channel stabilized variable threshold dis-
criminator 42 which acts to discriminate against all
portions of the conditioned signals which are below a thres-
hold level. The threshold, although the same for each
channel, is not fixed but is automatically varied up or down
depending on the average level of background noise in the
conditioned signals applied to the discriminator inputs.
Since the arc signal and the background noise are additive,
the high frequency pulses caused by brush arcing ride up
and down superimposed on the background noise wnich is also
relatively high in frequency. The variable -threshold
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~ 2 17GE 2987
effectively tracks the upper levels of the background noise
signal so that all of the arc pulses are passed while
the background noise is discrimina-ted against~ In
addition, means are provided within the discriminator 42
to fully stabilize the threshold against changes which
might otherwise be brought on by chanyes in ambient operating
temperatureO The two discriminator signals, after being
acted upon by the discriminator 42, are brought together
to form a single signal at the discriminator output. The
single of greater magnitude at any instant of time is
passed on to the non-linear amplifier 49.
The variable threshold for discriminator 42 is
established by feedback network 43 which includes an
auxiliary integrator 45 and a buffer amplifier 47. The
input signal to the feedback network 43 is taken from
non-linear amplifier 49 which has a gain characteristic
such that low input signal levels receive a large am-
plification while high input signal levels receive little
or no amplification. Auxiliary integrator 45 is a peak
integrator having a relatively long time constant and
provides a time varying dc output which is proportional
to the average value of the amplified signal from the non-
linear amplifier 49. The resultan-t time varying signal
is applied via buffer amplifier 47 to discriminator 42
which is adapted to adjust its threshold level up or down
depending on the signal level from the buffer amplifier 47.
The circuitry and operation of discriminator 42, feedback
network 43, and non-linear amplifier 49 are more fully
described herein below.
The output signal from the non-linear amplifier 49,
in addition to being applied to feedback network 43, is
also applied siMultaneously to first integrator 51 and -to a
~ 3 ~ 7GE-2987
reference network 53 which includes second integrator 55
and reEerence generator 57. The output signal from the
first and second integrators, 51 and 55 respectively,
are time varying dc signals indicative of the brush arcing
content of the composite brush signals applied to the input
terminals 22 and 24. IIowever, the time constants of
integrators 51 and 55 are considerably different and are
chosen so that first integrator 51 has a much shorter
time constant than does second integrator 55. Thus, first
integrator 51 is able to respond to short, quick brusts
of brush arcing and provide an output signal accordingly.
Second integrator 55 responds to longer bursts of arcing and
takes a considerably longer time to build up a proportional
dc output signal. It has been found for example that in
monitoring the brushes of large turbine driven generators,
time constants of one and thirty milliseconds for in-
tegrators 51 and 55, respectively, provide very satisfact-
ory results~
The output signal from second integrator 55 is applied
to reference generator 57 wherein the integrated signal i.s
summed with a preselected, fixed signal value so that the
output signal from reference generator 57 represents the
lntegrated signal from integrator 55 but which an elevated
base line. The base line is determined by the preselected,
fixed signal value of reference generator 57. The ou~put
signal from reference generator 5/ is applied as one input
to comparator 59.
The second input to comparator 59 is the short term
integrated signal from first integrator 51. Comparator 59
is operative to compare the magnitudes of -the two input
signals and to activate ala:rm 61 whenever the signal from
fixst integrator 51 is greater than -the si.gnal :Erom
~ 17G~-2987
reference generator 57. When arcing occurs at the brushes
(either brush 26 or brush 2~) first integrator 51 responds
relatively quickly, the first inpu-t to comparator 59
therefore increases faster than does the second input and
alarm 61 is triggered. :~f the brush arcing continues, the
output signal from second integrator ~5 will build up and
the integrated signal, elevated by fixed amount in reference
generator 57 will rise un-til it is substantially equal in
amplitude to the siynal from the first integrator. Thus, wi-th
the two inputs to comparator 59 substantially equal in mag-
nitude, comparator 59 will be reset or quanched and alarm
61 deactivatedO Alarm 61 is an audable/visual alarm to
alert operating personnel that brush arcing is occurring.
The rate at which alarm 61 is triggered on and off is an
indica-tion of the number of arc incidents per unit time.
A feedback amplifier 63 responds to the output of
comparator 61 and feeds back a slgnal to reinforce -the arc
indicative signal appearing at the output of first in-
tegrator 51. This arc indicative signal is applied in
parallel to an arc indicator 65 (which may simply be an
analog type meter) and to an output amplifier 67 which
produces an appropriate signal for recording or for serving
as an input to a computer. For example, the output signal
from output amplifier 67 ma~ be a conventional 4-20 milliamp
signal.
To permit testing of most of the circuitry described
above without actually inducing arcing of brushes 26 and
2~, noise generator 69 is provided. When coupled to
discriminator 42 by the closure of switch 71, noise
gene~ator 69 applies high frequency signals similar -to
low level brush arcing to discriminator 42 and, provided
the arc detection circuitr~ is functioning properly, will
~ `3~ 17GE-2987
result ln actuation of alarm 61 and arc indica-tor 65.
The noise generator 69 is in all respects equivalent to
the corresponding noise generator in the above referenced
U~S. Patent 4,163,227.
Referring now to Fig. 2, one of the conditioned brush
signals from the clipping network 36 (of Fig. 1) is applied
through coupling capacitor 80 to one channel of the stabili-
zed variable threshold discriminator ~2 while the other
conditioned signal is applied through coupling capacitor
82 to the second discriminator channel. Since both
channels of the discriminator are substantially identical,
only one will be described in detail. Thus the dis-
criminator network receiving a signal through capacitor
80 includes variable resistor 84; fixed resistors 86, 87,
and 88; temperature dependent resistor 89; and diode 90.
The resistance values of the fixed resistors 86-88 in
combination with the resistance setting of variable resistor
84 and the resistance of temperature dependent resistor 89
(at any given temperature) are selected such that the diode
90 is normally slightly forwaxd biased when no input
signal is present. That is, under ~ero signal conditions
the anode end of diode 90 is slightly more positive then
the cathode end. This forward bias condition ensures that
even very small positive siqnals will be conducted through
the diode 90 while all negative going components are
totally blocked. The forward biasing of diode 90 establishes
a signal threshold which is ordinarily very low. If the
background noise in the conditioned brush signal increases,
however, the threshold is automatically raised by a feed-
back signal which acts -to reduce the forward bias on diode
90 is proportion to the average valwe of the background noise.
Raisiny the threshold by an appropria-te amount causes the
-- 1 0 --
~ 17GE~2987
discriminator 42 to block the noise while passing -the
arcing content of the signal. The feedback signal which
regulates the discriminator threshold is applied to the
upper channel of the discriminator 42 through resistor 92
and to the lower channel through resistor 94. Operation
of the discriminator 42 and generation of the feedback
signal will be fully described herein below.
It may be noted at this point, however, that -the
discriminator 42 is also stabilized against the effects
of changes in ambient temperature. Temperature dependent
resistor 89 has a positive temperature coefficient of about
0.7 ohms/C over an ambient temperature range of about
-10C to -~ 150C. In the embodiment of the brush monitoring
apparatus o. Fig. 2, temperature dependent resistor 89 is
operable to apply a temperature varying reverse bias to
compensate for the inherent change in the threshold
characteristics of diode 90 of about 2MV/C. Thus, although
the discriminatox threshold varies under the influence of a
feedback signal, it remains unaffected by changes in ambient
temperature.
The output signals of the discriminator 42 pass by way
of coupling capacitor 96 and 98 to the input of non-linear
amplifier 49. The signals are combined into one signal at
junction 100 with the non-l~near amplifier responding to the
signal of greater instantaneous magnitude.
Non-linear amplifier 49 includes an input -transistor
stage 102 configured as an emitter follower having bias
resistors 104 and 106, and emitter resistor 108. The
emitter ~ollower serves as a buffer input stage between
the discriminator 42 and the amplifying stages of tlle non-
linear amplifier 49. The discrimninated signal is coupled
from the emitter resistor 108 through capacitor 110 into a
~ 17GE-2987
comm.on emitter stage including transistor 112, load resistor
114, bias resistors 116, 117, and 118, emitter resistor 120,
and bypass capacitor 122. The amplifier signal from tran-
sistor 112 is passed via coupling capacitor 124 to a
conventional Darlington type power amplifier including
transistors 126 and 128, and fixed resistors 130, 131, and
132. The amplified output signal is taken from across
emitter resistor 132.
The non-linear amplifier 49, and other parts of the
circuitry comprising the arc monitoring apparatus, are
supplied with operating power from dc power sources (not
illustrated) connected at +V and -V , each referenced to
a common grounding point. For example, plus and minus 15
volt sources may be used in the circuitry of Fig. 2.
Fig. 3 illustrates dynamic gain and output charac-
teristic cur-ves for non-linear amplifier 49. As can be
seen from the curves, gain and output are functions of the
input signal level. For low levels of input signal the
gain is high while the magnitude of the output signal is
relatively low. Conversely, at higher levels of the
input signal, gain is low while the output signal remains
relatively high. As will be apparent from the ensuring
discussion, the characteristics of the non-linear am-
plifier ~9 establishes an input signal level which is
determinative of the feedback signal to the discriminator
42 and which, therefore, has an effeet on the variable
threshold level of discriminator 42.
The characteristic curves of Fig. 3 are established
in a known-manner by selecting the operating condi-tions
for the common emitter stage incorporating transistor
112 as shown in Fig. 2. In one form of the invention,
satisfactory results have been obtairled by using a PNP
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17GE-2987
2N 2905 transistor for transist-or 112, ma~ing resistor 117
a lOOK ohm adjustable resistor, fixing resistor 116 at 56K
ohms, bias resistor 118 at 6.2K ohms, collector load
resistor 114 at 3.6K ohms,emitter resistor 120 at 3G~ ohms,
and bypass capacitor 122 at 5600 picofarads~ Power is
supplied at -15 volts dc. A wide range of non-linear
gain characteristics is obtained by varying resistor 117.
With further reference to Fig. 2, the output signal
from non linear amplifier 49 is passed simultaneously
through coupling capacitors 136, 137, and 138 to, re-
spectively, feedbac~ network 43 including auxiliary
integrator 45 and huffer amplifier 47, first integrator
51, and seeond integrator 55. Each integrator, 45, 51, and
55 functions as a peak integrator in a manner similar to a
filtered half-wave rectifier to produce a time varying dc
output signal whose amplitude is proportional to the peak
amplitude of the signal from non-linear amplifier 49.
Auxiliary i:ntegrator 45 of feedback network 43 is a
relatively long time eonstant integrator comprised of
resistor 140, bypass diode 141, rectifying diode 142,
filter capacitor 143, and adjustable output resistor 144.
The time constant of integrator 145 is determined prineip-
ally by the component values of capacitor 143 and re-
sistor 144 and may, for example, by on the order of ten
seconds or more. This relatively long time constant
prevents the integrator 45 from responding to short
bursts of brush arcing but allows the output signal at
resistor 144 to slowly build up in the face of sustained
arcing or during long periods of background high fre~uency
noise.
The output of auxiliary integrator 45 is applied to
buffer amplifier 47 configured as a emit-ter follower
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~3~ 17 OE-2987
formed from transistor 146 and resistor ]48. The buffer
amplifier 47 provides electrical isolation between auxiliary
integracor 45 and the discriminator 42. Discriminator 42
receives the auxiliary integrator signal through resistors
92 and 94, as discussed above, to effect changes in the
discriminator threshold.
Figs. 4a, 4b, and 4c illustrate the effect of the
variable threshold. Fig. 4a is a typical waveform for the
conditioned brush signal emerging from one channel of the
clipping network 36 of Fig. 1. The signal consis-ts of a
large amount of inheren-t background noise generally in
the form of a triangular wave. This background noise is of
relatively high frequency and low voltage so that it is not
removed by the signal conditioning networks, i.e., by
filter network 34 and clipping network 36. Furthermore,
the background noise is related to the load on the generator
being monitored and, under some generatcr operating con-
ditions may not be present. However, if it is present it
tends to increase in amplitude with generator loading.
The arcing components of the signal consist of high
frequency spikes superimposed on the background noise.
Fig. 4b illustrates the effect on the conditioned signal
of a discriminator network of a type known in the art and
having a fixed threshold level. The fixed threshold is at
some positive amplitude level low enough to ensure capture
of essentially all of the arcing spikes even in the absence
of background noise. As can be seen from Fig. 4b, however,
this also ensures that a portion of the background noise
is passed. Thus, in the absence of brush arcing the
background noise appears as a false indication of arcing.
On the other hand, if arcing is actually present -the
noise tends to obscure the arcing information.
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~ 2 17OE -2987
Fig. 4c illustrates the output of discriminator 42,
according to the present invention, which provides a
variable threshold. The threshold is automatically moved
up or down, depending on the background noise level, so that
substantially all the noise is removed and only the upper-
most arcing spikes remain in the signal. As is illustrated,
the high level of background noise has caused the threshold
to be moved up.
Returning -to Fig. 2, the variable threshold is
established most effectively by combining operation of
the feedback network 43 with operation of the non-linear
amplifier 49. As the background noise component grows in
amplitude the output of the non-linear amplifier 49
increases in magnitude even though it is momentarily
operating in a low gain region. See Fig. 3, for example.
If the background noise signal from the non-linear am-
plifier 49 is sustained for a period of time, it is con-
verted to a proportional time varying dc voltage by
auxiliary integrator 45. This voltage is applied through
buffer amplifier 47 and resistors 92 and 94 to the dis-
criminator 49 wherein the feedbac]c voltage is effective
to decrease the forward bias on the discriminator diode 90.
The discriminator diodes may in fact be reverse biased by
the feedback voltage. Thus, the discriminator threshold
is raised so that only the uppermost part of the signal
is passed as illustrated in Fig. 4c. With only the low
amplitude arcing spikes as an input signal the non-linear
amplifier 49 is returned to operation in the high gain
region of its gain characteristic. It will be realized
of course that all of this action, with the exception of
the integration action, occurs substantially instan-taneously.
The auxiliary integrator, 45 once having built up an outpu-t
17GE-2987
signal continues to hold that signal because of the
integrator's long time cons-tant. Variable resistor 144
ls adjusted under high background noise conditions to achieve
just enough feedback to maintain the non linear amplifier
49 in its high gain region, i.e., between about 1.5 and
3.5 volts on the output curve of ~ig. 3.
The first integrator 51 provides an output signal
indicative of brush arcing :Eor display and alarm purposes
and as illustrated in Fig. 2 includes input resistor 150,
bypass diode 152, rectifying diode 154, filter capacitor
156, and output resistor 158. The second integrator 55
provides an output signal for generating a reference signal
which is compared with the arc indicative signal from
first integrator 51 and includes input resistor 160, by-
pass diode 162, rectifying diode 164, filter capacitor 166,
and output resistor 168. It will be noted that the out-
puts of the first and second integrators 51 and 55 are
of opposite polarity. Further, as noted above, although
both the first integrator 51 and the second integrator
55 respond rather quickly (i.e., with a short time
constant), the second integrator 55 responds relatively
slower.
Other functional circuits of the brush monitoring
apparatus of Fig. 1 (with the exception of output amplifier
67 which is of conventional design), including reference
generator 57, comparator 59, alarm 61, feedback amplifier 63,
and arc indicator 65, are substantially identical to the
corresponding circuits in the above referenced U.S. patent
~Jo. 4,163,227 to Sawada et aI and need not be discussed
in detail herein. However, by giving the firsl and second
integrators, 51 and 55 respec-tively, much shorter response
time then are disclosed in the Sawada et al patent, and by
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~ 17OE--2987
making the relative response times be-tween these in-
tegrators about 30 to 1, it has been discovered that much
higher resolution can be attained with comparator 59,
alarm 61, and a~c indicator 55. Indeed, it has been found
that very short individual bursts of brush arcing can be
detected and resolved.
Fig. 5 may be referred to for an understànding oE
how the resolution time of comparator 59 of Fig. 51 is
determined. Curve 175 represents the time varying dc
voltage from the first integrator 51 which is applied to
one input of the comparator 59 during a burst arciny.
The curve of 175 rises from essentially a zero baseline
fairly rapidly at a rate determined by the time constant
of the first integrator 51. Curve 177 represents the time
varying dc voltage from the reference generator 57 which
is applied to the other input of the comparator 59.
Curve 177 rises at a slower rate from an elevated baseline
El which is a fixed increment provided by reference generator
57 in the manner described in the Sawada et al U.S. patent.
The rate of rise of curve 177 is determined by the time
constant of second integrator 55. At time Tl curve 175
exceeds the magnitude of curve 177 and the comparator 59
is activated. This triggers alarm 61 and, through feed-
back amplifier 63, arc indicator 65. Subsequently, at
time T2, curves 175 and 177 again are of equal magnitude
and the comparator is reset deactivating alarm 161.
It has been found, for example, that by setting
the time constant of first integrator 51 at about one
millisecond and second integrator 55 at about 30 milli-
seconds, brush arcing in large power generators can bedetermined on a unit -time basis. That is, the number of
arcs per minute or second can more readily be determ:ined.
~LlS~ 17GE-2987
This information provides an indicator of the condition of
the brushes which indication was not available with prior
art brush monitoring apparatus.
While the invention has been described in detail with
reference to a specific preferred embodiment, it is under-
stood that various modifications will be apparent to those
skilled in the art of brush monitoring systems. It is
intended to claim all such modifications which fall within
the true spirit and scope of the present invention.
