CA1194112A - High sensitivity brush arcing monitor for a dynamoelectric machine - Google Patents

Abstract

HIGH SENSITIVITY BRUSH ARCING MONITOR
FOR A DYNAMOELECTRIC MACHINE
ABSTRACT OF THE DISCLOSURE
High noise immunity instrumentation for monitoring brush arcing in a dynamoelectric machine is disclosed. In a preferred form the instrumentation includes a stabilizing variable threshold discriminator operative in conjunction with a feedback network and a non-linear high frequency amplifier to provide a discriminator threshold which in-creases with increasing levels of background noise so that only the arc indicative portions of the signal are passed. A substantial increase in the signal to noise ratio is obtained. The feedback network includes a relatively long time constant integrator and buffer amplifier.
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 signal from the non-linear high frequency amplifier.
This compensation provides for automatically resetting the alarm comparator. This is achieved by providing a second integrator having a time constant somewhat longer than the first integrator. By appropriately selecting the relative time constants of the first 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 of the brushes.

Description

~ 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.

~ 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 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 ~ 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 ~ 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

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 ~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.

~ 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 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 ~ 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.

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In apparatus for monitoring arcing of brushes in an electrical generator, such apparatus being of the type including means receiving from at least one brush a composite signal comprised of high frequency brush arcing components, low frequency components, noise spikes, and high frequency background noise whose amplitude depends upon generator loading; signal conditioning means removing said low frequency components and attenuating said noise spikes to a preselected amplitude to provide a conditioned signal;
a discriminator network having a signal threshold limiting said conditioned signal to signal components of one polarity above said threshold; an amplifier producing an amplified signal from the conditioned signal from said discriminator;
and a first integrator integrating said amplified signal to produce a first integrated signal indicative of brush arcing; an improvement comprising:
a feedback network including an auxiliary integrator for integrating said amplified signal to provide a feedback bias signal indicative of the background noise contained in said amplified signal, and a buffer amplifier for applying said feedback bias signal to said discriminator, said discriminator being responsive to said feedback bias signal to vary said threshold level up or down as a function of said signal.

2. The apparatus of claim 1 wherein said amplifier comprises a high frequency non-linear amplifier having higher gain at low levels of said conditioned signal and lower gain at high levels of said conditioned signal.

3. The apparatus of claim 2 further comprising:
a second integrator responsive to said amplified signal to produce a variable reference signal whose amplitude is indicative of brush arcing, said second integrator having a longer time constant than said first integrator to delay the response of said reference signal relative to said first integrated signal; and a comparator network operable to compare said first integrated signal with said variable reference signal to produce an alarm signal whenever said first integrated signal exceeds said reference signal.

4. The apparatus of claim 3 wherein said discriminator network includes a diode whose bias condition is effected by said feedback bias signal thereby to vary said threshold level

5. The apparatus of claim 4 wherein said buffer amplifier comprises an emitter follower transistor circuit.

6. The apparatus of claim 1, wherein said auxiliary integrator has a time constant which is long relative to the period of the arcing and background noise content of said composite signal.

7. The apparatus of claim 2, wherein said auxiliary integrator has a time constant which is long relative to the period of the arcing and background noise content of said composite signal.

8. The apparatus of claim 5 wherein said auxiliary integrator has a time constant which is long relative to the period of the arcing and background noise content of said composite signal.

9. The apparatus of claim 6 wherein the time constant of said second integrator is about 30 times as long as the time constant of said first integrator.

10. The apparatus of claim 7 wherein the time constant of said second integrator is about 30 times as long as the time constant of said first integrator.

11. The apparatus of claim 8 wherein the time constant of said second integrator is about 30 times as long as the time constant of said first integrator.

12. The apparatus of claim 9 wherein the time constant of said first integrator is about 1 millisecond.

13. The apparatus of claim 10 wherein the time constant of said first integrator is about 1 millisecond.

14. The apparatus of claim 11 wherein the time constant of said first integrator is about 1 millisecond.

15. The apparatus of claim 6 wherein said discriminator network includes temperature compensating means for stabilizing the threshold level against changes in ambient temperature.

16. The apparatus of claim 7 wherein said discriminator network includes temperature compensating means for stabilizing the threshold level against changes in ambient temperature.

17. The apparatus of claim 8 wherein said discriminator network includes temperature compensating means for stabilizing the threshold level against changes in ambient temperature.

18. The apparatus of claim 15, 16 or 17 wherein the high frequency nonlinear amplifier comprises a 3-stage transistor amplifier including an emitter follower first stage, a common emitter second stage determining the non-linear gain characteristics of said amplifier, and a power amplifier third stage.