Brush wear detection and warning system

A wear-indicating brush and warning circuit is adapted to convey an indication that a dynamoelectric machine is in need of maintenance before the brush wears to the point that a rotor contact surface is damaged by contact with electrical connections to the brush. A brush (10) having current carrying pigtail leads (16) embedded therein includes a wear-detecting wire (20) embedded in the brush a short distance (A) beyond the lowest extent of the pigtail leads (16). The wire (20) is insulated over its periphery, including its tip (24), the insulation (26) being abraded by contact with the rotor contact surface (50), so that the wire (20) senses the voltage present thereon. Non-latching warning circuits (66, 66a), with and without the use of an isolating resistor within the dynamoelectric machine to permit the use of a single wire between the machine and the warning circuit to provide a brush wear indication. Also disclosed is a warning circuit for indicating that an input is electrically connected to a voltage, for indicating that a line (20) has contacted a rotor contact surface. The voltage sense may be negative or positive, over a wide range, so that the positioning and number of brushes (10) is irrelevant. The circuit (92) includes an oscillating voltage source (122) and comparison circuits (124, 126, 128) giving a warning indication when an input line is connected to a voltage and is unable to follow the oscillating voltage.

CROSS REFERENCE TO RELATED APPLICATIONS 
The instant application is related to U.S. patent application Ser. No. 
118,944, filed Feb. 6, 1980, entitled "Brush-Wear Detection in 
Dynamoelectric Machines" and to U.S. patent application Ser. No. 118,943, 
filed Feb. 6, 1980, entitled "Voltage Detection for Brush Wear Detector 
System". 
FIELD OF THE INVENTION 
The instant application relates to the field of detecting wear of motor or 
generator brushes to signal the need of brush replacement before damage to 
a rotor contact surface occurs. In particular, the instant application 
relates to a modified brush that may be used in an conventional brush 
holder to provide a signal when brush wear reaches a predetermined amount, 
and a warning circuit for use with such brushes, suitable of use with a 
plurality of dynamoelectric machines. 
BACKGROUND OF THE INVENTION 
Rotating dynamoelectric machinery, such as motors and generators, whether 
utilizing or generating alternating current or direct current, have a 
rotor contact surface, such as a commutator or slip rings, fixed to the 
rotating armature or rotating field, and electrically connected to 
windings of the rotating armature or rotating field. The windings of the 
rotating armature or rotating field are connected through brushes or the 
like which make sliding electrical contact with the rotor contact surface, 
and may be connected to the fixed winding, or the fixed winding may be 
independently connected to an external power source. A fixed magnetic 
field may also be provided by permanent magnets. PG,5 
Brushes are typically made of a carbon particulate, such as graphite, and a 
binder material, and may also include metallic particles. One or more 
holes are made in an end of the brush, to accommodate current carrying 
wires, or pigtails. For higher current operation, angled, intercepting, 
holes are drilled, and a rivet through the brush is installed at their 
intersection. The pigtail is looped through the angled holes and around 
the rivet, and soldered to the rivet. For lower current operation, a hole 
is made in the end of the brush, the pigtail wire is inserted, and 
metallic or other conductive particles are tamped in the hole around the 
pigtail wire to hold it in place. 
In order to provide a sliding contact which does not cause significant wear 
to the rotor contact surface, the brushes are made softer than the rotor 
contact surface, which is often made of a copper alloy. However, the 
conducting wires, or pigtails, are also made of copper alloy, and may 
damage the rotor contact surface after a sufficient amount of the brush 
wears away. In the rivet style brush, the rivet may contact the rotor 
contact surface, causing rapid and severe damage. 
Many attempts have been made to avoid damage to the rotor contact surface, 
by providing a signal when a brush wears to a predetermined point, to 
allow the brush to be moved away from contact with the rotor contact 
surface when it has worn to a predetermined point. All these attempts have 
involved modification of a standard brush holder, requiring disassembly of 
the dynamoelectric machine, and substitution of new brush holding 
structure. U.S. Pat. No. 942,246, issued to Kimble on Dec. 7, 1909, 
entitled "BRUSH HOLDER" discloses a brush holder with a spring member 
which is normally seated in a cavity made in a brush, and is released from 
the cavity to hold the brush away from a commutator when the brush wears 
down to the point near the bottom of a cavity. U.S. Pat. No. 1,295,860, 
issued to Dean, on Mar. 4, 1919, entitled "BRUSH HOLDER", discloses the 
use of a flexible conductor attached to the pressure exerting device, 
rather than embedded in a brush, to conduct current, the conducting member 
being attach to the pressure exerting member, and the pressure exerting 
member being equipped with extention which contacted the brush box to 
prevent the flexible conductor from contacting the brush box. U.S. Pat. 
No. 2,193,172, issued to Hills, on Mar. 12, 1940, entitled "SAFETY DEVICE 
FOR GENERATORS", discloses the use of a bracket attached to a 
cantilevered, pivoting brush holder, the bracket opening a leaf-type 
switch when the brush wears to a predetermined point, the leaf switch 
members being formed of different types of metal so as to also open when a 
generator overheats. U.S. Pat. No. 2,691,114, issued to Lykins, on Oct. 5, 
1954, entitled "GENERATOR BRUSH WITH CONDUCTION INDICATOR", discloses the 
use of a spring-urged clip bearing on the brush, a resilient contact 
member being provided on, and insulated from, the spring-urged clip, the 
contact member being engagable with an end of the brush holder upon excess 
wear of the brush. U.S. Pat. No. 2,813,208, issued to Ritter, on Nov. 12, 
1957, entitled "ELECTRICAL CONTACT BRUSH", discloses the use of a 
spring-biased plunger in a recess in the body of the brush, the plunger 
being adapted to push the brush away from the commutator surface when the 
brush wears to the bottom of the recess. U.S. Pat. No. 3,523,288, issued 
to Thompson, on Aug. 4, 1970, entitled "BRUSH WEAR INDICATOR", discloses 
the use of a brush having a recess or protuberance thereon, a cantilevered 
resilient arm attached to the brush holder, which either bears a pin which 
falls into a recess in the brush when the brush has worn to a 
predetermined point, or falls off the end of a protuberance on the brush 
when the brush has worn to a predetermined point, operating an electrical 
switch. In this manner, although the lead wire is prevented from touching 
the commutator due to its length, an indication is given to allow 
replacement of brushes before arcing from the brush to commutator can 
damage the commutator. U.S. Pat. No. 3,609,429, issued to Thompson, on 
Sept. 28, 1971, entitled "BRUSH WEAR INDICATOR", discloses the use of a 
pin falling into a recess in the brush when the brush has worn to a 
predetermined point, allowing the contact arm to contact the brush holder, 
and provide a signal that the brush should be replaced. U.S. Pat. No. 
3,898,492, issued to Vassos et al, on Aug. 5, 1975, entitled "CURRENT 
INTERRUPTING BRUSH HOLDER ASSEMBLY", discloses the use of a current 
carrying trip member which is released from an arm on the brush when the 
brush wears down to predetermined length, and also discloses the use of a 
separate contact member sandwiched between a spring and the brush which 
engages stops on the brush holder, allowing the brush to fall away when it 
has worn to a predetermined length, and further discloses the use of a 
wedge-shaped contact sandwiched between the spring and the brush, which 
aligns with a recess in the brush holder when the brush has worn to a 
predetermined length, and is forced out of contact with the brush by the 
spring, stopping the motor. U.S. Pat. No. 4,024,525, issued to Baumgartner 
et al, on May 17, 1977, entitled "BRUSH WEAR INDICATOR", discloses the use 
of a motor brush having an elongated groove formed in one side, and a 
brush box having an insulated probe protruding into the groove. As the 
brush wears to a predetermined point, an end of the groove contacts the 
insulated probe, providing a signal to indicate that the brush has worn. 
The movement of the brush may also be restrained, creating the possibility 
of commutator damage from arcing. Also disclosed is a latching warning 
device connected to the insulated probe. U.S. Pat. No. 4,121,207, issued 
to Jones, on Oct. 17, 1978, entitled "SWITCH FOR INDICATING BRUSH WEAR", 
discloses the use of a conventional roller-arm microswitch adjacent a 
brush box, the roller bearing against a brush through a opening in the 
brush box. The switch is actuated when the brush has worn to the point 
where the roller falls off the end of the brush, actuating the switch, and 
providing an indication that the brush is worn. U.S. Pat. No. 4,172,988, 
issued to Lowther, on Oct. 30, 1979, entitled "BRUSH WEAR INDICATING MEANS 
WITH ENGAGABLE ELECTRICAL CONTACTS", discloses the use of a Z-shaped 
contact element bonded to the top of a brush, the contact element 
contacting an insulated contact provided on the brush holder when the 
brush has worn to a predetermined point. 
The instant invention provides a brush wear indicating means which avoids 
the deficiencies and difficulties of previous attempts to provide a signal 
indicative of brush wear to predetermined length. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a brush with an embedded, 
insulated wear detecting wire, which will contact the rotor contact 
surface, drawing power therefrom to operate a warning device, when the 
brush has worn to a predetermined point, and the insulation has worn from 
the embedded wire by sliding contact with the rotating rotor contact 
surface. 
It is an advantage of the invention that a means of indicating brush wear 
before damage to the rotor contact surface occurs may be provided without 
modification to the brush holding structure of a motor or generator. 
It is a feature of the invention that the insulated wire is insulated both 
around its periphery and at its tip, and is embedded in a brush to 
protrude into the brush a slight distance beyond the end of the pigtail 
wire. 
It is a further object of the invention to provide a warning circuit for 
detecting brush wear in a plurality of dynamoelectric machines, wherein 
each machine may have a single wire between it and the warning circuit, 
and every brush in the dynamoelectric machine may be simultaneously 
monitored for excessive wear. 
It is an advantage of the invention that no devices which are substantially 
affected by temperature need be provided within a dynamoelectric machine 
used in conjunction with the warning circuit. 
It is a feature of the invention that the warning circuit includes an 
oscillator, and compares a oscillating voltage with the voltage on the 
single wear detecting signal wire from each dynamoelectric machine to 
determined when a wear-detecting contact wire in that dynamoelectric 
machine has become connected to a voltage due to excessive wear of a 
brush. 
It is a further object of the invention to provide a system including wear 
detecting brushes and a warning circuit for detecting brush wear in a 
plurality of dynamoelectric machines. 
It is an advantage of the invention that the system provides a warning that 
a brush in a dynamoelectric machine has become excessively worn, and also 
provides a separate indication indicating which of a plurality of motors 
contains a brush that has become excessively worn.

DETAILED DESCRIPTION OF THE INVENTION 
Before referring to the drawings, it should be clearly noted that the 
disclosed embodiments are illustrative only, since the facets of the 
invention shown have application beyond that presented as illustrative. 
For example, a wear-detecting brush may be use in either motors or 
generators, and may be used directly, or with an intermediate circuit, 
with a varity of indicators, including visible indicators, audio 
indicators, and electro-mechanical indicators such as annunciator flags, 
or a combination thereof. Wear-detecting brushes according to the 
invention may also be used in series with the coil of a relay, to stop a 
dynamometer when a dynamoelectric machine undergoing testing on the 
dynamometer has brushes that have reached the end of their useful life, 
without allowing brush wear to proceed to the point where a rotor contact 
surface is damaged. 
The warning circuits illustrated as being connected to a single 
dynamoelectric machine may obviously be connected to more than one 
dynamoelectric machine, using diodes or, preferably, isolating resistors 
between wear detecting wires embedded in like polarity brushes of 
different dynamoelectric machines. The dynamoelectric machines may be 
constant speed or variable speed, the speed control forming no part of the 
invention, and may be series wound, compound wound, permanent magnet, or 
other type of motor or generator. Certain disclosed warning circuits are 
equally applicable to alternating current and direct current 
dynamoelectric machines. It should be particularly noted that the 
disclosed warning circuit which determines when a wear detecting wire has 
made contact with an armature contact surface by comparing the voltage 
present on that lead with a oscillating voltage source has general 
applications for determining when any electrical lead has become connected 
to a source of voltage which is either substantially constant, or which 
does not vary at the same rate as that of the source of oscillating 
voltage, or which varies at the same rate, but with a greater amplitude. 
Referring to the figures, FIGS. 1, 2, 3 and 4 show various embodiments of 
the invention in several configurations of brushes. FIG. 1 shows a brush 
10 of the riveted type. As shown, intersecting holes 12 are made in brush 
10. A loop of pigtail wire 16 is passed through hole 12, around rivet 14, 
and is usually subsequently soldered to rivet 14. Pigtail 16 is usually 
provided with insulating sleeves 18, for mechanical and electrical 
insulation. In accordance with the invention, an insulated sensing wire 20 
is embedded in brush 10, and extends into brush 10 a distance A of 
approximately 0.06 inches (0.15 cm) beyond the lowest extent of pigtail 
16, as measured from surface 22 of brush 10. Wear detecting wire 20 is 
insulated around its periphery, and at its tip 24. Insulation material 26 
is, in the preferred embodiment, a fluorocarbon compound such as Teflon, 
although other high temperature insulations are also usable. The 
insulation used should withstand a temperature of about 200.degree. C. 
As will be apparent, a brush 10 is placed in contact with a rotor contact 
surface, not shown, which rotates beneath it and causes frictional wear on 
brush 10 at rotor contact surface 28. As brush 10 wears, surface 28 moves 
closer to surface 22, and, as wear progresses, surface 28 approaches tip 
24 of sensing wire 20. As wear continues, the friction of the rotor 
contact surface will remove both material from brush 10 and insulation 
from tip 24 of wear detecting wire 20, allowing wire 20 to come into 
contact with the rotor contact surface. At this point, the voltage present 
on the rotor contact surface is also present on wire 20, which was 
previously insulated both from the brush and from the armature contact 
surface. In a brush according to the invention, wear of brush 10 may 
continue for a limited period of time following contact of wire 20 and the 
rotor contact surface, without incurring substantial damage to the rotor 
contact surface. Wire 20 is intentionally made with a soft stranded 
conductive material, such as a soft copper alloy or the like, and its 
diameter is chosen to be as small as practical, so that it can cause at 
most insubstantial damage to the rotor in comparison to the results of 
allowing the pigtail 16 or rivet 22 to contact the rotor. In the preferred 
embodiment, surface 28 of brush 10 may be worn away by the rotor contact 
surface for a distance A after wire 20 makes contact with the rotor 
contact surface before pigtail 16 or rivet 22 could come into contact with 
the rotor contact surface. This is to allow the dynamoelectric machine to 
be operated until the next opportunity for maintenance, and brush 
replacement, of the dynamoelectric machine, rather than causing a sudden 
shut down of the dynamoelectric machine, which is not desirable in most 
applications for dynamoelectric machines. 
FIG. 2 illustrates a brush 10 which is substantially indentical to that 
shown in FIG. 1, except that wire 20 passes through surface 22 of brush 10 
through hole 12 at surface 22. It is possible, although not desirable, 
that wire 20 could be lead into space 30 between the outer diameter of 
pigtail 16 and the inner diameter of hole 12, facing armature contact 
surface 28, since this would give poor practical control over the distance 
A, and could result in an undesirably small allowance for brush wear 
before maintenance of the dynamoelectric machine could be conveniently 
performed. 
FIG. 3 illustrates a brush 10 which is similar to the brush 10 shown in 
FIG. 1, except that it is a tamped-type brush, where pigtails 16 are 
inserted into holes 30 perpendicular to surface 22, and the space between 
pigtail 16 and hole 30 is filled with particulate material 23 which is 
tamped into hole 30 to retain pigtail 16 to brush 10. 
FIG. 4 is similar to FIG. 3 and illustrates a tamped-type brush having only 
a single pigtail 16, with wear detecting wire 20 being located in the same 
general location as a second pigtail lead shown in FIG. 3. As would be 
apparent to one skilled in the art, the wear detecting wire should be 
positioned to avoid substantial weakening of the brush, although the brush 
does not require substantial strength in the area of sensing wire 20 as 
shown, and allow room for a resilient means to urge brush 10 into contact 
with an armature contact surface. In FIG. 4, central area 22a of brush top 
surface 22 is pressed upon by a spring or the like, not shown, to urge 
brush 10 against an armature contact surface, so that the preferred 
location for sensing wire 20 is as illustrated in FIG. 4. 
Sensing wire 20 is preferably embedded in a brush in an aperture, not 
shown, which closely receives insulation 26 of sensing wire 20, and a high 
temperature glue is used to retain sensing wire 20 in brush 10. The 
retention of wire 20 in brush 10 may be improved by chemically etching 
insulation 26 of wire 20 before it is embedded in brush 10. 
FIG. 5 illustrates a brush wear detection system including a non-reversing 
series field dynamoelectric machine. As stated above, the type of 
dynamoelectric machine is not critical, and is not part of the invention. 
As shown, a rotor contact surface 50a is provided with a set of brushes 10 
identified as 10a, 10b, 10c and 10d for convenience, and provided with 
wear detecting wires identified as 20a, 20b, 20c, and 20d. As shown in 
FIG. 5, brushes 10a and 10b are arranged in a first polarity about the 
rotor contact surface, and brushes 10c and 10d are arranged in a second 
polarity. Wear detecting wires 20a, 20b, 20c, and 20d are embedded in 
brushes 10a, 10b, 10c, and 10d, respectively. A power supply or load 52 
supplies power to the dynamoelectric machine having rotor contact surface 
50a through series field F1 and brushes 10a and 10b, which are 
electrically joined together by wire 54. The return path to power supply 
or load 52 is from brushes 10c and 10d, which are joined together by wire 
54. Obviously, if the dynamoelectric machine having rotor contact surface 
50a is a generator, power supply or load 52 serves as a load, and current 
flows from, rather than to brush 10a and 10b, using conventional 
terminology. In the system illustrated, a wire 56 electrically connects 
power supply or load 52 to field winding F1, which is connected in turn to 
brushes 10a and 10b through wire 58, which is also connected to wire 54. A 
wire 60 is connected between power supply or load 52 and brushes 10c and 
10d. 
Wires 62 and 64 are electrically connected to brushes 10a and 10b, and 10c 
and 10d, respectively, to supply power to a warning circuit 66. Warning 
circuit 66 has two branches in parallel between wires 62 and 64, the first 
branch being the series combination of the emitter and collector of 
transistor Q101, resistor R101, and Zener diode Z101, the second branch 
being the series combination of resistor R102, the collector and emitter 
or transistor Q102, resistor R103, and light emitting diode LED101. The 
base of transistor Q102 is connected to the junction between resistor R101 
and Zener diode Z101, and also to a first terminal of resistor R104. The 
base of transistor Q101 is connected to a first terminal of resistor R105. 
The opposite terminal of resistor R104 is connected to wear detecting 
wires 20c and 20d. Until a brush 10 wears to a critical wear point 
determined by the protrusion of a detecting wire 20 into a brush, lines 
20a, 20b, 20c, and 20d will have no fixed voltage but rather will be 
"floating". Should a brush wear detecting wire 20a or 20b become connected 
to rotor contact surface 50a due to wear of brush 10a l or 10b, there will 
be, in the embodiment illustrated in FIG. 5, a positive voltage on wires 
20a or 20b, causing a current to flow through resistor R104 into the base 
of transistor Q102, turning transistor Q102 on. Current then flows from 
wire 62 through resistor R102, the collector and emitter of transistor 
Q102, resistor R103, and light emitting diode LED101, which gives a visual 
indication that a brush has reached the end of its useful life. Obviously, 
light emitting diode LED101 could be replaced with a commercially 
available audio indicator. Should a brush wear detecting wire 20c or 20d 
become connected to rotor contact surface 50b by reason of frictional wear 
to brushes 10c or 10d, the voltage appearing on lines 20c or 20d will 
cause a current to flow through resistor R105, from the base of transistor 
Q101, turning transistor Q101 on. Current then flows from line 62 through 
the emitter and collector of transistor Q101, resistor R101, to the base 
of transistor Q102, turning transistor Q102 on and illuminating light 
emitting diode LED101. It will be apparent from the disclosure herein that 
this warning circuit 66 could be used for a plurality of dynamoelectric 
machines, although it would not be able to identify which of the machines 
required maintenance, by putting isolation resistors between sensing wires 
20 connected to brushes 10 of opposite polarity or differing voltage. It 
will also be obvious that, while all brushes of a dynamoelectric machine 
may be monitored for excessive wear, fewer brushes may be monitored using 
the disclosed invention. Further it will be obvious that the 
dynamoelectric machine involved in the system of FIG. 5 may be either an 
alternating current or direct current machine, of any voltage, with 
appropriate selection of resistors R104 and R105 and Zener diode Z101 and 
the addition of appropriate rectifying or blocking diodes. Resistors R104 
and R105 are selected to restrict the current provided by sensing wires 
20a and 20b, or 20c or 20d respectively, to that necessary to cause 
saturation of transistors Q102 and Q101, respectively. Zener diode Z101 is 
selected to limit the voltage at the junction of resistor R104 and 
transistor Q102 to limit the current through transistor Q102 and light 
emitting diode LED101, or whatever warning device may be used in its 
place. 
FIG. 6 discloses a system similar to that shown in FIG. 5, with a different 
type of dynamoelectric machine, shown for illustration only, and with 
warning circuit appropriate for use with a reversible dynamoelectric 
machine. In FIG. 6 diodes D201, D202, D203, D204 form a fullwave retifier 
for supplying power to a warning circuit similar to that shown in FIG. 5, 
with equivalent components. Resistors R201, R202, R203, R204 and R205 are 
equivalent to resistors R101, R102, R103, R104, and R105 in FIG. 5. 
Transistors Q210 and Q202 are equivalent to transistors Q101 and Q102 in 
FIG. 5, respectively and zener diodes Z210 and light emitting diode LED210 
are equivalent to zener diode Z101 and light emitting diode LED101 in FIG. 
5. The dynamoelectric machine having a rotor contact surface 50b and 
compound field winding F2 is provided with a plurality of brushes 10, 
designated 10e, 10f, 10g, and 10h, provided with wear sensing lines 20e, 
20f, 20g, and 20h. Brushes 10e and 10f, in contact with rotor contact 
surface 50a, are connected to a first terminal of reversing switch 68 
through wires 70. Diodes D202 and D204 are also connected to brushes 10 
through wires 72. Brushes 10g and 10h are connected to a second terminal 
of reversing switch 68 through wires 74. Wires 76 connect diodes D201 and 
D203 through brushes 10g and 10h and the second terminal of reversing 
switch 68. Wires 62a and 54a, equivalent to wires 62 and 64 in FIG. 5, 
respectively, are connected to the junctions of diodes D203 and D204 and 
the junction of diodes D201 and D202, respectively and supply power to the 
warning circuit 66a. The power supply 78 supplies power to field winding 
F2 and to reversing switch 68 through wires 80 and 82. Wear detecting 
wires 20e and 20f are joined together at a point 84. Wear detecting wires 
20g and 20h are joined together at a point 86. An isolating resistor R206 
is connected between point 86 and point 84. Resistor R206 insures that, 
should brushes connected in opposing polarity groups reach the critical 
wear point at the same time, wear detecting wires 20e, 20f, 20g, and 20h 
will not serve to short opposing groups of brushes. Point 86 is connected 
to the cathode of diode D205. The anode of diode D205 is connected to 
resistor R205. Point 86 is also connected to the cathode of a diode D206. 
The cathode of diode D206 is connected to resistor R204. Therefore, a 
positive voltage appearing at point 86 due to a sensing wire 20e, 20f, 
20g, or 20h in a brush 10e, 10f, 10g, or 10h which is connected to a 
positive portion of rotor contact surface 50b, as determined by the 
position of the reversing switch 68, will cause a current to flow through 
diode D206 and resistor R204, turning transistor Q202 on and illuminating 
light emitting diode LED201. A negative voltage appearing at point 86 will 
cause current to flow through diode D205 and resistor R205, turning 
transistor Q201 on, thereby turning transistor Q202 on, and illuminating 
light emitting diode LED210. As before, other devices can be substituted 
for light emitting diode LED210 and any number of brushes 10, from a 
single brush 10 to all brushes 10 included in a dynamoelectric machine, 
maybe provided with a wear detecting wire 20, and the dynamoelectric 
machine may be either an alternating current machine or direct current 
machine, either a motor or generator, since diodes D201, D202, D203, D204, 
D205, and D206 insure that transistors Q210, Q202, zener diode Z201, and 
light emitting diode LED210 of warning circuit 66a operate with a correct 
voltage polarity. The use of isolating resistor R206 allows the use of a 
single wire 88 between a dynamoelectric machine and warning circuit 66a, 
with resistor R206 placed inside the motor, and all temperature sensitive 
components placed outside the dynamoelectric machine in a warning circuit 
66a. However, a less desirable alternate circuit is also illustrated. 
Resistor R206 could be removed by disconnecting it at point B and C, the 
junction between the cathode of diode D205 and the anode of diode D206 
broken at point D, and a diode D207 connected with its anode connected to 
point 86 and its cathode connected to the cathode of diode D206. A diode 
D208 maybe added, with its cathode connected to the anode of diode D206 
and to point 84, and its anode connected to the anode of diode D205. These 
connections are shown in broken lines. The use of these alternate 
connections, shown in dotted lines, results in the addition of two diodes 
and the necessity of bringing two sensing leads, 88 and 90, from the 
dynamoelectric machine having armature contact surface 50a to warning 
circuit 66a. 
FIG. 7 illustrates a preferred system emboding the invention. One 
application of the invention is for industrical lift trucks, which have a 
plurality of electric motors, although the invention is also usable with a 
plurality of generators, and either alternating current machines or direct 
current machines. In a conventional lift truck, there maybe one or more 
motors driving hydraulic pumps for operation of the hydraulic lifting and 
tilting cylinders, and for driving a power steering pump, as well as one 
or more motors supplying motive power to the industrial lift truck. FIG. 7 
illustrates, in schematic form, a system including three dynamoelectric 
machines having rotor contact surfaces 50c, 50d, and 50e, connected to a 
preferred embodiment of a warning circuit 92. A dynamoelectric machine 
having rotor contact surface 50c is provided with a field winding F3, 
which is electrically reversible by means of relay contacts K1, K2, K3, 
and K4. As can be seen, field F3 has a first magnetic polarity when 
contact K3 and K4 are closed, and second magnetic polarity when contacts 
K1 and K2 are closed, and K3 and K4 are opened. In the illustrative case 
of a dynamoelectric machine having a rotor contact surface 50c, current 
would flow from a power supply terminal 94, through wire 96, contacts K4 
and K3 to brushes 10m and 10n through wire 98, and return from bushes 10k 
and 10L through wires 100 to a power supply terminal 102. Assuming the 
opposite direction of rotation for the illustrative dynamoelectric machine 
having rotor surface 50c, current would flow from power supply terminal 94 
through contact K1, field winding F3, and contact K2 to brushes 10m and 
10n, and return to power supply terminal 102 from brushes 10k and 10L. 
Brushes 10k, 10L, 10m and 10n are provided with sensing wires 20k, 20L, 
20m, and 20n, respectively. Sensing wires 20k and 20L are joined together 
at a point 104. Sensing wires 20m and 20n are joined together at a first 
terminal of isolating resistor R301, the other terminal of isolating 
resistor R301 being connected to point 104. A wire 106 is connected from 
point 104 to warning circuit 92. 
The dyanmoelectric machine having rotor contact surface 50d is provided 
with a field winding F4, and is controlled by a relay contact K5. Power 
flows from power supply terminal 96 through contact K5, field winding F4, 
wire 108, to brushes 10p and 10r, and returns to the second power supply 
terminal 102 from brushes 10s and 10t, through wires 110. Brushes 10p, 
10r, 10s, and 10t, are provided with wear detecting wires 20p, 20r, 20s, 
and 20t, respectively. Wear detecting wires 20p and 20r are joined 
together at a point 108. Sensing wires 20s and 20t are joined together at 
first terminal of isolating resistor R302, the second terminal of resistor 
R302 being connected to point 112. Point 112 is connected to warning 
circuit 92 through a single wire 114. 
The dynamoelectric machine having armature contact surface 50e is 
illustrated as being identical to the dynamoelectric machine having 
armature contact surface 50d. It should be noted that all illustrated 
machines may be variable in speed, means for varying speed forming no part 
of the invention. In the case of the dynamoelectric machine having 
armature contact surface 50e, current flows from power supply terminal 94 
through contact K6, field winding F5, and through wire 116 to brushes 10u 
and 10v, and returns from brushes 10w and 10x through wire 110 to power 
supply terminal 102. Brushes 10u, 10v, 10w, 10x, are provided with wear 
detecting wires 20u, 20v, 20w, and 20x, respectively. Wear detecting wires 
20u and 20v are joined at a point 118. Wear detecting wires 20w and 20x 
are joined at a first terminal of a isolating resistor R303. R303 has a 
second terminal connected to point 118. A wire 120 is connected between 
point 118 in the dynamoelectric machine to warning circuit 92. Obviously, 
resistors R301, R302 and R304 may be removed from the dynamoelectric 
machines and placed in warning circuit 92, if it is desireable to bring 
two wires 106, 114 or 120 from each machine. 
Warning circuit 92 operates by sensing when one of lines 106, 114, or 120, 
connected in turn to one or more wear detecting wires 20 embedded in 
brushes 10, is connected to a voltage due to contact with an rotor contact 
surface. Since sensing wires 20 are insulated, until insulation 26 is worn 
away by contact with the armature contact surface, no fixed voltage is 
present on sensing wires 20. Therefore, sensing wires 20 are said to be 
floating. As will be described below, warning circuit 92 operates by 
generating an internal sweep or oscillating voltage, and comparing the 
voltage present on a line 106, 114, or 120 with the oscillating voltage. 
As long as no brush 10 has reached the critical wear point, connecting a 
wear detecting wires to rotor contact surface, wear detecting wires 20 
have theoretically infinite impendance, and will follow the voltage 
generated by the oscillator, and the comparitor will indicate no 
difference between the two. As soon as a sensing wire 20 makes contact 
with a rotor contact surface, the voltage on that wear detecting wire 20 
can no longer follow that generated by the oscillator, and the comparitor 
indicates this condition. In the preferred embodiment of warning circuit 
92, an oscillator 122 is connected to a plurality of comparitors, shown as 
three comparitors 124, 126 and 128. Latching or memory circuits 130, 132, 
and 134 are connected between warning devices drivers 136, 138 and 140, 
respectively. Warning devices 142, 144, and 146 are connected to warning 
device drivers 136, 138 and 140, respectively. A master warning device 148 
is connected to warning devices 142, 144, and 146. In this manner, each 
dynamoelectric machine is provided with a indication that it requires 
maintenance in the near future, and an operator is advised, by means of a 
master warning device, that one of the dynamoelectric machines will 
require maintenance in the immediate future. The master warning device 
should be located within the field of view of a operator, although the 
individual warning devices 142, 144, and 146 may be located in a separate 
location, preferably in proximity to an associated dynamoelectric machine. 
Comparing FIG. 7 with FIGS. 5 and 6 it will be noted that FIG. 7 includes 
memory or latching devices 130, 132, and 134. In light of this disclosure, 
it will know be apparent to one skilled in the art that latching devices 
130, 132, 134 are not strictly necessary to practice the invention, since 
a non-latching warning signal such as provided by the system shown in 
FIGS. 5 and 6 may be felt to be more effective in certain applications, 
due to the flickering or pulsation of a warning device as a wear detecting 
wire 20 begins to make firm contact with a rotor contact surface as a 
brush 10 wears. It will also be apparent that warning device drivers 136, 
138 and 140 are not strictly necessary to practice the invention, since 
latching devices 130, 132, and 134 or comparitors 124, 126 and 128 may 
have adequate capacity to directly energize warning devices 142, 144, 146, 
and 148. 
FIGS. 8, 8a and 8b illustrates an actual embodiment of the preferred form 
of warning circuit 92 shown in FIG. 7. In FIGS. 8, 8a and 8b logic gates 
IC1 and IC2, and associated components, form an oscillator such as 
oscillator 122 shown in FIG. 7. Logic gates IC3 and IC4 form a latch 
circuit such as shown as latching device 130 in FIG. 7. Similarly, logic 
gates IC3a and IC4a, and IC3b and IC4b, form latching circuits such as 
shown as latching devices 132 and 134, respectively, in FIG. 7. 
Transistors Q301 and Q302, and associated components, form a comparator 
such as shown as comparator 124 in FIG. 7. Similarly, transistors Q301a 
and Q302a, and transistors Q301b and Q302b, and their associated 
components, form comparators shown as comparators 126 and 128, 
respectively, in FIG. 7. Similarly, transistors Q303, Q303a, and Q303b, 
and associated components, form warning device drivers shown as 136, 138, 
and 140 in FIG. 7. Light emitting diode LED301 is illustrative of a master 
warning device as shown as 148 in FIG. 7, while light emitting diodes 
LED302, LED302a and LED302b are illustrative of warning devices shown as 
142, 144, and 146 in FIG. 7. It should be noted that preferred embodiment 
uses CMOS logic gates, such logic being relatively immune to being 
affected by random noise signals. Other types of logic, either integrated 
circuit or discrete devices may be substituted without varying from the 
scope of the invention. Due to the use of this type of logic, and its 
advantages, a separate warning device driver, such as transistor Q303, 
Q303a, or Q303b is required. Obviously, substitution of components having 
similar characteristics but higher output capacity would eliminate the 
need for a warning device driver such as transistors Q303, Q303a, or 
Q303b. It will also be apparent from inspection of FIGS. 8, 8a and 8b that 
the illustrated warning circuit is modular in form, illustrated as having 
three substantially identical warning circuits connected to three input 
lines, each intended to provide a warning based input lines, each intended 
to provide a warning based on the condition of one or more among a 
plurality of bruhses reaching a predetermined condition, such as critical 
wear. Of course, the utility of the circuit is not limited to sensing of 
signals from the devices disclosed. 
As illustrated in FIGS. 7, 8, 8a and 8b, the three illustrated identical 
comparator circuits are connected to a plurality of brushes in each of 
three dynamoelectric machines. Obviously, a single comparator could be 
connected to all brushes in the three illustrated dynamoelectric machines, 
with appropriate isolation resistors. 
As disclosed, it is immaterial to the circuits disclosed in FIGS. 8, 8a and 
8b what is the exact voltage present on a input lead such as 106, 114, 
120, as long as it does not produce a current which would be injurious to 
the solid state components shown, and can be scaled to within a broad 
range of voltages. The comparators are composed of Q301 and Q302 and 
associated components, or 301a and 302a, or Q301b and Q302b, and their 
respective associated components. The comparitors activated whenever there 
is a significant different between the voltage appearing on line 106 or 
114 or 120 and the output of the oscillator. 
An oscillator such as oscillator 122 is composed of logic gates IC1 and 
IC2, resistor R310 and capacitor C301. The output of logic gate IC1, point 
401, is connected to the input of logic gate IC2, point 402, through 
capacitor C301, and to the input of logic gate IC1 and output of logic 
gate IC2, point 400, through capacitor C301 and resistor R401. Therefore, 
output 401 provides positive feedback to input 402 through capacitor C301 
and point 400 provides negative feedback to input point 402 through 
resistor R301, positive feedback being initially predominant. For example, 
at initial power turn on, points 401 and 402 are both at a low voltage 
state, current flowing from the resulting high voltage at point 401 until 
capacitor C301 is charged to a value sufficient to affect input point 402. 
A high voltage at point 402 causes point 400 to become a low voltage, and 
output point 401 to assume a high voltage state. The high voltage at point 
401, acting through capacitor C301, maintains point 402 at a high voltage. 
Then, after a time determined by the R-C time constant of resistor R301 
and capacitor C301, the low voltage at point 400, acting through resistor 
R310, will pull point 402 to a low voltage, forcing point 400 to a high 
voltage, and point 401 to a low voltage. This is the same condition as at 
initial power turn on. This sequence repeats and continues as long as 
power is applied to the circuit, the voltage at point 401 oscillating 
between a high voltage state and a low voltage state at a rate determined 
by the time constant of capacitor 301 in series with resistor R310. 
Capacitor C302, besides serving a function to be described later, also 
serves to limit the rate of transition at point 401 between high voltage 
and low voltage states, capacitor C302 being connected between point 401 
and ground. A wire such as 106, connected to a plurality of brushes in a 
dynamoelectric machine in the illustrated embodiment of the invention, is 
normally unconnected, but may function as an antenna, and detect random 
noise pulses. Wire 106 is connected to a first terminal of resistor R314, 
a second terminal of R304 being connected to a first terminal of capacitor 
C302, which has a second terminal in turn connected to point 401 and 
capacitor C302. Noise pulses appearing on a line such as 106 will be 
attenuated passing through resistor 304, and pass to ground through 
capacitor C303 and capacitor C302. 
Should a brush wear detecting wire connected to line 106 be connected to 
any voltage in excess of that equivalent to the base-emitter diode voltage 
of transistor Q301 or transistor Q302, an indication of critical brush 
wear will be given. Note that the disclosed circuit is capable of 
accepting signals ranging from a very small signal to a signal of much 
greater magnitude, since transistors Q301 and Q302 can accept a 
significantly higher current than required to initially turn either one of 
them on. Should a wear detecting wire 20 in a brush 10 used as a positive 
brush come into contact with a rotor contact surface, and the voltage then 
present on the rotor contact surface being positive, a current will flow 
through resistor R9, and resistor R2, to point 401, but only if point 401, 
which is the output of the oscillator, is at a low voltage level at the 
time. The bases and emitters of transistor Q301 and Q302 are connected 
together across resistor R6, transistor Q301 and Q302 being shown as 
having opposite polarities, transistor Q301 being shown as PNP transistor 
and transistor Q302 being shown as an NPN transistor. A positive voltage 
greater than the base-emitter diode drop of transistor Q301, developed 
across R310, will turn transistor Q301 on, current flowing from an input 
403 of logic gate IC3 through diode D301. The function of the latching 
circuit, which includes logic gates IC3 and IC4 will be discussed below. 
Current flowing through Q301 flows to point 401, which is assumed to be at 
a low voltage state during its oscillation. The voltage at point 403 
having been low, the oscillator output point 401 may change to a high 
voltage level, stopping the flow of current through resistor R304, and 
turning Q301 off without affecting the warning signal. The function of 
diode D301 is to block reverse current through transistor Q301. An 
undesirably high or momentary voltage spike at the base of transistor Q301 
may force current through the collector of Q301, forcing point 403 to a 
high voltage level, and unlatching the latching circuit, after the current 
path through point 401 becomes unavailable as the oscillating output of 
logic gate IC1 forces point 401 to a high voltage level. Diode D301 
prevents this. 
Should a wear detecting wire 20 in a brush 10 used in a negative position 
become connected to an armature contact surface, and to the voltage 
present thereon, due to wear of the brush 10, current will flow to the 
wire 20 through line 106, resistor R304 and resistor R306, but only if 
point 401, the output of logic gate IC1 is at a high voltage level. 
Current flowing through R310, if capable of generating a voltage drop in 
resistor 306 in excess of the voltage drop of the diode formed by the base 
and emitter of Q302, will turn Q302 on, current then flowing from point 
401 through the emitter and collector of Q302, the collector of Q302 being 
connected both to an input 404 of logic gate IC4 and to a first terminal 
of resistor R307. The opposite terminal of resistor R307 in connected to 
ground. Resistor R307 serves as a pulldown resistor for point 404, an 
input to logic gate IC4. 
It should be noted that the collectors of transistors Q301 and Q302 are 
connected separately to logic gates IC3 and IC4 for economical reasons. A 
conventional NOR gate could be used to combine the outputs of transistors 
Q301 and Q302, for a single input to a standard latching device, or 
directly to a warning devide, if desired. 
In summary, the comparitor formed in part by transistors Q301 and 302 
provides a signal to the latch circuit composed of logic gates IC3 and 
IC4, from the collector of transistor Q401, when line 106 carries a 
voltage indicative of critical wear on a positive brush, and point 410, 
the output of oscillator logic gate IC1, is a low voltage level, and also 
provides a signal to the latch circuit from the collector of transistor 
Q302, when line 106 carries a voltage indicative of critical wear of a 
negative brush, and point 401 is at a high voltage level. 
The latching circuit, including logic gates IC3 and IC4, is set to a 
correct initial un-latched condition by means of capacitor C304 and 
resistor R309, forming a power-on-reset circuit. When the warning device 
is operating a positive voltage is applied to a terminal 419, current 
flowing through a resistor R310 connected to the terminal 410, the 
opposite terminal 421 of resistor R310 being connected to cathode of zener 
diode ZD1 and to a terminal of a capacitor C306, the other terminal of 
capacitor 306 and the anode of zener diode ZD1 being connected to ground. 
Zener diode ZD1 functions to stabilize voltage, passing current to ground 
if voltage at terminal 412 exceeds the zener voltage. Capacitor C306 
limits the effect of negative transients at terminal 410. A line 414 is 
connected to terminal 412 and carries power to power the logic gates, and 
is also connected to the power-on-reset circuit formed by capacitor C304 
and resistor R309. When power is initially applied, current flows through 
capacitor C304 and resistor R309, causing a positive voltage at the 
junction 416 between capacitor C304 and resistor R319, the opposite 
terminal of resistor R309 being connected to ground. Junction 416 is 
connected to a input 418 of logic gate IC3. A high voltage level appearing 
at this point causes a low voltage to appear on line 420, connected to the 
output of logic gate IC3, as well as to an input 422 of logic gate IC4, 
thereby causing the output 424 of logic gate IC4 to become a high voltage 
level. This high voltage at point 424 is applied to the base of transistor 
Q303 through a resistor R312, holding transistor Q303 off, and is also 
applied to input 403 of logic gate IC3 through a resistor R314, forcing 
input 403 to a high voltage level, maintaining line 420 at a low voltage 
level, unlatching the latching circuit. As the capacitor C304 charges, the 
voltage at junction 416 will drop to zero but, as can be seen, if the 
latching circuit was latched and warning device driver transistor Q303 was 
on, it will be turned off, and the latching circuit will be unlatched. In 
this manner, turning the warning circuit on and off serves to clear the 
circuit after maintenance of a dynamoelectric machine, as well as to check 
for erroneous indications. 
Then, a low voltage applied to input 403 of logic gate IC3 by transistor 
Q310 in response to a positive voltage appearing on line 106 while point 
401 was at a low voltage will force line 420 to a high voltage state, 
which is applied to input 422 of logic gate IC4, making output 424 of 
logic gate IC4 a low voltage level, which is applied through resistor R312 
to the base of transistor Q303, turning transistor Q303 on. Current then 
flows from line 414 through resistor R316 to the emitter of transistor 
Q303, and from the emitter of transistor Q303 through light emitting 
diodes LED303 and LED301. The illumination of light emitting diode warning 
device LED302 indicated that line 106 has detected a fixed voltage 
indicative of critical brush wear, and the illumination of warning device 
light emitting diode LED310 indicates that one of input lines 106, 114, or 
120 has detected a fixed voltage indicative of critical brush wear. It 
should be noted that a voltage called a fixed voltage is any voltage that 
does not change at substantially the same rate as the voltage at the 
output of oscillator logic gate IC1, or has the same rate and phase but 
greater amplitude. 
A negative voltage appearing on line 106 while point 401 is at a high 
voltage level will provide a high voltage level at input 404 of latching 
circuit logic gate IC4, forcing its output 424 to a low value, and 
energizing a warning device as described above. The low voltage also is 
applied to input 403 of logic gate IC3 through resistor R314, forcing line 
420 and input 422 to a high voltage level, maintaining output 424 at a low 
voltage level, causing the latching circuit to latch with the warning 
devices energized. 
FIGS. 8, 8a and 8b show that the warning circuit 92 shown in FIG. 7 is 
modular in form, having a module 400 and a module 500 which are identical 
to the circuit described above except for the omission of an oscillator 
circuit. A line 430 is connected to point 401, the output of oscillator 
logic gate IC1, and provides an oscillating comparison voltage to module 
400 and 500. A line 432 is connected to junction 416 of the power-on-reset 
circuit described above, and provides this signal to modules 400 and 500. 
A line 434 is connected to the cathode of warning devices, light emitting 
diodes LED302a and LED302b, in modules 400 and 500 to the anode of the 
master warning device, light emitting diode LED310, so that the master 
warning device will give an indication whenever warning device light 
emitting diodes LED302, LED302a or LED302b are illuminated. In all other 
respects, modules 400 and 500 are identical to that discribed and 
explained above, with the omission of an equivalent for capacitor C302, 
whose primary function is to slow the output of oscillator logic gate IC1, 
a noise path for modules 400 and 500 being provided through lines 430 to 
capacitor C302. Although, as indicated, the warning circuit in FIGS. 8, 8a 
and 8b can be expanded indefinitely, only a master section and two modular 
sections are shown. Resistors R301, R304, R306, R307, R309, R310, R312, 
R314, and R316, are equivalent in description and function to resistors 
R301a, R304a, R306a, R307a, R309a, R310a, R312a, R314a, and R316a in 
module 400, and R301b, R304b, R306b, R307b, R309b, R310b, R312b, R314b, 
and R316b in module 500. Likewise, capacitors C301, C302, C303, C304 and 
C306 are the equivalents of capacitors C301a, C303a, C304a, and C306a in 
module 400, and capacitors C301b, C302b, C303b, C304b, and C306b in module 
500. Transistors Q301, Q302, and Q303 are repeated as transistors Q301a, 
Q302a, and Q303 in module 400, and transistors Q301b, Q302b, Q303b in 
module 500. Warning device LED302 is repeated in module 400 as LED302a, 
and in module 500 LED302b. Likewise, input points and lines 403, 404, 418, 
420, 422, and 424 are repeated as 403a, 404a, 418a, 420, 422a and 424a in 
module 400, and as 403b, 404b, 418b, 420b, 422, and 424b in module 500. 
And, as will be apparent, line 114 in module 400, and line 120 in module 
500, are the equivalent of line 106, and are connected to a source, such 
as wear detecting lead in a brush of a dynamoelectric machine, which is 
either an open circuit or connected to a voltage which maybe either 
positive or negative or varying at a rate different or amplitude than that 
established by output 401 of oscillator logic gate IC1. 
Numerous modifications and variations to the embodiments of the invention 
discribed above will be obvious to one skilled in the art, and may be made 
without departing from the scope and spirit of the invention.