Note: Descriptions are shown in the official language in which they were submitted.
~17415~
METHOD OF AND APPARATUS FOR TESTING AND ADJUSTING
D. C. ROTARY MACHINES
Backaround of the Invention
The inventions shown and described herein relate to
methods of and apparatus for testing and adjusting direct
current rotating machines such as motors and generators.
One of the problems involved in testing and adjusting
D.C. machines is to select the proper positions for the
brushes. Many years ago a brush was shifted back and forth
in an effort to find the best position. As stated in the
text Electrical Machinery, by Fitzergerald and Kingsley,
published by McGraw-Hill Book Co. (1952), at page 241:
"Shifting the brushes was at one time a
common way of securing good commutation. The
method is now obsolete, however."
The heart of a D.C. machine is the armature. All
power conversion energy flows through it, and its
capability to convert mechanical rotation from or to
undirectional currents and magnetic fields is largely
controlled by geometrical factors. This means that the
coupling medium comprised of the air gap magnetic field
should be marshalled into perfectly symmetrical areas
through which energy is allowed to flow (the main poles)
and out of other areas where the commutation process must
take place (the commutation zones).
The armatures of D.C. machines may be lap or wave
wound. The armatures have multiple parallel current paths,
one or more paths per pole for those having simplex or
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multiplex windings. If the air gap magnetic fluxes are
perfectly symmetrical, the parallel currents should have
equal magnitudes, a factor that should contribute to good
commutation. Asymmetrical air gap conditions will result
in unbalanced currents.
Under steady load conditions, the currents that flow
from the positive to the negative pole brushes are
continuous. However, internal to each brush and at the
commutator-to-brush interface the currents are continuously
changing. There are three current components, two of which
must be m;n;m;zed or ideally, completely eliminated for
good commutation. These are described below:
The armature load current is divided proportionately
between the commutator bar areas bridged by the brush. The
shorted coils provide a low resistance bypass such that the
current division between the bars shorted by the brush is
controlled by the brush-to-bar contact resistance, which is
proportional to contact area. Under this condition alone
the rate of change of current across the brush face is
uniform resulting in uniform brush contact temperatures.
Armature parallel path imbalance currents are caused
by asymmetrical air gap conditions and improper positioning
of the brushes. These currents are divided between the
shorted coils and the brush contact areas. Since these
currents must flow from one side of the armature to the
other they result in increased current densities toward the
le~; ng or lagging edges of the brushes.
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Since high current densities cause high contact
temperatures which in turn lower the brush contact
resistance and further increase current densities, the
current imbalances result in sparking and burning of the
brush contact faces and commutator surfaces.
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Summar~ of the Invention
The current flowing to and from the armature W;n~;ng,
of a D.C. rotating machine such as a motor or generator,
produces a magnetomotive force (mmf) that produces a
magnetic field. This mmf varies from the leading and to
the trailing end of a brush. This current flows through
the risers that are adjacent a given brush. The current
flow through the riser produces a magnetic flux adjacent to
the given brush. The risers, taken as a group, form a
cylinder that is coaxial with the machine. A probe that
detects magnetic fields is rotated about the axis of the
machine, at a radius slightly greater than the radius of
the risers. The magnetic fields produced by the currents
flowing through the risers, that are adjacent a given
brush, are sensed by a probe as the probe moves in its arc
around the risers. The probe detects the relative
positions of the magnetic field component of armature
current adjacent the "givenN brush. Thus, if a probe is
rotated around an arc of a radius greater than the radius
of the risers, it can find the angle where the magnetic
field adjacent a given brush, is a maximum. I can then
attempt to set the brush to the same angle around the axis
of the machine as the angle of said maximum magnetic field
reading. This attempt to set the brush at the maximum may
change the angular position of the maximum flux. If so, I
then readjust the angle of the brush, and/or probe several
times, if needed, until the angle of the maximum field is
217415~
the same as the angle of the midpoint between the le~; ng
and trailing ends of the portion of the brush that contacts
the commutator. Such a brush setting will provide optimum
commutation.
To assist in the testing of D.C. machines I have
invented special apparatus as follows. I provide a ring
coaxial with the axis of rotation of the machine. Probes
and other devices on the ring can assist in the making of
tests. With this special apparatus the field at any point,
where knowledge of the magnitude of the field is useful,
can be measured. To get the special apparatus into the
casing, the apparatus enters through one or more holes in
the casing, in disassembled form, and is assembled in the
casing. The special apparatus feeds the information that
it acquires to instruments, outside the casing, which
process, display and record the information.
One purpose of the new method is to test and adjust a
D.C. machine in order to eliminate destructive and other
unwanted armature current components which flow through the
commutator-brush contact areas.
The new method overcomes two specific problems:
1. It is not practical to achieve perfectly
symmetrical air gap flux distributions.
2. The armature circuit is a very high admittance
network having as many current paths as there are poles.
The high parallel admittances make it impractical to
control the current distributions and balances by
conventional methods.
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My new method overcomes the above problems by
correlating air gap field distributions with armature
current distributions and making adjustments which
eliminate the destructive and unwanted current component
flows.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figure 1 is a schematic drawing of a typical D.C.
machine on which the present method may be applied.
Figure 2 is a schematic drawing similar to Figure 1,
except that it has my test apparatus installed therein.
Figure 3 is a front view of the test apparatus as
installed in a machine.
Figure 4 is a schematic view of my test apparatus as
applied to the commutator 22 and riser 23 of a typical D.C.
machine.
Figure 5 is a schematic drawing of two field poles and
an interpole of a d.c. machine.
Figure 6 is a schematic diagram of the circuitry
associated with the two probes 34a and 34b.
Figure 7 is an air gap field mapping mat of a typical
D.C. machine.
Figure 8 is a diagram useful in expl~; n; ng how to set
a brush to its optimum position.
Figure 9 is a block diagram of test equipment.
Figure 10 is a block diagram of another form of test
equipment.
Figure 11 is a block diagram of the laser pointer.
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Figure 12 is a block diagram of a so-called horizontal
probe.
Figure 13 is a block diagram of another form of test
equipment.
DETAI~ED DESCRIPTION
In the practices of my new method, I prefer to use
particular apparatus which I will describe before the
method is described. The method can be performed with
apparatus other than that described below.
I will describe my test apparatus as well as the test
method, as applicable to a 200 HP D.C. motor having either
a wave or lap wound armature. The invention is, however
applicable to any d.c. motor or generator, irrespective of
the type of the armature w;n~l;ng. The term "D.C. machine/'
is generic to both d.c. motors and generators.
Figures 1 and 2 show the frame 20 of the d.c. machine,
the field pole piece 21, the commutator 22, the riser 23,
the brush riggings 24, brushes 25, the portions 26 of the
armature w; n~; ng that connects to the commutator segments
(not shown). It is customary to have one riser 23 for each
commutator segment. The riser 23 electrically connects it
complementary commutator segment to the proper armature
w;n~;ng. The armature 26a is mechanically connected to the
commutator 22 and to the portions 26, so that all of these
parts rotate together.
In order to facilitate my test method, I have added
the parts described in this paragraph, to the D.C. machine.
The expansion clamp 30 is attached to the frame 20 and is
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capable of exp~nd~ng or contracting, under human control,
so as to move support ring 31, positioning ring 32 and
probe 34 toward or away from the axis of the machine. The
probe 34 is mounted in a socket 36 on the positioning ring
32, so that the probe 34 may be inserted into or removed
from the socket.
There are two identical probes 34, namely probes 34a
and 34b, exactly 180- apart as shown in Figure 3. A
permanent magnet 40 may be temporarily bonded to the riser
23. The positioning ring 32 is mounted for 360 degree
angular motion about the axis of rotation of the armature.
This angular motion is effected by motor and gearbox
(Figure 2). The motor and gearbox 33 is controlled by the
controller 92 of Figures 9 and 10. The controller 92 may
start, stop or reverse the angular motion of positioning
ring 32. The apparatus of Figures 9 and 10 is, of course,
something that I have added to the motor per se.
The apparatus 33 may take other forms, such as a
stepping motor or a gearhead motor.
In Figure 6, the circuitry to the left of wires 53, 54
and 55 produces a constant current in wires 53, 54 and 55.
The probes 34a and 34b are devices operating on the
principle of the Hall effect and in each of these probes a
constant current flows through the device, passing into pin
3 and out pin 1. When there is a magnetic field through
such a probe, there is an output current flowing from one
of pins 2 and 4 to the other of those pins. Thus, probes
34a and 34b each have an output related to the density of
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the magnetic field passing across the probe. The output
circuit of probe 34a is hereinafter call ~h~nnel 1 and the
output of probe 34b is hereinafter called Channel 2.
A suitable Hall effect probe 34 is manufactured by F.
W. Bell, Model No. BH-200. A number of other manufacturers
sell a suitable Hall effect generator. Indeed, any so-
called Hall effect generator meeting IEEE Standard No. 296
will work.
The magnitude of the constant current, referred to
above, is determined by potentiometer 50. The amplifier 51
(LLF 356) controls the excitation of transistor 52 (SK
3024) to maintain the current in conductors 53, 54 and 55
constant.
Since the support ring 31 and the positioning ring 32
are not a part of the motor being tested, they must be
added. Accordingly, the support ring 31 and the
positioning ring 32 come in two parts each of which will
extend around the axis of the motor for 180-. One of these
two parts is 31a-32a. The other part is 31b-32b. These
two parts are separately inserted through holes in the
motor casing 20 and assembled in that casing to form a 360
degree support ring 31 and a positioning ring 32 ext~n~;ng
for 360- around the motor axis. See Figure 3. The support
ring 31 is fixed to the casing and supports and guides ring
32 which rotates under the control of controller 92.
One of my tests uses a device (Figure 12) which may be
called a horizontal probe 34c. That probe has a
construction and function similar to the probe 34a of
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Figures 3 and 6. This horizontal probe 34c is mounted on
one end of a horizontal arm 48 that has another end 49 that
fits in socket 36 on the positioning ring 32. The probe
34c extends into the air gap between any field pole piece
and the armature. The probe 34c may be a Hall effect
generator and be identical to probes 34a and 34b.
Moreover, this horizontal arm 48 can be moved back and
forth along lines parallel to the axis of rotation of the
armature. Since this horizontal probe is mounted on the
positioning ring 32 it may be rotated in an angular
direction coucentric to the axis of rotation. Thus, the
horizontal probe may measure the magnetic field density at
any point between any field pole and the armature.
An interpole, also known as a commutating pole, is a
well known part of D.C. machines, for example, see the
St~n~l~rd Handbook for Electrical Engineers Sixth Edition
(1933) published by McGraw-Hill Book Co., Secs. 8-54 to 8-
58.
The interpoles consist of narrow iron cores with coils
which are series connected with the armature circuit and
excited by the machine's load current. The interpoles are
located between main poles of opposite polarity, thus their
name. See interpole 2la of Figure 5, for example.
The interpole neutralizes any residual flux in the
commutating zone and supplies a small reverse flux
sufficient to cancel the inductive effects.
~17~15~
In the description of my tests, as set forth below, it
will be assumed that the D.C. machines under test have
interpoles even though they are not shown.
The laser pointer 62 (Figure 11) is mounted on an arm
61 whose left and 60 includes a plug 63 that fits in socket
36 in place of the probe 34a. The laser pointer 62 emits a
laser beam of very small width (0.5mm) and which will
rotate around the axis of rotation of the armature 26a.
During such rotation the laser beam 64 is directed inwardly
into the machine and will illuminate the side wall of the
poles at points on the poles 21 at or very near their free
ends. This very small beam of light 64 will be visible to
the human eye and will permit the engineer making the test
to see the spot on the poles illuminated by the beam 64.
When the positioning ring 32 rotates about the axis of the
machine the beam 64 forms a cylinder.
The laser pointer 62 (Figure 11) is mounted on arm 61
and has a socket 60 that may be inserted in socket 36 but
points its beam horizontally and at the free edges of the
iron of the field poles 21. The positioning ring 32 may be
rotated so that the beam 64 of the laser pointer defines a
cylinder. The laser pointer 62 plus the digital counter 41
may be used to plot horizontal lines of Figure 7. As the
laser pointer 62 is rotated by positioning ring 32, the
digital counter 41 counts. For example, the vertical line
45 (Figure 7) is the starting position of the laser pointer
62 as it is rotated by ring 32. This starting position has
a zero count on the digital counter 41. As the rotation of
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the laser pointer 62 by the positioning ring 32 progresses
the digital counter 41 counts through the graduation marks
on line 32 including counts 5, 10, 15 etc. As the laser
beam 64 passes a first pole 21 of the D.C. machine it
illuminates one end edge of that pole. The digital counter
41 is then read and let it be assumed that at that point
the count on digital counter 41 is 1Ø Thereupon, the
engineer makes a vertical mark 46 on Figure 7. As the
laser beam 64 advances further it illuminates the edge at
the other end of the first pole and the engineer then
observes the digital counter 41 and notes it is reA~; ng 8.0
and so he draws line 47 to represent said other end of the
first pole 21. Therefore, the space between vertical lines
46 and 47 is span 48. The positioning ring 32 continues to
rotate the laser pointer 62 until that pointer illuminates
one edge of an interpole. The engineer notes that this
occurs at count 9.0 and so he draws line 49 to represent
one end of the first interpole (IP-l). Similarly, the
other end of the first interpole is marked 50. Thus, span
51 represents the first interpole (IP-l).
The process of plotting the map (line 44 ) of the
d.c., machine under test then continues until the entire
360 degrees of the rotation of the positioning ring 32 and
the laser pointer 62 is complete.
After all of the poles and interpoles have been
identified along line 44 of Figure 7, it is now possible to
determine the field strength at any point in the air gaps
between field poles 21 and the armature 26a. This can be
.
- 2174152
done by substituting the horizontal probe 34c for the laser
pointer assembly (Figure 12, 13). The horizontal probe 34c
may be positioned at any desired point in the air gap that
is between the field poles 21 and armature 26 as follows.
The horizontal probe 34c may be moved back and forth
parallel to the axis of the machine. Moreover, the probe
34c may be rotated in a arc by the positioning ring 32.
The location of the probe 34c along the arc may be
determined by reading the digital counter 41. For example,
if the digital counter 41 reads 5, the angular position of
probe 34c would be obtained by drawing a vertical line
through number 5 on Figure 7. This vertical line will
intersect the first pole which covers span 48 of Figure 7.
Thus, the field strength may be measured at any point in
the air gaps between the poles 21 and the armature 26a.
If tests with the probe 34c indicate that the fields
produced by the field poles 21, and/or interpoles 21a, are
imbalanced, correction may be made by well known
conventional procedures.
Figure 8 shows how a brush 25 may be aligned with a
probe 34a, or 34b. Socket 36 holds the probe 34a or 34b
and is moved in a path adjacent to and concentric with rise
23. See Figures 1 and 2. The midpoint of the portion of
the brush 25 which contacts the armature 22 is on a line
parallel to the axis of rotation of the d.c. machine. That
line in turn intersects a straight line through the probe
which intersects the axis of rotation of the machine. If
the angular position where the probe detects the maximum
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magnetic flux does not coincide with the angular position
of the center of the brush footprint on the commutator, the
pointer 81 provides visual indication of such displacement.
Pointer 81 is attached to the socket 36 and moves with
probe 34.
The words "brush footprintn refer to the area of the
commutator that is in contact with the brush.
Figure 9 is the apparatus used for mapping the air gap
flux distributions (as hereinafter explained) and
performing the "second test" hereinafter described. An
external motor M drives the test machine at rated speed.
The exciter 91 energizes the main field 21 at a suitable
excitation current as indicated by ammeter 91a. A well
known conventional controller 92 causes a gear motor drive
33 to move a positioning ring 32 (which carries the probe)
such that the armature riser 23 may be scanned over 360
degrees. The digital counter 41 indicates the angular
position of the probe 34. Cable 37 (figures 3 and 9)
carries channels 1 and 2 (see Figures 6 and 9) signals
emanating from the probes 34 to the instrument amplifier
95. The amplified ch~nnels 1 and 2 signals are transmitted
to the analog oscilloscope 94 and to the personal computer
PC (which has a digital oscilloscope 93). The analog 94
and digital 93 oscilloscopes provide usable indication and
recording of the data gathered by the probes 34.
Figure 10 is the apparatus used for the "third testn
hereinafter described. Similar parts of Figures 9 and 10
have similar reference numbers and they function in the
14
- 2~7415~
manner described for Figure 9. Figure 10 has an additional
part which is a load box 101 used to induce load currents
in the test machine while it is externally driven as a
generator.
Figure 13 is the apparatus used for the "fourth test"
hereinafter described. Figures 9, 10 and 13 have similar
reference numbers for similar apparatus. Each of these
parts of Figures 9, 10 and 13 have the same function and
mode of operation. However, in Figure 13, the electrical
power source 131 drives the test machine as a motor, and
dyn~mom~ter 132 acts as a load for the test machine.
Hereinafter, when I refer to data that represent
values, it is to be understood that these would be the
typical figures that an engineer would expect to use or
find, as the case may be, in tests on the 200 HP D.C.
machine referred to above.
Before my test procedures and adjustments are made the
usual st~n~rd tests on the d.c. machine should be made as
follows:
Visual inspection of insulation
Inspection and measurement of the commutator
Inspection and measurements of the brushes and
brush holders
Insulation resistance measurements
Field wln~;ng resistance measurements
Air gap width measurements
The tests described below evaluate the air gap
magnetic fields and armature current flows, as the
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controlling factors of commutation and brush wear. These
factors are geometrically interrelated, such that
adjustments made to correct conditions indicated by one
test may cause problems in other areas. This means that to
get reliable results, the different tests and adjustments
will need to be performed repeatedly as part of an
iterative process. By this process the variables that need
to be controlled are adjusted and tested, one at a time,
until all variables coverage to within acceptable limits.
This will happen after several iterations have been
completed.
The first test that I will describe is one that
evaluates the field in the air gap. The purpose of this
test is to evaluate the uniformity and symmetry of all air
gap magnetic fields by direct measurement and comparison
methods. This test uses the following equipment previously
described:
a. Oscilloscopes, Digital 93 and Analog 94
b. Instrument amplifier 95
c. Digital Counter 41
The test equipment may be installed inside of the
machine under-test in accordance with the following:
Installation of positioning ring 32:
Preferred installation technique preferably should not
disturb existing electromagnetic configuration of the
machine, except for lifting of brushes.
16
217glS~
Positioning ring 32 allows probe sc~nn;ng of all
relevant magnetic field and current flow distributions with
the machine at rest and operating under load.
The probe cable 35, 37 is able to reel in and out
unobtrusively to allow probe sc~nn;ng of 360 mechanical
degrees.
Positioning ring 32 installation is preferably
adjusted to result in the following:
Probes to function in a plane perpendicular to
armature's rotational axis within test instrument tolerance
limits.
The positioning ring 32 is accurately centered. This
will be accomplished ~;m~n~ionally relative to the
commutator riser 23 and with the help of the
instrumentation system and a permanent magnet 40 affixed to
the commutator riser 23 as a final refinement.
Installation of probes:
The preferred position of the probes 34 permits an
engineer to detect either the air gap field, leakage static
magnetic fields, or mmf's produced by currents flowing
through the armature. Preferred detection of armature
current mmf's will be in close proximity to armature
w; n~; ng connections 26 to the commutator risers 23.
Probe socket 36 shall be positioned such that a probe
can be aligned axially with either the machine's main field
poles, commutating poles or the brush footprint on the
commutator. This shall be assisted by laser pointer 62, a
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mechanical pointer, or other visual aid capable of
m;n;m;zing parallax errors.
The probes shall be oriented to result in
perpendicular magnetic flux line detection at key positions
relative to the static fields and armature current mmf's.
Calibration of instrum~entation channels:
~ h~nnel l shall be designated as a reference and is
bench calibrated for (a) zero offset and (b) a preselected
gain with a specific probe.
Ch~nnel 2 calibration shall be matched to Channel l
while installed in the machine. Calibration matching may
be verified in four position quadrants. If calibration
drifts in the four position quadrants, perpendicularity of
the Hall probe relative to the axis of the rotation must
corrected.
The test procedure is as follows:
a. Lift all of the commutator brushes and install
the test setup as indicated in Figure 9. Install the
positioning ring 32 as shown in Figures 2 to 8. Do not
install the sensing probes 34 and cable 35, 37 at this
time. Center the positioning ring 32 ~;mpn~ionally by
using the edges of the commutator risers 23 as reference
points.
b. Thoroughly inspect the top surface of the
armature 26a inside of the moving ring 32 of the
positioning ring for obstructions (high points) to ensure
adequate clearance for the probes.
18
217415~
c. Thoroughly check the installation of test
components internal to the machine to prevent damage when
the rotor is turned. Rotate the machine with the external
drive to perform the steps that follow. This is required
to cancel residual magnetism effects of the armature 26a.
d. Center the positioning ring 32 by use of the
instrumentation setup as follows:
(1) Bond a permanent magnet 40 to the commutator
risers 26 as shown in figure 4. Note: this step should be
taken with enough lead time to ensure adequate drying or
curing time for the bonding agent, say, several hours or a
day in advance.
(2) Install probe 34a and cable 35, 37 as shown in
Figure 3. Install a bypass plug in the socket 35 for probe
34b to deactivate channel 2. Initial probe clearance from
the top of the commutator riser 23 should be ample to avoid
damage to the probe 34 from any high point on the armature
surface. Cautiously rotate the positioning ring 32 with
the gearhead motor drive 33 and controller 92 to verify
probe clearances and note any spots where m;n;mllm
clearances are observed. Verify probe clearance from the
armature 26a by slowly sp;nn;ng the rotor. Do this by
bumping the external drive M with a quick on-off cycling of
its motor starter.
(3) Rotate the armature with the external drive M.
(4) Using probe 34a, find the maximum signal produced
by the magnet 40 and select oscilloscope setting for best
resolution.
19
217~
(5) Adjust centering of the positioning ring 32 until
the signal amplitudes observed with probe 34a positioned at
the top, bottom right and left are the same, within scope
re~-l; ng accuracy.
(6) Remove the permanent magnet 49.
(7) On the instrument amplifier 95 adjust the drive
current as needed.
(8) Turn off the instrument amplifier 95 and remove
the probe 34a.
(9) Install the provided laser pointer 62 in place of
the probe 34a. Align the laser pointer 62 to spot the
lower edges of the iron cores of the main poles 21 and
interpoles.
The next steps of this first test are as follows:
a. Using the laser pointer 62, hereinabove
explained, prepare a mapping matt of the field poles 21 and
the interpoles, for future use by use of the computer
plotter program, as explained below.
(1) Using the remote drive, initially position the
laser beam 64 between a main pole 21 and an interpole with
the probe cable 37 fully unreeled. Zero the digital
counter 41 at this position. All subsequent reference
points in the air gap map will be incremented from this
positlon .
(2) Using the digital counter 41, determine the
positions of the left and right edges of the main poles and
interpoles, as shown in Figure 7. Divide the spans of the
main poles into an odd number of counts, say eleven, and of
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the interpoles, say three, for plotting resolution. A
typical span is 48 on Figure 7 and is the distance between
the left and right edges of the iron portion of pole 21.
(3) Print the air gap reference map (Figure 7) with
the above identified position counts for use for field
plotting.
Note: Evaluate the physical quartering of the main
poles around the air gap and the centering of the
interpoles between the main poles for use later in the
procedure.
(4) Remove the laser pointer 62 and install the
provided horizontal probe 34c.
(5) Center the horizontal probe 34c for a penetration
of 1.0 inch from the front edge of a main pole 21 into the
air gap.
(6) Turn on the instrument amplifier 95 and check
proper instrument system operation.
b. Set the two oscilloscopes 93 and 94 for DC, 500
mv per division, and 1 ms per division.
c. Adjust the external field excitation source 91
for an output of 5.00 amperes, with all main poles
energized.
d. Using the reference air gap map, the remote
drive, the digital counter and the oscilloscope readings,
plot the radial air gap field distribution. Record the
probe signal levels and read at the reference points
identified in step a as the air gap is scanned.
'- ~174152
e. Print the radial air gap plot and evaluate the
following (Correlate evaluation to physical quartering and
centering as noted above):
(1) Accuracy of quartering main radial fields
(2) Zero radial field crossing under the interpoles.
(3) Difference in amplitudes, shapes, and areas of
the radial field poles.
f. Use the following acceptance criteria:
(1) Quartering of the main poles to be within a
predetermined number of counts on the digital counter.
(2) Zero crossings at centers of interpoles.
(3) Main field amplitudes matched within five percent
and areas matched within seven percent.
g. Turn off the instrument amplifier and remove the
horizontal probe.
The second test is desirable in the event that the
D.C. machine under test has 4,8 or 12 poles and a lap wound
armature. If this test is performed before my test to
determine the optimum brush positions is run it will
expedite the determination of best brush locations, when
the D.C. machine is one of the types specified in the first
sentence of this paragraph.
I will next explain the second test. This test is
optional.
The second test will show good positions for the
brushes at which all currents, other than the load current,
in the brushes are a m;nlmllm. The third test, which is
described later will show an even better way to adjust the
22
~17415~
brushes. The armature of a D.C. machine may have
circulating currents due to the fact that the emf's induced
in the armature by the air gap field may not be
symmetrical. The "second testN now to be described will
characterize the circulating current and pin-point the
location of the brushes at which the unwanted currents are
a m;n;ml]m.
To perform this "second test", an external
driving/motor is employed to rotate the d.c. machine under
test. The field of the D.C. machine under test is supplied
by a DC source outside of the DC machine being testing. In
the case of the 200 HP d.c. machine referred to above, the
field is excited by 5.0 amperes by the external D.C. source
91 .
As shown in Figure 1 and 2, each brush holder receives
three brushes 25. For the purposes of this test, only the
center one of the three brushes is used in two dramatically
opposite brush holders. The brush holder in the said two
diametrically opposite positions are then adjusted a small
distance around the periphery of the commutator until the
circulating current signal is reduced to a m; n; ml]m .
The step of adjusting positions of the brush holders
until the circulating current is m;n;m; zed will now be
explained in more detail.
If the brush holders, on the d.c. machine under test,
are not individually adjustable around the periphery of the
commutator, a substitute brush rigging should be provided
which permits the desired adjustment. Both oscilloscopes
21741S2
93, 94 of Figure 9 should be set to: AC 20mv, 2ms., auto,
triggered at approximately -8 mv. The probes 34a and 34b
should be rotated (using the controller 92 and the
positioning ring 32) until they are at the signal peaks as
displayed on the analog scope. Record in the computer, for
future reference, the signal traces that are on the
computer oscilloscope, and enter in the file of the
personal computer all pertinent information on the tests.
At each of the positions at which each pair of brushes is
tested, the following three steps should be repeated. (1)
Set both scopes to 93, 94 to AC, 20 mv., 2 ms., auto
triggered at about -8 mv. (2) rotate the positioning ring
32 and set the probes 34a and 34b to the maximum signal
positions as indicated on the analog oscilloscope, and (3)
record the signal traces and all other pertinent data, in
the computer PC.
The brushes may be set to positions relative to each
other in which the circulating current is a m;n;mllm, or
zero, using the data taken during this second test. The
procedure described above may be performed on each pair of
brush sets until every brush has been set to a good
positlon .
I will next describe the third test. If the D.C.
machine under test has a lap wound armature and 4,8 or 12
poles, the first step of this third test is to use the data
obtained during the second test and determine which pair of
brushes has the lowest circulating current signal level,
this, however, is optional. After this brush pair has been
24
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selected, the second step is performed. Brushes are
installed in the applicable brush holder so that the
brushes of this brush pair are in their customary contact
with the commutator. In the event that each brush holder
is built to receive three brushes, only the center brush is
installed for this third test.
After the pair of brushes (of the same polarity) has
been installed, the third step occurs as follows: Install
a third brush in a brush holder of opposite polarity to the
polarity of the two brush pair. The fourth step is to set
the computer scope 93 to DC, 200 mv/division, lms., and the
analog scope 94 to AC, 50 mv/division, 1 ms. Next, the
fifth step uses an external motor M which rotates the
machine under test at its normal operating speed. The
sixth step is to gradually increase the load on the D.C.
machine under test (while operating it as a generator), by
decreasing the resistance of the load box 101 and/or
increasing the field current (as shown in ammeter 91a) to
the machine under test, until the machine under test is
delivering a current to the load box of 100 amperes.
The seventh step of the third test involves rotating
the positioning ring 32 to place probe 34a near radial
alignment with said third brush after which the probe 34a
is rotated, by positioning ring 32, back and forth a small
amount so as to find the location in which it yields the
maximum alternating voltage signal on the oscilloscopes.
This is the optimum position for said third brush during
.
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normal operating conditions. That brush should now be set
to the optimum position.
The positioning of the brush to its optimum position
may be described in another way, thus: with the D.C.
machine being operated as a generator and driven at its
rated speed and load current, the probe 34a is rotated back
and forth, by positioning ring 32, above the brush to
determine the position at which the mmf through the brush
is a maximum. The brush is then reset so that the maximum
mmf passes through the brush midway between the leading and
trailing ends of the brush.
The ninth step is to save the signal traces on the
computer oscilloscope for future reference. Log the PC
file identification and all pertinent information in test
forms.
Repeat the above nine steps for the opposite "third
brushl'. This process must be repeated until the center
brushes of all poles have been adjusted to result in
maximum mmf~s, and maximum fields, at the center of the
brush footprint.
Install all center brushes and perform a load test at
output current of 100 amps. Observe current flow through
centers of the brush footprints and current balance.
Record the signal traces for future reference.
Repeat all ten steps to cover the inside and outside
brush paths to verify performances equal to center brush
path.
26
217~152
Upon completion of the third test, secure the machine
and install all of the brushes.
Direct current machines with wave wound armatures can
be tested and the brushes adjusted in the m~nner described
above, however, in those cases it does not expedite the
testing and adjusting to perform the second test as part of
the procedure.
In D.C. machines (with either wave or lap wound
aramtures) having 2, 6, or 10 poles (or any other number of
poles which divided by 2 yields an odd number), the brushes
180 degrees apart are opposite plarity. The brushes of
these machines may be adjusted to their optimum positions
as follows.
Operate the D.C. machine as a generator at rated speed
with the normal current that would flow through each brush
when the machine is delivering its rated maximum current.
Select two brushes 180 degrees apart and remove all
other brushes. Position the probe 34c to sense the
position of the maximum mmf through the brush. Move the
brush so that the maximum mmf passes through the brush
midway between the leading and trailing ends of the brush.
Repeat the prescription just given for the brush 180
degrees from the brush that was just tested. Thereafter,
select another set of brushes that are 180 degrees apart.
Remove all other brushes and test and adjust these two
brushes in the manner described for the first two brushes.
Continue to test and adjust two brushes at a time until all
brushes are tested and adjusted.
217~1~2
When, during the discussion of the third test, I refer
to an mmf aligned with the brush, it should be understood
that such mmf is almost entirely the result of the current
flowing through the armature.
As stated above the numerous risers 23 of the machine
form a cylinder whose center is coaxial with the axis of
the machine. Moreover, as shown in Figure 2, probe 34 has
a path concentric with the outer surface of the risers.
The space may be one to two millimeters. For the purpose
for the third test the probe can be at other positions
dep~n~;ng on the construction of the machine under test.
For example, in many machines, the probe 34 (Figures 2
and 4) may move in a circle to the left or to the right of
the path it moves in Figure 2.
The purpose of the fourth test is to evaluate the
performance of the interpoles and verify balanced armature
currents after the air gap field and brush positioning are
adequately adjusted.
The following instruments are used in this test:
a. Instrument amplifier 95
b. Oscilloscopes (Digital 93 and Analog 94)
c. DC Ammeter 91a
d. Digital Counter 41
The test setup is illustrated in Figure 13.
the following equipment is used during this test:
a. DC power source 131 rated at the full input power
requirement of the test motor (600 amps. at 250 Vdc).
~174152
b. Dyn~m~m~ter 132 capable of absorbing 200 HP.
The test procedure is as follows:
a. Set the computer scope 93 to DC, 200
mv./division, 1 ms. and the analog scope 94 to AC, 50
mv./division, 1 ms.
b. Start and operate the machine at normal operating
speed while engaged to the dyn~mom~ter.
c. Gradually apply load to the machine by control of
the dyn~m~ter 132 and the d.c. power source 131. Repeat
the steps that follow at 25 percent current increments.
for each load increment:
(1) Check maximum current flow at or near the
position of the brush set for each pole. Park probe 34a in
front of brush face at point where maximum signal peaks are
evident in the analog scope 94.
(2) Verify maximum current flow at the center of the
brush footprints for each brush set.
(3) Verify balanced current flow through the
equipotential brushes. Record the signal traces for future
reference.
(4) If tests (2) and (3) do not show satisfactory
results, verify the results by repeating the three brush
tests that constitutes the third test.
(5) Observe and record zero signal crossings by use
of probe 34 and counter 41 indications.
d. If zero signal crossings are not evident at the
centers of interpoles within the 100 percent load range,
the interpole air gap will require correction.
29
