Meter methods of identifying the type of lap or wave winding
So far, we have identified the winding as
either lap or wave. We now need to know the type of lap or wave winding that we have.
It is essential that we know the exact type
of winding prior to performing the tests that will tell us if the winding is open,
shorted, or grounded. The type of winding, including plex and reentrancy, will dictate how
these tests are conducted and the results are analyzed.
Use one of two meter methods: milliammeter or
low-reading ohmmeter.
Of the two methods, the ohmmeter method is
the easiest to conduct. The ohmmeter method does not require a separate source of test
voltage (battery), a rheostat, or configuration of test leads.
We will discuss the milliammeter method for
the following reasons:
• Personnel may not always have access to a low-reading ohmmeter.
• Once the milliammeter method is understood, the transition to the ohmmeter
method will be relatively simple.
The step-by-step procedures for the two
methods are exactly the same. The only difference is that the readings obtained by the two
methods will be exactly opposite. This is because current and resistance are
inversely proportional.
Winding identification indications for both
the milliammeter and ohmmeter methods are contained in the table for lap wound
armatures and in the table
for wave wound armatures.
Milliammeter method of lap winding
identification
 The 500 mA
dc function of a Simpson 260 or comparable multimeter can be used in the place of a dc
milliammeter. Shift to a lower mA scale as necessary.
Equipment required
• dc milliammeter (0 to 500
milliampere (mA) deflection) with meter leads
• Low-voltage dc supply (6V or 12V battery depending on armature size)
• Test leads
• Alligator clips
• Switch
• Rheostat
• Chalk
• Paper and pencil
Preparation
 Sand
the commutator with very fine sandpaper only. Never use emery cloth.
Inspect the commutator bars and risers for
cleanliness. Lightly sand them with very fine sandpaper, as necessary, to ensure good
electrical contact.
Inspect the slots between the bars to ensure
that they are not shorted by copper, carbon dust, or dirt. Clean the slots with a
toothbrush, a scribe, or slot shaver. Use the procedures from the lesson on inplace repair
of dc motors to undercut
the slots.
Mark any bar with a piece of chalk. This bar
will be referred to as bar 1.
Write individual numbers on a piece of paper to correspond to the number
of commutator bars. See example.
Obtain the equipment listed and connect as
shown in the figure at right.
Ensure that the milliammeter is connected in series
with the dc power supply before beginning the test and that the switch is in the OFF
position.
Test procedure
Turn the rheostat to the full resistance position.
Turn the switch to the ON position.
Place one test probe (stationary) on bar 1 and the other probe (movable)
on the adjacent bar in the clockwise direction. We will call this bar 2.
Adjust the rheostat until one-half scale reading is obtained.
If no reading is obtained, return the rheostat to
the full resistance setting and place test probes on segments 1 and 3.
Adjust the rheostat to half-scale milliammeter reading. Return the
movable test lead to bar 2. Record the reading, including zero, if applicable.
Continue to move the movable test lead clockwise from bar to bar until
all bars have been read. Rotate the armature and record the readings.
If zero deflection was obtained on adjacent segments, retest the armature
by reading bars 1 and 3, 1 and 5, and so on until all odd- numbered bars have been read.
Record the results.
Next, read bars 2 and 4, 2 and 6, and so on until all even-numbered bars
have been read. Record the results.
Analyzing the results of the lap winding test
Simplex lap winding indications (typical simplex lap-wound
dc armature)
A simplex lap winding is the only lap winding that
will give the indications below.
Readings between bar 1 and all adjacent bars decreased proportionately
until the movable lead was directly opposite the stationary lead on bar 1 (one-half the
armature coils, maximum resistance) . Mark this bar in preparation for a bar-to-bar test.
Readings increased proportionately as the movable lead was placed on
adjacent bars of the untested half of the armature (decreasing coil resistance, increasing
current). The maximum reading was registered on adjacent bars (one coil, minimum
resistance).
Duplex single reentrant lap winding indications (typical duplex
single-reentrant lap-wound dc armature)
Readings between bar 1 and odd-numbered bars decreased proportionately as
the commutator was circled (increasing coil resistance, decreasing current) .
A duplex single reentrant lap winding is the only lap winding that will
give the indications below.
Readings between bar 1 and even-numbered bars increased proportionately
as the commutator was circled (decreasing coil resistance, increasing current).
The maximum reading was registered on alternate bars (one coil, minimum
resistance).
The minimum reading was registered on adjacent bars (two paths, maximum
resistance).
Duplex double reentrant lap winding indications (typical duplex
double-reentrant lap-wound dc armature)
It is possible to obtain a very slight meter indication on adjacent bars.
This is an indication that the insulation between the windings is weak.
Zero reading on adjacent bars indicating winding is double reentrant (two
separate windings insulated from each other).
Readings between bar 1 and odd-numbered bars decreased proportionately
until movable lead was placed on odd numbered bar directly opposite bar 1 (one-half the
armature coils, maximum resistance). Mark this bar in preparation for a bar-to-bar test.
A duplex double-reentrant lap winding is the only lap winding that will
give the indications below.
Readings between bar 1 and odd-numbered bars of the untested half of the
armature increased proportionately (decreasing coil resistance, increasing current).
Readings between bar 2 and even-numbered bars gave identical indications
as between bar 1 and odd-numbered bars (two identical but separate circuits). Mark the
even-numbered bar opposite bar 2 in preparation for a bar-to-bar test.
The maximum reading was registered on alternate odd or even numbered bars
(one coil, minimum resistance).
Analyzing results of a wave winding test
Simplex wave winding indications (typical simplex
wave-wound dc armature)
A simplex wave winding is the only wave winding that will give the
indication below.
Readings between bar 1 and adjacent bars proportionately decreased and
then increased P/2 times in one trip around the commutator (coil resistance increases
until test probes are one pole pitch apart, decreases until probes are two pole pitches
apart and then repeats).
The maximum current reading was registered on bars two pole pitches apart
(one coil, minimum resistance). Mark the bar where maximum reading was obtained in
preparation for a bar-to-bar test.
The high reading was registered on adjacent bars (two coils in series,
low resistance).
The illustration of a typical simplex wave-wound dc armature shows an
armature designed for a four-pole machine. Simplex wave windings will always have the same
number of coils connected in series between adjacent bars as pairs of poles or P/2.
The minimum reading was registered on bars approximately one pole pitch
apart (maximum resistance, minimum current). Mark this bar in preparation for a bar-to-bar
test.
Duplex single reentrant wave winding indications (typical duplex
single-reentrant wave-wound dc armature)
Readings between bar 1 and odd-numbered bars decreased once and then
increased once in one trip around the commutator (increasing and decreasing coil
resistance).
A duplex single reentrant wave winding is the only wave winding that will
give the indications below. Reading increases and decreases will always be equal to P/2.
Readings between bar 1 and even-numbered bars increased once and then
decreased once in one trip around the commutator (decreasing and increasing coil
resistance).
Maximum readings were registered on bars two pole pitches apart (one
coil, minimum resistance).The high reading was registered on alternate bars (P/2 coils in
series, low resistance).
A duplex single reentrant wave winding is the only wave winding that will
give the indication below.
The minimum reading was registered on adjacent bars (maximum coil
resistance, minimum current).
Duplex double reentrant wave winding indications (typical duplex
double-reentrant wave-wound dc armature)
It is possible to obtain a very slight meter indication on adjacent bars.
This is an indication that the insulation between windings is weak.
Zero readings on adjacent bars indicating winding is double reentrant
(two separate windings insulated from each other)
Readings between bar 1 and odd-numbered bars decreased and then increased
P/2 times in one trip around the commutator.
A duplex double reentrant wave winding is the only wave winding that will
give the indications below.
Readings between bar 2 and even-numbered bars decreased then increased
P/2 times in one trip around the commutator. The maximum reading was registered on odd or
even bars two pole pitches apart (one coil, minimum resistance). The high reading was
registered on alternate bars (P/2 coils in series, low resistance). The minimum reading
was registered on odd or even bars one pole pitch apart (maximum coil resistance, minimum
current).
Mark these bars in preparation for a bar-to-bar test. |
