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Drop Test for checking state of
armature winding in a DC motor.... |
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Drop test or more correctly, Voltage
Drop Test, is often carried out for checking the condition of the Armature
winding. You need a DC power source of low voltage capable of feeding up to
10 amperes or so, and a sensitive milli voltmeter to carry out this test. Two
persons sitting on stools in front of the armature make a perfect combination
for the test. One person goes on touching the DC source probes on the
segments with long chalk mark, and the other person goes on reading the
voltage drop across successive segments. The armature of the DC motor under test
is taken out and laid horizontal on two supports, so that it can be turned by
hand. Commutator segments are marked by white chalk as shown below. Every sixth
or seventh ( not specific ) segment is marked with a longer mark. Gaps
between two long marks should be same over entire commutator. The last gap
could be different, depending upon the number of segments available. It does
not matter.
A DC source is used to pass current
through segments which are marked long and which near each other. Milli
voltmeter is then used to measure the
voltage across successive pairs of segments, one by one. All reading should
be same. Value of the mill volts does not matter. After confirming that all
readings within a group of two long marks are same, the white marks are wiped
out by finger, and test is carried out for next pair of long marks. The
process is repeated till all segments are covered. Any mismatch between the
voltage is an indication of faulty winding. Since the armature is supported on two
stands it is convenient to turn it slowly to bring the segments under probe
to top position. Technically, drop
test is a measurement of voltage drop developed across series of equal
resistances, when same steady current is passed through all of them. Unequal
voltage drops indicate unequal resistance indicating a faulty condition of
winding.
Another good method of testing the
armature winding is to test in on a Growler. Growler is a special transformer
with only primary winding. Magnetic stampings do not complete the magnetic
path as in normal transformer, but instead a large air gap is purposely
created where the armature to be tested can be placed. A hacksaw blade is
kept at different positions on the armature stampings and checked if it gets
attracted to the stampings making a chattering noise. No noise or no
attraction is an indication of healthy winding. Electrically, it is doing
nothing but passing alternating magnetic flux through armature. Any short in
the winding acts as a short circuited secondary of the transformer. A meter
connected in primary side shows sudden rise in the current when shorted secondary is forced to generate
voltage. Here is a video
of how Growler Test is carried out.
The sketch below shows the arramgement to explain the principle of
working. Flux produced in the magnetic arms on which the armature is kept
resting, causes voltage to be induced in the armature winding. If there is a
short circuit in the armature winding, it produces another magnetic flux
which causes the hacksaw blade kept on the armature stampings to vibrate.
This makes a chattering noise. If the winding is not having any short
circuit, the blade can be moved over the stampings as if it is moved on a
surface of a table.
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