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DC motor gives jerks while running...
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...Read further to know more about the reasons |
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Jerks observed while running the
motor are dangerous to the drive as well as to the motor and connected load.
It is harmful to continue running the motor without trying to locate the
source. Jerks, many a times, appear to
be arising from the mechanical parts but such conclusions are deceptive.
Immediate checking of current waveform will give some clue. Note the
difference in the two waveforms. Sharp rise only in one peak is normally not
possible due to mechanical parts. Waveform as shown in the sketch
on the left hand side is also possible if a thyristor has collected a lot of
dust around gate cathode area. Keeping ceramic insulator around gate clean is
one solution to such cases. Thyristors misfire with such dust getting
accumulated and can create a big sound from the gears connected to the motor.
If one is not aware of this "electrical possibility", it misleads
people to think that the gears have developed some problem. Printed circuit boards also
accumulate black dust around the tracks which causes similar symptoms. When any
contactor in the panel drops out, some times, the motor gives jerks. Dropping
of a big contactor causes electrical noise to be generated which travels
along and is picked up by sensitive “Gate” leads. Such jerks can be reduced
by twisting the gate cathode leads with very tight twist. The twisting makes
the gate cathode leads immune to picking up of noise. Shorter lengths of
these wires also helps. Long leads routed through cable trays only “look”
esthetically nice but are bad electrically. Short lengths of gate-cathode
leads, running directly in air are good, although they may not look good! R-C
circuits across the contactor coils also help in reducing these
jerks. R-C circuit eats up the noise generated right at the source. |
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Download PDF
version of these pages.
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Speed control not satisfactory
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