The most common causes of dc motor arcing in commutator
-rotor winding fault
-commutator surface is not smooth
-commutator is not perfectly round
-commutator short-circuit between the lamellas (dirty)
-worn carbon brushes
-too small spring pressure to brushes
( brushes vibrations )
( in bigger motors drive fault, field current controller ,rotor current controller)
Some general guidelines...................
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Maintenacne World: Maintenance and Troubleshooting of Electric Motors
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How to measure rotor winding & commutator............
The commutator has several segments, and there are several
armature windings, each one connected between 2 commutator
segments.
The simplest way to test it is to put the meter across the mains
plug pins (NOT plugged in),( or across brushes) and turn the tool round very slowly so
you see the R of each section of the motor armature.
Analogue meters are much easier to use for this.
Comparing Rs wil pick up some faults, though not all. If this
doesnt work you'll need to compare L values rather than R, as L
nosedives with a shorted turn or 2, whereas R can stay almost the
same. Its easiest to do this by connecting the tool in series
with a lightbulb, and putting the pair across a low voltage ac
supply, eg 12v. Measuring the voltage across the tool will show
up a bad winding section as the motor is turned. The lightbulb's
non-linearity accentuates the fault. Your psu v needs to be low
enough that the tool doesnt spin.