20
DC Motors
Commutation
Intermittent sparking due to overloads or slight visible sparking does not
necessarily indicate poor commutation. Poor commutation exists when
there is excessive sparking requiring abnormal maintenance. Every case of
excessive sparking should be investigated to determine the cause and
correct it. The chart in
may help in analyzing commutation
problems. DC motors and generators are brushed for full load current. If
unit or units are consistently operated at less than 1/2” rated load a
condition known as threading will result.
Table K Commutation Problem Quick Check
Symptom
Identified By
Possible Cause
1. Excessive sparking at motor
or generator commutator.
Sparking
1. Dirty or corroded commutator due to dirt,
ambient contaminants, oil or oil mist, etc.
2. Brushes incorrectly seated.
3. High or feather-edged mica.
4. Faulty machine adjustment.
5. Interpoles failed or improperly adjusted.
6. Loss of brush spring tension.
7. Brushes sticking in brush holder.
8. Unit overload.
9. Defective commutator or armature.
10. Unequal spacing of holders around commutator.
2. High commutator bars
produce a rough commutator.
Generally associated with sparking and noisy operation of
the brushes on the commutator.
1. Loose commutator.
3. Low commutator bars
produce rough commutator.
Generally associated with sparking and noisy operation of
the brushes on the commutator.
1. Loose commutator.
2. High mica.
3. Open or high resistance connection at
commutator.
4. Streaking or threading of
commutator surface.
Rough commutator with associated sparking. Fine lines in
brush track.
1. Low average current density in brushes due to
light machine loading.
2. Contaminated atmosphere.
3. Oil on commutator or oil mist in air.
4. Humidity too low.
5. Lack of film forming properties in brush.
6. Brush too abrasive.
5. Bar etching or burning.
Rough commutator with associated sparking and eventual
flashover.
1. High mica.
2. Operation of machine with brushes off neutral.
3. Commutator dirty.
4. Incorrect spring tension.
5. Machine overload or rapid load change such as
plugging.
6. Bar marking at pole-pitching
spacing.
1. Two bars marking 180 degrees C (356 degrees F)
apart on 4-pole machine at start.
2. Three bars marking 120 degrees C (248 degrees F)
apart on 6-pole machine at start.
3. As pitch bar marking progresses, it will eventually
show at all bars on the machine.
4. Associated sparking and eventual flashover.
1. Shorted commutator bars or coils.
2. Open armature or field circuit.
3. Unequal air gap.
4. Cyclic disturbance either electrical or
mechanical.
7. Bar marking at slot-pitch
spacing.
Sparking and marking of one or more bars at equal
spacing around commutator according to bar-per-slot ratio
with eventual flashover.
1. Unequal compensation of armature coils. The
energy unbalance is reflected into the last coil in
the slot to undergo commutation, and will result
in a spark at the brush.
8. Rapid commutator or brush
wear.
Bright commutator surface.
1. Abrasive material under brush.
2. Too abrasive a brush.
3. Low average brush current density due to light
machine loading.
4. Contaminated atmosphere.
5. Humidity too low.
6. Incorrect brush tension.
