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Examoogle • User • info@examoogle.com • EE-2024-8821
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What is the cause of the cavity on the commuter?
Hitting
Short circuit
Sparkling
Over voltage
Sparkling
Quick Summary: Cavities or pitting on a DC machine commutator are primarily caused by persistent sparking at the brush-commutator interface. This sparking leads to localized electrical erosion and metal transfer, gradually creating small craters or cavities on the copper segments.
Cavities or pitting on a DC machine commutator are primarily caused by persistent sparking at the brush-commutator interface. This sparking leads to localized electrical erosion and metal transfer, gradually creating small craters or cavities on the copper segments.
Er=L⋅dtdi — Reactance voltage causing commutation issues
Pspark=I2R — Power dissipation leading to thermal erosion
Sparking occurs during commutation when the current reversal in the armature coil is not instantaneous, often due to high reactance voltage. The high-energy discharge between the brush and the commutator segment causes localized high temperatures (I2Rt losses), leading to vaporization and physical removal of the copper material. Over time, these micro-arcs result in deep pits or cavities known as pitting.
Sparking is often due to incorrect brush position or high brush contact resistance.
Carbon brushes are used because they have higher contact resistance compared to metal brushes, aiding in commutation.
Cavities promote further sparking, creating a regenerative degradation cycle.
Regular maintenance includes cleaning the commutator and ensuring proper spring tension.
Proper commutation ensures smooth torque production
Reduces electromagnetic interference (EMI) caused by sparks
Deep pitting requires expensive commutator re-surfacing or replacement
Excessive sparking reduces the overall lifespan of the DC machine
DC Generators
DC Motors in traction systems
Universal motors in appliances
Option B (Short circuit) is a major fault but usually causes sudden winding failure rather than specific commutator pitting.
Option A (Hitting) refers to mechanical vibration/chatter which can worsen sparking but is not the primary cause of electrical cavity formation.
Option D (Over voltage) typically leads to insulation breakdown rather than localized copper erosion on the commutator.
C is correct — Cavities on the commutator are caused by continuous sparking occurring between the brush and the commutator segments.
In exams, remember that commutator surface issues are almost always linked to mechanical or electrical commutation defects, not general voltage or load faults.