--- title: "Commutator and carbon brush functions" subject: "Electrical Machine" chapter: 3 key: "semester_4/electrical_machine:commutator-brush-functions" source: https://www.bctengineer.com/q/semester_4/electrical_machine/commutator-brush-functions site: BCT Engineering --- # Commutator and carbon brush functions Electrical Machine · Chapter 3: DC Generator - Asked in: 2075 Baisakh, 8 marks - Video: [Commutators: Basics on AC and DC Generation](https://www.youtube.com/watch?v=ATFqX2Cl3-w) — Edison Tech Center - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/commutator-brush-functions - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-3 ## Question Explain the functions of commutator and carbon brushes in d.c generator with neat sketch. ## Short answer The commutator converts the alternating emf induced in the armature into direct current at the terminals by reversing coil connections at the instant of emf reversal. Carbon brushes collect current from the commutator, are self-lubricating to reduce wear, and their high contact resistance limits sparking during commutation. ## Answer In a DC generator the emf induced in the armature conductors is alternating. The commutator and brushes together convert it into unidirectional emf at the external terminals and also carry the current from the rotating armature to the stationary load. #### Sketch ```text N S +-----+ +-----+ | | armature coil | | | | +-----------+ | | +-----+ | | +-----+ | | seg1| |seg2 commutator +--+--+ +--+--+ (split ring) |/////| |/////| ===B1=== ===B2=== carbon brushes | | +---[LOAD]---+ ``` #### Commutator It is a cylinder of wedge-shaped hard-drawn copper segments, insulated from one another and from the shaft by mica, mounted on the shaft. 1. **Rectifies the emf**: it reverses the connection of each coil to the external circuit at the instant the coil emf reverses, so the current in the load is always in one direction. 2. **Collects current**: it connects the armature winding to the brushes. 3. **Keeps the armature emf and current in the proper phase**, so the torque and the output polarity stay constant. Without the commutator the machine would give AC at the brushes (as an alternator with slip rings does). #### Carbon brushes They are blocks of carbon or graphite held in brush holders by spring pressure, and rest on the commutator surface. 1. **Carry current** from the rotating commutator to the stationary external circuit. 2. **Self-lubricating**: carbon has a low friction coefficient, so wear of the commutator is small and the brush is replaced cheaply. 3. **High contact resistance** helps commutation (reduces the sparking current in the short-circuited coil). 4. **Soft**: the brush wears instead of the costly copper commutator. 5. Spring pressure keeps good contact despite vibration. The brushes are placed at the magnetic neutral axis (MNA), where the coil sides are not cutting flux, so that the coil is short-circuited when its emf is zero and sparking is minimum. | Part | Material | Main duty | |---|---|---| | Commutator | Copper segments + mica | Converts AC to DC | | Brush | Carbon / graphite | Takes current to the load | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main function of the commutator in a DC generator? A. Convert the alternating emf induced in the armature coils into unidirectional emf at the terminals B. Increase the magnitude of the induced emf C. Produce the main field flux D. Reduce the armature reaction Answer: A. It reverses each coil's connection to the external circuit as the coil emf reverses, so the load current stays in one direction. (Key point: Rectification) 2. Which property of carbon brushes protects the costly commutator? A. They are harder than copper so they never wear B. They are soft and self-lubricating, so the brush wears instead of the copper C. They have zero contact resistance D. They are magnetic and attract the segments Answer: B. Carbon is soft with low friction, so the cheap brush wears first and commutator wear is small. (Key point: Why carbon for brushes) 3. Why is the high contact resistance of carbon brushes useful? A. It raises the output voltage B. It increases the generated emf C. It helps commutation by reducing the sparking current in the short-circuited coil D. It removes the need for mica insulation Answer: C. High contact resistance limits the current circulating in the coil short-circuited by the brush, so sparking falls. (Key point: Brush contact resistance) 4. Where are the brushes placed and why? A. Under the pole centre, where flux is maximum B. On the shaft, to carry the field current C. Anywhere, since position does not matter D. On the magnetic neutral axis, so the coil is short-circuited when its emf is zero and sparking is minimum Answer: D. At the MNA the coil sides are not cutting flux, so the short-circuited coil has zero emf and sparking is least. 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