--- title: "Commutator, brushes; start shunt generator no load" subject: "Electrical Machines" chapter: 3 key: "semester_4/electrical_machines:commutator-brush-function-no-load-start" source: https://www.bctengineer.com/q/semester_4/electrical_machines/commutator-brush-function-no-load-start site: BCT Engineering --- # Commutator, brushes; start shunt generator no load Electrical Machines · Chapter 3: DC Generator - Asked in: 2077 Chaitra, 8 marks - Video: [Commutator Action In Generator | Basic Concept | Electrical Machines](https://www.youtube.com/watch?v=vlTETZfdrmo) — ENGINEERING TUTORIAL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/commutator-brush-function-no-load-start - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3 ## Question Explain the functions of commutator and carbon brushes in d.c. generator. Explain why dc shunt generator should be started without load. ## Short answer The commutator mechanically rectifies AC armature emf to DC and collects current. Carbon brushes make sliding contact, are self-lubricating, and reduce sparking. Shunt generator must start on open circuit: a load diverts the small initial armature current from the field, and armature reaction weakens residual flux, preventing voltage buildup. ## Answer #### Function of the commutator - It acts as a **mechanical rectifier**: it converts the alternating emf induced in the armature conductors into a unidirectional (DC) voltage at the brushes. - It collects the current from the armature conductors and passes it to the brushes. - It connects the correct coil to the brush at the correct time. The copper segments are separated by mica insulation. #### Function of the carbon brushes - They make sliding contact with the commutator and carry the armature current to the external load. - Carbon is self-lubricating and causes little wear on the commutator, and its high contact resistance reduces sparking. - They are placed at the magnetic neutral axis for sparkless commutation. #### Why a shunt generator is started without load A shunt generator is self-excited. The emf builds up from the **residual magnetism** like this: residual emf $\to$ small field current $\to$ more flux $\to$ more emf, until the field-resistance line meets the magnetisation curve. If the load is connected during starting: 1. The terminal voltage is very low, so a large part of the armature current goes to the load and the **field current is reduced** ($I_{sh} = V/R_{sh}$), because the load is in parallel with the field. 2. The voltage drop $I_aR_a$ in the armature takes away a larger share of the small emf, so the terminal voltage stays near zero. 3. The armature reaction of the load current weakens the small flux further, so the machine may fail to build up its voltage at all. Hence the load is connected only after the voltage has reached its rated value. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The commutator in a DC generator functions as A. A mechanical rectifier converting armature alternating emf into unidirectional voltage B. A transformer C. An amplifier of flux D. A speed regulator Answer: A. It also collects the current and connects the correct coil to the brush at the proper time. (Key point: Commutator function) 2. Brushes are placed at the magnetic neutral axis in order to A. Maximize emf B. Obtain sparkless commutation C. Increase friction D. Reduce residual flux Answer: B. Near the neutral axis the emf in the coil being commutated is nearly zero, so sparking is minimal. (Key point: Brush position) 3. If a shunt generator is started with the load connected, the field current falls because A. The load is in parallel with the field, and the small voltage drives current into the load B. The field winding opens C. The load adds flux D. The speed rises Answer: A. Ish = V/Rsh depends on terminal voltage, which stays near zero when the load draws current and the armature drop takes a larger share of the small emf. (Key point: Load at starting) 4. With the load connected at starting, the likely result is that the generator A. Builds up faster B. Fails to build up voltage, and a heavy overload current may flow C. Runs at a higher speed D. Delivers double voltage Answer: B. Hence the load is connected only after the voltage has reached its rated value. 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