--- title: "DC motor principle and role of commutator and brushes" subject: "Electrical Machines I" chapter: 4 key: "semester_4/electrical_machines_i:dc-motor-principle-commutator-brushes" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-motor-principle-commutator-brushes site: BCT Engineering --- # DC motor principle and role of commutator and brushes Electrical Machines I · Chapter 4: DC Motor - Asked in: 2078 Baisakh, 8 marks - Video: [#6 - DC machines Basics - Working of DC motor ( Importance of commutator )](https://www.youtube.com/watch?v=2TcsX5iSKp8) — ELECTRICAL IS EASY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-motor-principle-commutator-brushes - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4 ## Question Explain the operating principle of a DC motor and clarify the functions of commutator segments and carbon brushes in dc motor. ## Short answer A current-carrying conductor in a magnetic field experiences force (F = B I l), creating torque. The commutator reverses coil current each half-turn to maintain unidirectional torque. Carbon brushes feed current from the stationary circuit to the rotating commutator and aid commutation by providing contact resistance and self-lubrication. ## Answer A DC motor converts DC electrical energy into mechanical energy. Its operating principle is that **a current-carrying conductor placed in a magnetic field experiences a force** $F = B I l$, whose direction is given by **Fleming's left-hand rule**. #### Operating principle 1. The field winding on the poles, fed with DC, sets up a steady magnetic flux. 2. DC supply is fed to the armature conductors through **brushes and commutator**. 3. Each armature conductor carrying current in the field experiences a force $F = BIl$. Conductors under the N pole carry current in one direction and those under the S pole in the opposite direction, so all forces act in the same rotational sense and produce a **torque** $T = \dfrac{P\phi Z I_a}{2\pi A}$. 4. As the armature turns, a conductor moving from N pole to S pole has its current reversed by the commutator, so the torque remains unidirectional and the armature keeps rotating. 5. The rotating conductors cut the flux and an emf is induced which opposes the supply: the **back emf** $E_b$, with $V = E_b + I_a R_a$. Back emf limits the current and makes the motor self-regulating with load. ```text N +---------------+ | x x x x | <- force (left) | ( armature ) | rotation | . . . . | -> force (right) +---------------+ S x: current in, .: current out ``` #### Functions of commutator segments The commutator is a cylinder of copper segments insulated by mica, each joined to coil ends. - **Mechanical inverter:** the supply is DC, but the current in each conductor must reverse as it passes from one pole to the next to keep torque in one direction. The commutator reverses the current in each coil at the correct moment. - **Unidirectional torque:** by this reversal, conductors under a given pole always carry current in the same direction. - **Connection of coils:** it joins the coils in series to form the parallel paths (lap or wave). - **Sliding contact surface** for brushes, transferring current to the rotating armature. #### Functions of carbon brushes Brushes are blocks of carbon or graphite held in brush holders pressed on the commutator by springs. - **Lead current** from the stationary supply terminals to the rotating commutator. - **Short-circuit the coil under commutation** briefly, helping current reversal. - **Good commutation:** carbon has fairly high contact resistance, which helps current reversal (resistance commutation) and reduces sparking. - **Self-lubricating and soft:** graphite lubricates the commutator, wears itself rather than the costly commutator, and is easily replaced. | Part | Material | Main function | |---|---|---| | Commutator | Copper segments, mica insulation | Reverse coil current, keep torque one way | | Brushes | Carbon/graphite | Feed current, help commutation | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the operating principle of a DC motor? A. A current-carrying conductor in a magnetic field experiences a force B. A changing flux induces a current in a stationary coil C. A rotating field drags the rotor along D. Eddy currents produce torque Answer: A. Direction by Fleming's left-hand rule. (Key point: Principle) 2. What is the main function of the commutator? A. Increase supply voltage B. Reverse coil current at the right moment to keep torque unidirectional C. Reduce iron loss D. Cool the armature Answer: B. It acts as a mechanical inverter. (Key point: Commutator function) 3. What does a brush do? A. Produces the main flux B. Reverses the field C. Conducts current between the stationary supply and the rotating commutator D. Provides mechanical support to the shaft Answer: C. Carbon contact resistance also helps commutation. (Key point: Brush function) 4. Why are brushes made of carbon/graphite? A. It has zero resistance B. It is magnetic C. It is harder than copper D. Contact resistance helps commutation, and it is self-lubricating and wears instead of the commutator Answer: D. It is soft, self-lubricating and replaceable. 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