--- title: "Short note: Armature reaction in DC machine" subject: "Electrical Machine" chapter: 3 key: "semester_4/electrical_machine:armature-reaction-dc-machine-note" source: https://www.bctengineer.com/q/semester_4/electrical_machine/armature-reaction-dc-machine-note site: BCT Engineering --- # Short note: Armature reaction in DC machine Electrical Machine · Chapter 3: DC Generator - Asked in: 2068 Magh, 4 marks - Video: [Armature Reaction in DC Machines](https://www.youtube.com/watch?v=Gp3rhS0CSBA) — Binary Stack - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/armature-reaction-dc-machine-note - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-3 ## Question Write a short note on armature reaction in dc machine. ## Short answer Armature reaction distorts the main field flux and shifts the magnetic neutral axis, causing sparking and flux reduction. Remedies include interpoles, compensating windings, brush shifting, and high reluctance pole tips. ## Answer **Armature reaction** is the effect of the magnetic field set up by the armature current on the main field flux of the machine. #### Cause On no load the main flux is symmetrical about the pole axis and the magnetic neutral axis (MNA) coincides with the geometrical neutral axis (GNA). When the armature carries current, it produces its own flux, acting at right angles to the main flux (along the interpolar axis). ```text main flux: ---> armature flux: | v resultant flux is distorted: weakened at one pole tip, strengthened at the other ``` #### Effects (generator) 1. **Cross-magnetising effect (distortion)**: the flux is weakened at the leading pole tip and strengthened at the trailing tip (for a generator). The flux density becomes non-uniform. 2. **Shift of MNA**: the neutral axis shifts in the direction of rotation for a generator (opposite to rotation for a motor). The brushes are then no longer at zero-emf position and sparking results. 3. **Demagnetising effect**: when brushes are shifted forward, part of the armature mmf directly opposes the main field. This reduces the total flux and the generated emf. 4. In saturated machines, strengthening at one tip is less than weakening at the other, so a net reduction of flux occurs, which lowers the terminal voltage. #### Remedies - **Brush shifting** to the new MNA (not practical for varying load). - **Interpoles (commutating poles)** in series with the armature, to neutralise the cross-magnetising mmf in the commutation zone. - **Compensating winding** in pole-face slots, in series with the armature, with opposite current direction, for large machines. - Using a higher air-gap or high reluctance pole tips. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is armature reaction? A. Reaction of the brushes on the commutator B. The effect of load on the prime mover C. The effect of the armature current's magnetic field on the main field flux D. Reaction of the field on the yoke Answer: C. The armature mmf acts at right angles to the main flux and distorts it. (Key point: Definition) 2. Which is an effect of armature reaction in a generator? A. MNA shifts against rotation B. Flux becomes perfectly uniform C. Brush sparking disappears D. MNA shifts in the direction of rotation, causing sparking Answer: D. The neutral axis shifts, so brushes are off the zero-emf position. (Key point: Effects) 3. When does the demagnetising effect appear? A. When the brushes are shifted forward, part of the armature mmf opposes the main field B. When the brushes are on the GNA C. When the machine is unloaded D. When the field is removed Answer: A. Shifted brushes make a component of armature mmf directly oppose main flux. (Key point: Demagnetising) ## Related in Electrical Machine - [Short note: Armature reaction in dc machine](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Anote-armature-reaction-dc) — Electrical Machines, chapter 3 - [Armature reaction in DC machines and its remedies](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aarmature-reaction-effects-remedies) — Electrical Machines I, chapter 3 - [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aarmature-reaction-sync-generator-loads) — Electrical Machine, chapter 6, asked 2× - [Synchronous generator working; armature reaction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-working-armature-reaction) — Electrical Machines, chapter 6 - [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aarmature-reaction-sync-gen-loads) — Electrical Machines, chapter 6 - [25 kW separately excited DC machine as motor or generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Asep-excited-25kw-125v-terminal-voltage) — Electrical Machines I, chapter 3 ## Related in other subjects - [True/False: armature reaction and ampere conductors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-armature-reaction-ampere-conductor) — Electric Machine Design, chapter 5 - [Armature reaction at different load power factors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aarmature-reaction-power-factor-effect) — Electric Machines II, chapter 1, asked 7× - [Terminal voltage above emf and armature reaction](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-greater-ar-effect) — Electric Machines II, chapter 1 - [Internal emf and armature reaction reactance](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ainternal-emf-armature-reactance) — Electric Machines II, chapter 1