--- title: "Short note: Armature reaction in dc machine" subject: "Electrical Machines" chapter: 3 key: "semester_4/electrical_machines:note-armature-reaction-dc" source: https://www.bctengineer.com/q/semester_4/electrical_machines/note-armature-reaction-dc site: BCT Engineering --- # Short note: Armature reaction in dc machine Electrical Machines · 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_machines/note-armature-reaction-dc - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3 ## Question Write a short note on armature reaction in dc machine. ## Short answer Armature reaction is the distortion and reduction of main field flux by the armature mmf. It shifts the magnetic neutral axis, causes demagnetization, sparking, and lowered emf. Remedies: brush shift, interpoles to neutralize cross-magnetization, and compensating windings in pole faces. ## Answer **Armature reaction** is the effect of the magnetic field set up by the armature current on the main field flux of the machine. When the generator is loaded, the armature carries current and produces its own mmf. This acts on the main field and has two effects. 1. **Distortion (cross-magnetising effect):** the armature field is at right angles to the main field. It strengthens the flux under one half of each pole and weakens it under the other half. The flux distribution becomes uneven, and the **magnetic neutral axis (MNA)** shifts away from the **geometrical neutral axis (GNA)**, forward in the direction of rotation for a generator (backward for a motor). 2. **Demagnetising effect:** if the brushes are shifted to the new MNA, the armature mmf has a component directly opposing the main field. This reduces the net flux per pole. ```text Main flux --> | Armature flux (cross) | Resultant: crowded at one pole tip, weak at other MNA shifted from GNA by angle theta ``` **Harmful effects:** - Reduced total flux, so the generated emf is lower. - Shifted neutral axis, so brushes spark if kept on the GNA. - Flux crowding at pole tips causes saturation and extra iron loss. **Remedies:** - Shift brushes to the MNA (small machines). - Use **interpoles** (commutating poles) to neutralise cross-magnetisation in the commutation zone. - Use **compensating windings** in the pole faces on large and fast-varying machines. - Increase the air gap or use a laminated pole tip with slots. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Armature reaction is: A. The reaction of the brushes with the commutator B. The heating of the armature winding C. The reaction of the field winding to supply voltage D. The effect of the armature current's magnetic field on the main field flux Answer: D. It is the influence of armature mmf on the main flux. (Key point: Definition) 2. The cross-magnetising effect of armature reaction causes: A. Flux strengthened under one half of each pole and weakened under the other, shifting the neutral axis B. Equal reduction of flux under the whole pole C. A decrease in armature resistance D. No change in the neutral axis Answer: A. The armature field is at right angles to the main field, distorting the flux and shifting GNA to MNA. (Key point: Distortion effect) 3. The demagnetising effect appears when: A. The brushes stay on the GNA exactly B. The brushes are shifted to the new MNA C. The machine is unloaded D. Interpoles are fitted Answer: B. After brush shift, a component of armature mmf directly opposes the main field, reducing net flux. (Key point: Demagnetising effect) 4. Which is a remedy for armature reaction on large, fast-varying machines? A. Increasing armature current B. Reducing the air gap C. Compensating windings in the pole faces D. Removing the interpoles Answer: C. Compensating windings neutralise armature mmf; increasing the air gap, not reducing it, also helps. 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