--- title: "Armature reaction in DC machines and its remedies" subject: "Electrical Machines I" chapter: 3 key: "semester_4/electrical_machines_i:armature-reaction-effects-remedies" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/armature-reaction-effects-remedies site: BCT Engineering --- # Armature reaction in DC machines and its remedies Electrical Machines I · Chapter 3: DC Generator - Asked in: 2081 Chaitra (new course), 2+2+2 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_i/armature-reaction-effects-remedies - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3 ## Question Define armature reaction in DC machine. Explain its effects on the operation of DC machine. What measures can be taken to reduce its effect? ## Short answer Armature reaction is distortion and weakening of main flux by armature current. Effects: shifts magnetic neutral axis causing sparking, reduces net flux per pole (demagnetizing), and may cause flashover. Remedies: compensating windings, interpoles, strong main field, chamfered pole tips, or brush shift. ## Answer #### Armature reaction **Armature reaction** is the effect of the magnetic field set up by the armature current on the distribution of the main field flux in a DC machine. On no load only the main field acts; on load the armature conductors carry current and produce a cross field that distorts and weakens the main field. ```text main flux (N to S) ---> N | ____ | S | / \ | GNA: geometric neutral axis --------( arm. )------ MNA shifts in the direction of | \____/ | rotation (generator) or against rotation (motor) armature flux acts along the brush axis (90 deg elec.) ``` #### Effects 1. **Cross-magnetising effect:** armature MMF is at 90° to the main field. It strengthens flux at one pole tip and weakens it at the other, distorting the flux. 2. **Shift of magnetic neutral axis (MNA):** the MNA shifts forward in the direction of rotation for a generator (backward for a motor). If brushes stay on the geometric neutral axis, coils undergoing commutation have EMF induced in them, causing **sparking**. 3. **Demagnetising effect:** if brushes are shifted to the new MNA, part of the armature MMF directly opposes the main field, reducing net flux. This **reduces the generated EMF** (generator) or **increases speed** (motor). 4. Because of saturation at the strengthened pole tip, the increase there is less than the decrease at the other tip, so the **net flux per pole falls** even without brush shift. 5. Uneven flux raises voltage between adjacent commutator segments, risking **flashover**, and increases iron loss in teeth. #### Measures to reduce armature reaction - **Compensating winding:** placed in slots in the pole faces, connected in series with the armature, with current opposite to the adjacent armature conductors. It cancels the cross field under the poles (used in large machines). - **Interpoles (commutating poles):** small poles between main poles, in series with the armature. They neutralise the cross field in the commutating zone and induce the reversing EMF for sparkless commutation. - **High reluctance in the cross-flux path:** pole tips chamfered or made with laminations having slots, or a larger air gap at pole tips. - **Strong main field:** design the main field MMF to be large compared with the armature MMF. - **Brush shift:** moving brushes to the new MNA reduces sparking (only works for a fixed load; not used in modern machines). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Armature reaction is: A. The effect of armature-current flux on the main field flux distribution B. Reaction of brushes with commutator C. Reduction of armature resistance D. Heating of armature Answer: A. Armature current produces a cross field that distorts and weakens the main flux. (Key point: Definition) 2. If brushes remain on the geometric neutral axis, the shift of the magnetic neutral axis causes: A. Sparking B. Higher EMF C. Lower speed D. Zero loss Answer: A. Coils under commutation have EMF induced in them, causing sparking. (Key point: MNA shift) 3. Why does net flux per pole fall even without brush shift? A. Saturation makes the gain at one tip less than the loss at the other B. Armature resistance rises C. The air gap grows D. The field current is cut Answer: A. Saturation limits the increase at the strengthened tip, so the net flux decreases. (Key point: Net flux falls) 4. Which winding cancels the cross field under the pole faces? A. Compensating winding B. Shunt field C. Damper winding D. Series interpole only in the commutating zone Answer: A. The compensating winding is in pole-face slots in series with the armature with opposite current, cancelling armature MMF under poles. 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