--- title: "Choice of specific loadings in DC machine" subject: "Electric Machine Design" chapter: 5 key: "semester_5/electric_machine_design:dc-specific-loadings-selection" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-specific-loadings-selection site: BCT Engineering --- # Choice of specific loadings in DC machine Electric Machine Design · Chapter 5: DC Machine Design - Asked in: 2080 Bhadra, 8 marks; 2075 Chaitra, 8 marks (asked 2 times) - Video: [Electrical Machine Design: Choice of specific electrical and magnetic loading | Unorthodox Learning|](https://www.youtube.com/watch?v=Dynadd4gggQ) — Unorthodox Learning - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-specific-loadings-selection - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-5 ## Question Explain various factors that should be considered while selecting the value of specific electrical and magnetic loading in dc machine. ## Short answer Specific magnetic loading (B_av) is limited by tooth saturation, iron loss (increases with frequency and square of flux density), field mmf, and armature reaction. Specific electric loading (ac) is limited by temperature rise, speed, insulation requirements, armature reaction, commutation (reactance voltage), and copper loss. They are chosen together to balance iron and copper. ## Answer The output coefficient $C_0 = \pi^2B_{av}\,ac\times10^{-3}$ shows that high specific loadings give a small machine. Their upper limits are set by the factors below. #### Specific magnetic loading ($B_{av}$), usually 0.4–0.8 Wb/m² 1. **Flux density in teeth**: the teeth carry the gap flux through a smaller area. Tooth density should not exceed about 2.1–2.2 Wb/m² in dc machines; otherwise the teeth saturate and need a large field mmf. 2. **Frequency of flux reversal**: iron loss grows with frequency and $B^2$. High-speed or many-pole machines (high $f$) need a lower $B_{av}$. 3. **Iron loss and efficiency**: higher $B_{av}$ raises iron loss, lowers efficiency and raises temperature. 4. **Size of machine**: higher $B_{av}$ means smaller $D^2L$ and lower cost. 5. **Field mmf**: higher gap density needs more field ampere-turns, so more field copper and larger poles. 6. **Armature reaction**: a higher $B_{av}$ (stronger main field) keeps the armature mmf relatively smaller, which helps against distortion. #### Specific electric loading ($ac$), usually 15000–50000 A/m 1. **Temperature rise**: more copper loss per unit surface; good ventilation allows a higher value. 2. **Speed**: fast machines cool better and can take a higher $ac$. 3. **Voltage**: thicker insulation at high voltage leaves less room for copper, so a lower $ac$. 4. **Size of machine**: large machines can take higher $ac$. 5. **Armature reaction**: armature AT per pole $= ac\,\pi D/2P$; high $ac$ means strong armature reaction. 6. **Commutation**: high $ac$ gives high reactance voltage and poor commutation. 7. **Copper loss and efficiency**: increase with $ac$. #### Summary | Factor | Limits $B_{av}$ | Limits $ac$ | |---|---|---| | Teeth saturation | Yes | No | | Iron loss, frequency | Yes | No | | Copper loss, heating | Little | Yes | | Armature reaction | Helps if high | Worse if high | | Commutation | Little | Yes | | Voltage (insulation) | No | Yes | The ratio $B_{av}/ac$ also fixes the ratio of iron to copper in the machine, so the two are chosen together. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is Bav limited by teeth? A. Teeth have larger area, so density is low B. Teeth do not carry flux C. Teeth carry gap flux through smaller area, so density above about 2.1-2.2 Wb/m^2 saturates them D. Teeth density does not depend on Bav Answer: C. Saturation demands a large field mmf. (Key point: Tooth density limit) 2. Why do high-speed or many-pole machines need lower Bav? A. Copper loss rises B. Commutation worsens C. Insulation space shrinks D. Higher frequency raises iron loss, which grows with f and B^2 Answer: D. Iron loss depends on frequency and B^2. (Key point: Frequency and Bav) 3. Which factor limits ac but not Bav? A. Voltage (insulation thickness) B. Teeth saturation C. Iron loss D. Frequency of flux reversal Answer: A. High voltage insulation takes slot space, limiting copper; the summary table marks only ac. (Key point: Limit on ac) 4. How does a higher Bav affect armature reaction? A. It worsens armature reaction B. The stronger main field keeps armature mmf relatively smaller, which helps C. It eliminates commutation problems D. No relation Answer: B. A high ac, in contrast, strengthens armature reaction. 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