--- title: "Choice of number of armature slots" subject: "Electric Machine Design" chapter: 5 key: "semester_5/electric_machine_design:dc-armature-slots-selection" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-armature-slots-selection site: BCT Engineering --- # Choice of number of armature slots Electric Machine Design · Chapter 5: DC Machine Design - Asked in: 2074 Asoj, 6 marks - Video: [Lecture Video4_18EE55_Module-2_Choice of armature slots _Gopinath K](https://www.youtube.com/watch?v=_eCq7cv5ins) — Sairam EE Video - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-armature-slots-selection - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-5 ## Question Explain the factors to be considered when selecting the number of armature slots in dc machine. ## Short answer Slot pitch (25–35 mm), cooling, flux pulsation (integer plus half slots under pole arc), commutation (slot loading ≤1500 ampere-conductors), tooth flux density (≤2.1–2.2 Wb/m²), cost, and suitability for the winding are key factors. ## Answer After the armature diameter is fixed, the number of armature slots $S$ is chosen to suit the winding, commutation, cooling and cost. #### 1. Slot pitch Slot pitch $y_s = \pi D/S$ is normally **25–35 mm**. Too small a pitch gives weak, narrow teeth; too large a pitch gives deep, wide slots with high leakage. #### 2. Cooling More slots spread the copper over a larger surface and improve heat dissipation, so the temperature rise is lower. #### 3. Flux pulsation As the armature turns, the number of teeth under a pole face changes, causing flux pulsation, iron loss and noise. To reduce this, the number of slots under a pole arc is made (integer + ½), and the slots per pole are kept at least about 9. #### 4. Commutation Fewer conductors per slot means lower ampere-conductors per slot and lower reactance voltage. Slot loading is limited to about **1500 ampere-conductors**. More slots therefore improve commutation. #### 5. Tooth flux density More slots give narrower teeth, so tooth flux density rises. Teeth must not saturate beyond about 2.1–2.2 Wb/m². #### 6. Cost More slots mean more coils, more insulation and more labour, so higher cost. Insulation also takes a larger fraction of slot space (poorer space factor). #### 7. Suitability for winding - For a lap winding the slots per pole pair should suit equalizer connections. - For a wave winding the number of coils must satisfy the winding rule ($C = (P/2)y_c \pm 1$). - $S$ must be compatible with the number of commutator segments ($C = u \times S$, with $u$ coil sides per layer). #### Example For $D = 0.30$ m, a slot pitch of 25–35 mm gives $S = \pi \times 300/35 \approx 27$ to $\pi \times 300/25 \approx 38$. The rule of at least 9 slots per pole, with (integer + ½) slots under the pole arc, points to the upper end of this range. A 4-pole machine might use $S = 37$: this gives 9.25 slots per pole, a slot pitch of about 25.5 mm, and $0.7 \times 9.25 \approx 6.5$ slots under a pole arc of $0.7\tau$. The choice is then checked against the winding rule and the number of commutator segments. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What slot pitch range is normally used? A. 2.5-3.5 mm B. 100-150 mm C. 25-35 mm D. 250-350 mm Answer: C. Too small gives weak teeth, too large gives deep slots with high leakage. (Key point: Slot pitch) 2. How is flux pulsation reduced? A. Make slots per pole arc an integer B. Make slots per pole arc (integer + 1/2) C. Use fewer than 9 slots per pole D. Use skew-free large slots Answer: B. Also keep at least about 9 slots per pole. (Key point: Flux pulsation) 3. Why do more slots improve commutation? A. It increases iron loss B. Fewer conductors per slot reduces slot ampere-conductors and reactance voltage C. It shortens the commutator D. It reduces cost Answer: B. Slot loading is limited to about 1500 A-conductors. (Key point: Commutation) 4. How does more slots affect cost? A. Reduces cost B. Raises cost: more coils, insulation, labour, and worse space factor C. No effect D. Reduces insulation Answer: B. More slots also narrow the teeth. 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