---
title: "Selection of number of stator slots"
subject: "Electric Machine Design"
chapter: 4
key: "semester_5/electric_machine_design:stator-slot-number-selection"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/stator-slot-number-selection
site: BCT Engineering
---

# Selection of number of stator slots

Electric Machine Design · Chapter 4: Three phase induction motor design

- Asked in: 2076 Chaitra, 6 marks; 2076 Asoj, 6 marks; 2074 Asoj, 6 marks; 2071 Shrawan, 6 marks (asked 4 times)
- Video: [Selection of stator slots](https://www.youtube.com/watch?v=ofmq_kBuVhY) — Praful Nandankar 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/stator-slot-number-selection
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-4

## Question

Explain the considerations to be made while selecting the number of stator slots for a 3-phase induction motor.

## Short answer

More slots lower leakage reactance, pulsation losses, noise, and improve cooling, but raise cost and may weaken teeth. Slot pitch 15–25 mm, q ≥2 integer, avoid harmful slot combinations with rotor to prevent cogging and crawling.

## Answer

The number of stator slots $S_s$ is fixed after the main dimensions are known. A large number of slots and a small number of slots each have advantages, so the choice is a compromise.

#### Advantages of a large number of slots
- Lower leakage reactance (conductors spread over more slots), so higher overload capacity.
- Lower tooth pulsation losses and less magnetic noise.
- Better heat dissipation, because the copper is spread over more slot surface.
- Smaller slots, so fewer conductors per slot and lower slot insulation stress.

#### Disadvantages of a large number of slots
- More slots means more insulation and more labour, so higher cost.
- Teeth become narrow and mechanically weak, and the tooth flux density may exceed the limit.
- A larger fraction of the slot area is taken by insulation, so the slot space factor is poor.
- Magnetising current increases because the teeth may saturate.

#### Practical rules for selecting $S_s$
1. **Slot pitch**: $y_s = \pi D/S_s$ should normally be between 15 mm and 25 mm. For small motors it can be as low as 10–15 mm.
2. **Integral slot winding**: $S_s = 3 \times P \times q$, where $q$ (slots per pole per phase) is a whole number. Use $q \ge 2$, and preferably $q \ge 3$ for a good emf waveform.
3. **Tooth flux density**: the minimum tooth width must keep $B_t \le 1.7$ Wb/m².
4. **Conductors per slot**: $6T_s/S_s$ must be a whole number (even for a double-layer winding).
5. **Slot combination**: $S_s$ must suit the rotor slot number $S_r$. Always choose $S_s \ne S_r$, and avoid certain differences between them, to prevent cogging, crawling, noise and vibration.

#### Example
For a 4-pole motor with $D = 0.18$ m, try $q = 3$: $S_s = 3 \times 4 \times 3 = 36$. The slot pitch is $\pi \times 180/36 = 15.7$ mm, which lies in the normal range, so 36 slots are chosen.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is an advantage of a large number of stator slots?
   A. Lower cost and less insulation
   B. Stronger, wider teeth
   C. Lower leakage reactance and less tooth pulsation loss
   D. Higher slot space factor
   Answer: C. Conductors spread over more slots lower leakage reactance and pulsation loss; cost, tooth strength and space factor get worse. (Key point: Advantages of many slots)

2. What rule is used for slots in the example of a 4-pole motor with q = 3?
   A. Ss = P*q = 12
   B. Ss = 3*P*q = 36, with slot pitch between about 15 and 25 mm (15.7 mm for D = 180 mm)
   C. Ss = 3*q = 9
   D. Ss = 6*P*q = 72
   Answer: B. Integral slot winding gives Ss = 3Pq = 36 and the slot pitch pi*180/36 = 15.7 mm lies in the normal range. (Key point: Slot pitch and integral slots)

3. Why must Ss differ from Sr (and certain differences be avoided)?
   A. To increase the leakage reactance
   B. To reduce the number of conductors
   C. To avoid cogging, crawling, noise and vibration
   D. To allow a smaller air gap
   Answer: C. Poor slot combinations give locking and harmonic torques causing cogging, crawling, noise and vibration. (Key point: Stator and rotor slot combination)

## Related in Electric Machine Design

- [Unequal stator/rotor slots and output equation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslots-unequal-output-equation) — Electric Machine Design, chapter 4
- [Rotor slots and turns, 90 kW motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Arotor-design-90kw-slip-ring) — Electric Machine Design, chapter 4
- [2.2 kW cage motor design with stator core data](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-2-2kw-winding-factor) — Electric Machine Design, chapter 4, asked 3×
- [Choice of number of armature slots](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-armature-slots-selection) — Electric Machine Design, chapter 5
- [Stator design of 120 kW 750 rpm motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Astator-120kw-750rpm) — Electric Machine Design, chapter 4, asked 2×
- [Stator bore, length and conductors of 70 hp motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Astator-bore-70hp-motor) — Electric Machine Design, chapter 4

## Related in other subjects

- [Induction motor current with load; stator connection](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-current-adjust-load-stator-connection) — Electrical Machines, chapter 5
- [Electrical considerations in conductor selection](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aconductor-electrical-considerations) — Transmission and Distribution System Design, chapter 4, asked 3×
- [Selecting main stations; number of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afactors-main-station-number-of-offsets) — Surveying I, chapter 3
- [Split phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asplit-phase-im) — Electrical Machine, chapter 7