Selection of number of stator slots

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.