Choice of number of armature slots

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.