Motor Core 10: Slot/Poles – How They Shape Stator and Rotor Design Choices

When engineers argue about motors, they usually talk about magnets, copper fill, or inverter tricks. But quietly, underneath all of that, one very simple decision is steering almost everything:

How many stator slots, and how many rotor poles, are you using?

That slot/pole combination decides whether your motor hums smoothly or screams, whether your magnets run cool or cook, and whether your machine is easy to manufacture or a nightmare. Recent research keeps coming back to the same conclusion: slot/pole choice is the first design decision you should get right, especially with modern fractional-slot concentrated winding (FSCW) machines.

• By the end of this article, you’ll be able to:
  • Read a slot/pole notation like “12s/10p” and immediately picture what that means for winding, cogging, and torque.
  • Understand how slots and poles jointly constrain stator tooth geometry, rotor magnet layout, and losses.
  • Compare 10 real-world slot/pole combinations and see which families are suited to EVs, drones, pumps, or direct-drive.
  • Ask much sharper questions when a supplier proposes a “standard” lamination or rotor pole count.

1. Slots, Poles and q: the 30-second mental model

Let’s fix terminology for a 3-phase machine (most of what follows generalises easily):

• Slots (Q) – teeth and slots in the stator core where your copper lives.
• Poles (2p) – north/south magnetic poles around the rotor (or stator, in some topologies).
• Slots per pole per phase (q) – the key ratio:

[ q = \frac{Q}{m \cdot 2p} \quad \text{(with } m = 3 \text{ for a 3-phase machine)} ]

This single number, q, tells you whether your winding is “integral-slot” (integer q) or “fractional-slot” (non-integer q). Fractional-slot concentrated winding (FSCW) machines – now common in EVs, aerospace, and generators – deliberately choose q < 1 for high torque density and short end turns.

• What slot/pole choice really controls (in plain language):
  • Torque density – more poles usually mean more torque per volume at low speed, but also lower base speed.
  • Cogging and torque ripple – certain combinations give nasty torque pulsations; others almost “average them out”.
  • Winding factor – how effectively your MMF fundamental adds up; poor combinations waste copper and magnet volume.
  • Noise & vibration – some slot/pole patterns drive strong radial forces into the stator, leading to acoustic noise.
  • Manufacturability – complexity of coil insertion, need for skew, lamination variety, and magnet segmentation.

2. Integral vs Fractional-Slot: the fork in the road

Historically, big industrial motors started with integral-slot distributed windings:

• Example: 36 slots / 4 poles, 3-phase
  • q = 36 / (3·4) = 3
  • Many small, overlapping coils → very sinusoidal MMF, low harmonic content, low torque ripple.

Then high-pole-count PM machines and direct-drive applications arrived. To keep copper short and simplify winding, designers moved to fractional-slot concentrated windings (FSCW) where each tooth carries a concentrated coil and q is fractional.

This wasn’t just a winding fashion change – it fundamentally changed how we pick slot/pole pairs. Instead of “whatever gives a nice distributed winding”, you now target:

• A high fundamental winding factor (≈ 0.9 or better for many designs).
• A high cogging torque frequency (so individual cogging pulses are small).

• Quick mental classification using q:
  • q ≥ 2 → “classical” distributed winding (e.g., 36/4): smooth torque, but more copper length and more complex winding.
  • 1 ≤ q < 2 → compact distributed or semi-concentrated; often used in industrial PMSMs.
  • 0.25 ≤ q < 1 → fractional-slot concentrated; dominates modern high-pole PM machines and wheel-hub motors.
  • q < 0.25 → extreme fractional; usually too many poles for the slot count, bringing strong parasitics unless very carefully designed.

3. How slot/pole combinations reshape stator design

Once you pick Q and 2p, your stator geometry space collapses down to a smaller set of viable options:

• Tooth width & saturation.
  • Fewer slots (small Q) → wide teeth that may saturate if you push flux density, limiting torque.
  • More slots → narrower teeth; easy to saturate, but you get better control of MMF shape and more options to tweak slot openings.
• Slot opening & harmonics.
  • The ratio of tooth tip width to slot opening controls air-gap permeance variation – and therefore cogging torque.
  • Close slot openings and particular Q/2p combinations can drastically reduce cogging, but may complicate manufacturing and insertion.
• Thermal path and fill factor.
  • High slot counts give more perimeter for heat to escape but also more insulation interfaces.
  • Fractional-slot windings may simplify coil shapes and improve copper packing in each slot, balancing out the smaller slot area.

• Stator-side checklist when you’re staring at a proposed slot/pole pair:
  • “Is q inside a comfortable range (≈0.25–3) for my manufacturing and winding type?”
  • “Can I achieve the target tooth flux density without pushing into deep saturation?”
  • “Do I have enough slot area for copper and insulation at my required current density?”
  • “Do I need tricks like slot skew, dummy slots, or tooth notches to manage cogging for this combination?”
  • “Does this Q allow me to reuse existing lamination tooling or does it imply a new punch set?”

4. Rotor consequences you can’t ignore

Change the slot/pole combination and you change the rotor’s entire job: how it carries flux, how magnets are sized and placed, and which harmonics hit the magnets and shaft.

For surface-mounted PMSMs and SPM machines, recent comparative studies show that pole/slot choices strongly affect:

• Back-EMF waveform shape.
• Cogging torque amplitude and frequency.
• Losses in magnets and rotor core.

For interior PM (IPM) or reluctance machines, the same slot/pole pair dictates where you can put flux barriers and how well your d- and q-axis inductances separate – crucial for field-weakening.

• Rotor-side questions to ask for any slot/pole proposal:
  • “What pole arc (magnet span) will I need to get good torque without over-saturating stator teeth?”
  • “Is the cogging torque frequency high enough that its amplitude stays small?” (Higher LCM of slots and poles → higher frequency, smaller amplitude.)
  • “Will sub-harmonics from this slot/pole pair drive troublesome vibration modes in my housing or shaft?”
  • “Can I segment magnets or skew the rotor without making assembly or cost impossible?”
  • “Does this combination fit my maximum mechanical speed (centrifugal stress vs magnet density)?”

5. Ten real-world slot/pole combinations – and what they do

Below is a practical snapshot of 10 common or illustrative slot/pole combinations for 3-phase motors. These aren’t “good vs bad” labels – they’re starting points to think about how stator and rotor choices are linked.

q is calculated for 3-phase (m = 3): q = Q / (3·2p)

The big picture: as you slide from (36/4) down towards (48/40), you trade speed for torque, and “pretty sinewaves” for compact high-pole-count machines that demand careful harmonic and mechanical control.

• When someone proposes a slot/pole combination, ask yourself:
  • “Which row in that table does it ‘feel’ closest to?”
  • “Am I closer to the ‘industrial 36/4 world’ or the ‘48/40 direct-drive world’?”
  • “Is my lamination and magnet technology mature enough for the high-pole side of that spectrum?”

6. What research says about “good” slot/pole combinations

A lot of academic work has tried to answer “what’s the best slot/pole pair?” for different machine types. The honest summary is: it depends on your priorities – but there are patterns.

Key findings from recent literature on PM machines with concentrated windings:

• High winding factor + high cogging frequency = strong candidates.
  • Studies on FSCW machines show that combinations where the number of slots is close to the number of poles can achieve winding factors above 0.95, if the layout is symmetrical.
  • At the same time, a high least common multiple (LCM) of slots and poles raises cogging torque frequency and usually lowers its amplitude.
• But some “high-winding-factor” layouts are troublemakers.
  • Classic work by Libert & Soulard shows that combinations like Qs = 9 + 6k with p = Qs ± 1 (which includes 9/8) can have very high torque ripple and unbalanced magnetic forces unless carefully mitigated.
• Design rules of thumb for BLDC / PM machines:
  • One widely cited guideline for BLDCs is to ensure:
    • Integer number of slots per unit winding per phase.
    • 3-phase symmetry satisfied.
    • q > 0.25.
    • Pitch factor > 0.5 and overall winding factor > 0.85.
• Application-specific optimisation matters.
  • 2023–2025 studies show that “optimal” combinations differ between:
    • Drone generators (obsessed with weight and efficiency).
    • Drilling PMSMs (low speed, huge torque, strong field-weakening).
    • Radiator fan motors (must be compact, quiet, and durable).
  • In each case, slot/pole is chosen together with rotor topology, cooling concept, and control constraints – never in isolation.

• How to translate all that research into one mental rule:
  • Start with combinations that:
    • Give a fractional q between ~0.3 and 0.7 if you want compact FSCW designs.
    • Avoid notorious “unbalanced” patterns (e.g., some 9/8-type layouts) unless you know how you’ll handle torque ripple and noise.
    • Have a large LCM(Q, 2p) to push cogging frequency high and amplitude low.

7. A practical selection workflow (stator + rotor together)

Here’s a human-friendly way to pick a slot/pole combination for a new motor, that reflects what high-end research and real design offices actually do.

1. Fix the easy stuff first
   • Target speed–torque point at rated operation and maximum speed.
   • Decide on machine type: SPM, IPM, synchronous reluctance, etc.
   • Decide roughly if you’re in the “distributed” (q ≥ 1) or “concentrated” (q < 1) camp.
2. Choose a short list of candidate Q / 2p pairs
   • Use your application family:
     • Pump / fan / general purpose → start near 12/4, 24/4, 36/4.
     • Compact servo / actuator → try 12/8, 12/10, 18/16.
     • High-torque low-speed → look at 24/22, 27/22, 36/30, 48/40.
   • For each candidate, compute q and quickly reject anything outside your manufacturing comfort zone.
3. Evaluate stator-side performance
   • Compute winding factor and MMF harmonics (even quick analytical tools or spreadsheets help).
   • Check slot fill, tooth flux density, and approximate copper loss.
   • Identify what stator tricks you’d need: skew, notches, auxiliary slots, tooth-tip shaping.
4. Evaluate rotor-side performance
   • For each candidate, sketch magnet layout, pole arc, and segmentation.
   • Estimate cogging torque level and frequency (LCM-based) and check against application sensitivity.
   • Look at mechanical stress at max speed and cooling implications for magnets and rotor iron.
5. Run quick FEA on 2–3 finalists only
   • Recent papers stress that FEA is where you see saturation and leakage correctly, but you don’t need to simulate every possible combination – just the promising ones.
   • Compare:
     • Average torque and torque ripple.
     • Back-EMF shape and THD.
     • Losses and thermal hotspots.
6. Choose the “least painful” compromise
   • Rarely is there a perfect winner; the best choice is the one that:
     • Meets performance targets with margin.
     • Is manufacturable with your lamination, winding, and magnet supply chain.
     • Leaves you options (e.g., you can later notch teeth, adjust pole arc, or skew slightly without redesigning everything).

• If you remember nothing else from this article, remember this:
  • Slot/pole combinations are not just a winding table curiosity – they’re the first design lever that locks in what your stator and rotor are even allowed to do.
  • Once you commit to Q and 2p, every later optimisation is just damage control or refinement.

8. Bringing it back to “Motor Core 10”

If we think of “Motor Core 10” as the tenth essential design decision, slot/pole combinations probably belong in the top three. Everything else – magnet grade, inverter sophistication, cooling – is built on this foundation.

So next time a datasheet casually says “12-slot, 10-pole”, don’t just nod and move on. Pause and ask:

• “What does this tell me about the stator’s tooth geometry and thermal path?”
• “What does it force the rotor to do – in pole arc, segmentation, and mechanical integrity?”
• “Is this combination aligned with my priorities: silence, torque, cost, or efficiency?”

Once you start seeing slot/pole combinations as design levers instead of just numbers, you’ll find it much easier to out-engineer competitors – and to have better, more grounded conversations with your lamination suppliers and motor manufacturers.