The winding should convert phase current into the required rotating magnetomotive force efficiently.
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PMSM ELECTROMAGNETIC DESIGN
How stator slots, rotor poles and winding layout influence torque density, back-EMF harmonics, cogging torque, NVH, copper loss and manufacturability.
ENGINEERING CONTEXT
The number of stator slots and rotor poles determines how coils are distributed around the air gap, which spatial harmonics are produced and how often magnet edges interact with slot openings. It also influences coil pitch, end-turn length, electrical frequency, lamination geometry, sensor resolution and production tooling.
A combination that provides a high fundamental winding factor may still create unacceptable rotor loss, unbalanced magnetic force or acoustic noise. Selection must be evaluated with the complete winding, magnet shape, air gap, current waveform, control strategy and manufacturing tolerance stack.
The winding should convert phase current into the required rotating magnetomotive force efficiently.
Slotting, winding harmonics, saturation and inverter current all contribute to periodic torque variation.
Pole count changes iron-loss frequency while spatial harmonics can heat magnets, sleeve and rotor steel.
Slot opening, tooth width, fill factor, winding route and end turns affect repeatability and cost.
MOTOR ARCHITECTURE
FIRST SCREENING METRIC
The value q distinguishes integer-slot and fractional-slot winding families. For a three-phase machine, q is found from stator slots Q, pole-pair count p and phase count m = 3. An integer q commonly supports distributed windings, while fractional q often supports concentrated or short-pitched layouts.
COMBINATION SCREENING
| Slots / Poles | q for 3 Phases | Typical Winding Family | Potential Advantages | Items Requiring Careful Review |
|---|---|---|---|---|
| 9 / 6 | 0.5 | Fractional-slot concentrated | Compact coils and simple tooth winding | Harmonic content, radial force and torque ripple |
| 9 / 8 | 0.375 | Fractional-slot concentrated | High pole count in compact package and short end turns | Rotor harmonic loss, force harmonics and mechanical frequency |
| 12 / 8 | 0.5 | Fractional-slot concentrated | Production-friendly coil grouping and balanced three-phase layout | Specific cogging orders, space harmonics and tooth saturation |
| 12 / 10 | 0.4 | Fractional-slot concentrated | Compact windings and commonly useful winding factor | Subharmonics, unbalanced force sensitivity and rotor eddy loss |
| 18 / 16 | 0.375 | Fractional-slot concentrated | Many poles for low-speed torque and short end turns | High electrical frequency, small tooth pitch and manufacturing tolerance |
| 24 / 4 | 2 | Integer-slot distributed | Distributed MMF and conventional winding options | Longer end turns, larger winding process and low cogging-period count |
| 36 / 6 | 2 | Integer-slot distributed | Flexible coil pitch and sinusoidal waveform potential | End-turn copper, slot count, tooling and winding complexity |
These observations are screening guidance only. Exact performance depends on winding layout, magnet pole arc, skew, air gap, saturation, rotor topology, control current and mechanical dimensions.
WINDING FACTOR
The winding factor combines distribution and coil-pitch effects for each spatial harmonic. The fundamental winding factor influences back-EMF and torque production, while harmonic winding factors influence parasitic fields, rotor loss, vibration and control behavior.
COGGING TORQUE
Cogging torque occurs without current as rotor magnets seek minimum-reluctance positions relative to stator slots. The least common multiple of slot and pole counts helps identify how many cogging periods occur per mechanical revolution, but amplitude requires magnetic analysis.
Opening width and tooth-tip shape control permeance variation at the air gap.
Arc ratio changes average air-gap flux and interaction with slot harmonics.
Rotor or stator skew can average selected harmonics but may reduce torque and complicate manufacturing.
Magnet segmentation and chamfers change edge fields and local harmonic content.
Static and dynamic air-gap variation can add orders not predicted by an ideal symmetric model.
Slot, magnet and assembly variation can shift both cogging amplitude and phase.
TORQUE RIPPLE
| Ripple Contributor | Exists at No Load? | Slot-Pole Link | Mitigation Direction |
|---|---|---|---|
| Cogging torque | Yes | Slot/pole interaction and air-gap permeance | Pole arc, slot opening, skew, shaping and tolerance control |
| Back-EMF harmonics | Voltage exists at no load; torque effect needs current | Winding and magnet spatial harmonics | Winding choice, magnet shape and current waveform |
| MMF harmonics | No | Coil distribution and phase-belt arrangement | Winding layout, coil pitch and current control |
| Saturation | Limited | Tooth/yoke dimensions linked to slot and pole pitch | Steel geometry, current loading and local flux management |
| Current ripple | No | Electrical frequency and machine inductance | Switching strategy, inductance, bus voltage and control tuning |
| Geometric asymmetry | May exist | Combination determines sensitivity and force orders | Runout, concentricity, magnet placement and assembly control |
ELECTRICAL FREQUENCY
Electrical frequency rises with pole-pair count and speed. Higher pole count can support torque in a given diameter, but it also increases stator iron-loss frequency, inverter fundamental frequency and the number of magnet transitions seen by each tooth.
NVH & RADIAL FORCE
Air-gap flux harmonics interact to create radial forces with specific spatial orders.
A modest force can become loud when its frequency intersects a structural mode.
Low-order radial forces can efficiently excite the stator and housing.
Rotation, current harmonics and eccentricity create families of acoustic orders.
Connection, turn and current imbalance can add unbalanced magnetic pull.
Stack joining, press fit, housing ribs and bearings determine radiated response.
MANUFACTURING IMPACT
| Design Variable | Potential Benefit | Manufacturing Challenge | Critical Control | Validation Evidence |
|---|---|---|---|---|
| High slot count | More winding-distribution options | Narrow teeth/slots and more tooling features | Punch accuracy, burr, tooth alignment and insulation | Lamination capability and wound-stator sections |
| High pole count | Torque at lower mechanical speed | Smaller magnets, polarity control and placement | Magnet dimensions, magnetization and rotor runout | Flux map, back-EMF and balance results |
| Concentrated coils | Short end turns and modular winding | Slot fill, tooth insulation and coil insertion force | Turns, tension, conductor damage and phase connection | Resistance, surge, sections and thermal tests |
| Distributed winding | Flexible harmonic shaping | Longer end turns and complex insertion/lacing | Coil pitch, placement, crossover and end-turn geometry | Winding map, phase balance and end-turn inspection |
| Skew | Reduced selected cogging/force harmonics | Indexing, stack alignment and winding difficulty | Skew angle, step position and cumulative error | CMM/optical map and torque/NVH correlation |
VALIDATION PLAN
Confirm coil table, phase sequence, winding factors, balance and harmonic spectrum.
Map torque, back-EMF, cogging, saturation, losses and demagnetization across the operating envelope.
Transfer radial-force harmonics into stator, housing and mount response.
Use copper, stator, magnet and rotor losses from the selected combination and control strategy.
Measure back-EMF, torque ripple, efficiency, temperature and order-tracked NVH.
Evaluate magnet placement, air-gap eccentricity, steel geometry, winding and assembly variation.
SELECTION WORKFLOW
Torque-speed, voltage, package, efficiency and NVH.
Slots, poles, layers, coil pitch and winding table.
Winding factor, cogging periods, forces and frequency.
Geometry, magnets, current, loss and tolerance.
Test correlation, DFM, controls and production limits.
RFQ CHECKLIST
FAQ
There is no universal best combination. The answer depends on torque-speed targets, winding method, voltage, efficiency, NVH, thermal limits, package and manufacturing capability.
More poles can support useful torque density in some packages, but they also increase electrical frequency and may increase iron or rotor losses. Geometry and current loading still determine the result.
No. They can generate strong spatial harmonics, but noise depends on force orders, structural modes, current control, geometry and manufacturing variation. Careful combination and structure design can produce acceptable NVH.
Not by itself. Winding factors describe harmonic coupling, while torque ripple also depends on magnet field, slotting, saturation, current harmonics, control angle and tolerances.
Skew should target identified cogging or force harmonics after considering torque reduction, axial effects, rotor loss and manufacturing difficulty. It should not be used as an automatic correction.
FROM SLOT-POLE SCREENING TO VALIDATED MOTOR
Ningbo Vanguard Technologies Co., Ltd supports motor architecture, winding synthesis, electromagnetic FEA, laminations, magnets, rotors, stators, rapid prototyping, test correlation and production development.
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