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Cogging torque

Cogging torque is the pulsating, position-dependent torque generated in permanent magnet (PM) electric machines due to the magnetic interaction between the rotor's permanent magnets and the stator's slotted structure, even in the absence of stator current. This phenomenon arises from the tendency of the rotor to align its magnetic poles with the lowest-reluctance paths provided by the stator teeth, resulting in a no-load torque variation that repeats periodically with rotor rotation.^[1] The primary cause of cogging torque stems from the harmonic interaction between the magnetomotive force (MMF) produced by the rotor's permanent magnets and the air-gap permeance variations induced by the stator slots. In PM machines, such as synchronous motors and generators, the discrete slots create spatial harmonics in the magnetic field, leading to detent torque that does not contribute to the average output torque but introduces undesirable oscillations. The magnitude and frequency of cogging torque are influenced by key design parameters, including the number of stator slots and rotor poles, the slot opening width, and the magnet pole-arc ratio; for instance, combinations where the least common multiple of slots and poles is large tend to produce lower cogging torque.^[2]^[3] Cogging torque significantly impacts machine performance, particularly at low speeds and light loads, by causing torque ripples, speed fluctuations, acoustic noise, and mechanical vibrations that can reduce efficiency and precision in applications like electric vehicles, wind turbines, and servo systems. These effects are more pronounced in high-pole-number designs and can exacerbate wear on bearings and couplings over time. To mitigate cogging torque, various techniques are employed, such as skewing the rotor or stator to average out the torque variations, optimizing slot-pole combinations for minimal harmonic content, and modifying magnet or slot geometries (e.g., notching or auxiliary slots) to disrupt the alignment forces, often achieving reductions of up to 80-90% without substantially compromising other performance metrics.^[1]^[4]

Fundamentals

Definition

Cogging torque, also referred to as detent torque or no-current torque, is the oscillatory, position-dependent torque generated in permanent magnet (PM) machines due to the interaction between the rotor's permanent magnets and the stator's slots or teeth. This phenomenon manifests as a reluctance torque at no load, where the rotor experiences preferred angular positions as it aligns with the varying magnetic reluctance paths defined by the stator geometry.^[5]^[6] A key characteristic of cogging torque is its periodic variation with rotor position, with the frequency of oscillation determined by the least common multiple of the number of stator slots (N_s) and rotor poles (p), resulting in multiple pulses per mechanical revolution. The amplitude of this torque depends on the specific combination of p and N_s, as well as factors like the air-gap flux density. Although it averages to zero over a complete cycle, cogging torque produces a discontinuous torque profile that can disrupt smooth rotation.^[6]^[5] Unlike reluctance torque, which arises from rotor saliency and requires stator current for excitation, or torque ripple, which encompasses load-dependent variations including current harmonics, cogging torque is inherent to the machine's unenergized state and independent of electrical excitation. It is predominantly observed in PM synchronous motors (PMSM), brushless DC (BLDC) motors, and stepper motors, where slotted stators are common.^[7]^[8]

Causes

Cogging torque in permanent magnet (PM) motors originates from the electromagnetic interaction between the rotor-mounted permanent magnets and the stator's slotted structure, specifically the variation in magnetic reluctance along the air gap as the rotor rotates. When the rotor magnets align with the stator teeth, the magnetic flux path experiences lower reluctance, allowing easier flux penetration into the iron core and minimizing the system's magnetic energy. Conversely, misalignment with the slots increases reluctance, creating a higher energy state that the system seeks to avoid, resulting in a detent-like torque that prefers certain angular positions. This reluctance variation is inherent to slotted stator designs and occurs even under no-load conditions with zero stator current.^[9]^[10] The geometry of the motor plays a central role in determining the characteristics of this torque, particularly through the interplay between the rotor PM flux distribution and the stator iron teeth. The periodicity of the cogging torque waveform is governed by the least common multiple (LCM) of the number of stator slots N_s and the number of poles p, where p is the total number of poles; this LCM defines the number of cogging torque cycles per mechanical revolution. For instance, in a PM motor with 4 poles (p = 4) and 12 stator slots, the LCM(4, 12) = 12, leading to 12 discrete alignment positions per revolution where the reluctance is minimized. Such geometric configurations create a repeatable pattern of torque pulsations tied directly to the slot-pole arrangement.^[11]^[12] Several design parameters further influence the magnitude and profile of cogging torque by modulating the reluctance variation. The slot opening width affects the sharpness of the flux path transitions, with narrower openings typically increasing the peak reluctance changes and thus amplifying cogging amplitude. Similarly, stator tooth width determines the saturation levels in the iron, altering flux concentration and energy storage. Non-uniform air gaps exacerbate uneven reluctance paths, while the pattern and strength of PM magnetization influence the flux density distribution across the rotor surface, contributing to asymmetric torque components. Motors with higher slot-to-pole ratios often exhibit more pronounced cogging effects due to increased interaction frequency between slots and poles, leading to a greater number of reluctance minima per revolution that can compound if not balanced.^[13]^[14]^[15]

Mathematical Modeling

Principles

The foundational principle for modeling cogging torque in permanent magnet (PM) machines relies on the energy method, which treats the torque as the negative derivative of the magnetic co-energy with respect to the rotor angular position. Specifically, the cogging torque is expressed as T_{\text{cog}}(\theta) = -\frac{\partial W_{\text{co}}(\theta)}{\partial \theta}, where W_{\text{co}} represents the co-energy stored in the magnetic field under constant current conditions.^[16]^[17] In no-load operation, where stator currents are zero, this co-energy arises solely from the interaction between the PMs and the stator slots, capturing the reluctance variations in the air gap.^[16] An alternative approach employs the Maxwell stress tensor to compute the torque by integrating the tangential stress components across the air-gap surfaces. The torque is derived from the surface integral of the electromagnetic stress tensor, incorporating the magnetic field intensity vectors \mathbf{H} and \mathbf{B} in the air gap, which yields the detent torque due to geometric misalignment between rotor poles and stator slots.^[18] This method emphasizes the force densities acting on the rotor, providing a direct electromagnetic perspective on the cogging phenomenon.^[18] The waveform of cogging torque exhibits a periodic harmonic nature, typically represented as a Fourier series: T_{\text{cog}}(\theta) = \sum_{k=1}^{\infty} T_k \sin(k N_c \theta + \phi_k), where T_k and \phi_k are the amplitude and phase of the k-th harmonic, respectively, and N_c = \frac{\mathrm{LCM}(N_s, 2p)}{2p} denotes the number of cogging torque cycles per mechanical revolution, with p as the number of pole pairs and N_s as the number of stator slots.^[19]^[20] This harmonic decomposition highlights the dominant spatial frequencies arising from slot-pole interactions.^[19] These modeling principles operate under key assumptions, including the neglect of magnetic saturation, eddy currents, and end effects, which simplifies the analysis to linear magnetic behavior.^[21] Such approximations are particularly valid for surface-mounted permanent magnet (PM) motors, where the air-gap flux distribution remains relatively uniform.^[21]

Calculation and Simulation

Analytical methods for cogging torque calculation rely on closed-form expressions derived from the virtual work principle or air-gap permeance functions, providing efficient predictions without extensive computational resources. The virtual work principle determines cogging torque as the negative derivative of the magnetic co-energy with respect to the rotor angular position, expressed as T_{cog} = -\frac{1}{2} \frac{d}{d\theta} \sum_i \Lambda_i \Phi_i^2, where \Lambda_i represents the permeance of the i-th flux path, \Phi_i is the corresponding magnetic flux, and \theta is the mechanical angle.^[22] Permeance functions model the variation in air-gap reluctance due to stator slots, typically incorporating Fourier series to capture harmonic components of the permeance waveform.^[23] The amplitude of the cogging torque in these models is proportional to B_g^2 A_g r l / \mu_0, where B_g denotes the air-gap flux density, A_g the effective pole area, r the rotor radius, l the axial stack length, and \mu_0 the permeability of free space; this scaling highlights the quadratic dependence on flux density and the influence of machine geometry. Numerical simulations employ the finite element method (FEM) to solve Maxwell's equations for detailed 2D or 3D modeling of the magnetic field distribution, capturing nonlinear effects and complex geometries that analytical approaches may approximate. In FEM, cogging torque is computed by integrating the Maxwell stress tensor over an air-gap surface or applying the virtual work principle to the stored energy, yielding precise torque-versus-angle curves over one electrical period.^[24] Commercial software like ANSYS Maxwell uses low-frequency electromagnetic solvers with adaptive meshing and cyclic symmetry to efficiently simulate permanent magnet machines, enabling evaluation of torque ripple and harmonic content under no-load conditions.^[24] Similarly, JMAG performs 2D FEM analysis to predict cogging torque waveforms, such as in surface-mounted permanent magnet motors, where the torque period corresponds to the least common multiple of pole and slot numbers divided by the mechanical speed.^[25] Hybrid approaches integrate analytical Fourier-based models with FEM to accelerate design optimization while maintaining accuracy, particularly for parameter sensitivity studies like slot skew angle effects. Analytical Fourier decomposition of the air-gap flux density and permeance provides initial estimates of torque harmonics, which FEM refines through iterative simulations to minimize discrepancies.^[23] For example, sinusoidal magnet shaping derived from Fourier analysis can eliminate dominant cogging harmonics, with FEM validation confirming reductions up to 100% in torque amplitude without compromising average torque output.^[23] Validation of these methods involves direct comparison of simulated waveforms against theoretical predictions, focusing on peak amplitude, period, and harmonic spectra to quantify model fidelity. FEM-generated torque profiles typically align closely with analytical results, with errors below 5% in peak values for well-modeled geometries, as verified in permanent magnet synchronous machines.^[26] Harmonic analysis via fast Fourier transform on the waveforms reveals that optimized designs suppress higher-order components, such as the least common multiple harmonic, corroborating theoretical expectations and guiding further refinements.^[26]

Effects on Performance

Operational Impacts

Cogging torque induces torque and speed ripples in permanent magnet motors, resulting in jerky motion that is particularly evident at low speeds below 500 rpm, where the rotor experiences intermittent "jumps" between magnetic alignments.^[27]^[28] This ripple degrades overall performance by reducing positioning accuracy in servo applications, such as electro-optical systems, where even minor fluctuations can compromise targeting precision and data quality.^[27] These ripples generate secondary effects, including increased vibration and acoustic noise at harmonic frequencies, which arise from the periodic torque variations exciting mechanical resonances in the motor structure.^[29] The resulting vibrations contribute to additional mechanical losses and heating in bearings, as uneven forces accelerate wear and frictional dissipation, further reducing system efficiency.^[30]^[31] At the system level, cogging torque amplifies control challenges in electric vehicles (EVs), where it causes torque fluctuations that increase energy consumption, accelerate battery degradation, and diminish regenerative braking effectiveness in permanent magnet synchronous motors (PMSMs).^[32] In drones and high-precision robotics, it limits operational efficiency by introducing positional errors and excessive vibration, hindering tasks requiring smooth, low-speed motion and precise positioning.^[33]^[34] The impacts vary contextually; cogging torque is more pronounced in slotted axial flux permanent magnet machines due to their planar magnetic paths, which exacerbate torque ripple compared to radial flux designs.^[35] However, at high speeds, rotor inertia smooths these effects, rendering cogging torque negligible as the mechanical averaging damps the ripple.^[28]

Measurement Techniques

Direct measurement of cogging torque typically involves the use of high-precision torque transducers, such as strain gauge-based rotary sensors, mounted on the motor's output shaft in a locked-rotor configuration where the stator is fixed and the rotor is rotated slowly—either manually or via a low-speed drive—to capture the angular position-dependent torque variations.^[36]^[37] This setup isolates cogging by ensuring no electrical excitation or mechanical load, allowing the sensor to record the torque required to overcome the reluctance variations, often at speeds as low as 1-10 rpm for resolutions down to 0.018° angular precision.^[37] Strain gauge transducers are favored for their ability to provide dynamic and static measurements with accuracies better than 0.1% of full-scale range, essential for low-torque permanent magnet motors where cogging amplitudes may be in the millinewton-meter range.^[37] Alternative direct approaches include measuring the reaction torque on the stator using a clamped prototype with a balance bar attached to the shaft, which leverages mechanical equilibrium to quantify cogging without direct shaft coupling, suitable for both single-phase and three-phase permanent magnet machines handling symmetrical or asymmetrical waveforms.^[38] For multi-slot/pole configurations, a lever-arm method employs a weighted arm on the rotor shaft, a digital force gauge to record contact forces, and a resolver for position tracking, calculating cogging torque as the product of force and arm length while compensating for preload and friction.^[20] These techniques ensure high fidelity in capturing peak-to-peak values, with systems like precision geared motor-driven test benches providing automated rotation and data logging for repeatable results.^[37] Indirect methods infer cogging torque from secondary effects, such as integrating no-load back-electromotive force (back-EMF) waveforms to derive harmonic content linked to reluctance variations, or using vibration sensors like accelerometers mounted on the motor housing to detect radial forces induced by cogging harmonics.^[39]^[40] Dynamometer testing under no-load conditions at low speeds further enables indirect assessment by isolating torque ripples on the shaft, often combined with encoder feedback to correlate vibrations or EMF distortions with cogging amplitude.^[41] These approaches are valuable when direct access to the shaft is challenging, though they require calibration to map secondary signals accurately to torque. Standardized setups follow guidelines in ISO 21782 series, which specify low-speed dynamometer operation to waveform-measure cogging torque in permanent magnet motors, emphasizing precision instruments with minimal mechanical losses and angular resolutions sufficient for harmonic analysis.^[41]^[42] For permanent magnet motors, setups demand torque measurement accuracies below 0.1% full-scale to resolve low-amplitude cogging, often incorporating environmental controls to mitigate external influences like friction or misalignment.^[37] Post-measurement data processing commonly employs fast Fourier transform (FFT) analysis to extract the peak-to-peak cogging amplitude and dominant harmonics from the raw torque waveform, filtering out noise and identifying the fundamental cogging period related to slot-pole least common multiple.^[37] Averaging multiple rotational cycles—typically 6 to 12—via curve-fitting tools like MATLAB enhances signal-to-noise ratio, enabling precise quantification of cogging magnitude for performance evaluation.^[20]

Reduction Strategies

Design Modifications

One effective geometric modification for mitigating cogging torque involves skewing, where the stator stack or rotor magnets are axially skewed by one slot pitch to average out reluctance variations across the alignment positions. The skew angle is typically calculated as \alpha_{skew} = \frac{360^\circ}{N_s}, where N_s is the number of stator slots, ensuring that the magnetic interaction between rotor magnets and stator teeth is distributed over the skew length rather than peaking synchronously. This technique can reduce cogging torque by up to 90% in surface-mounted permanent magnet synchronous motors (PMSMs), though it introduces a trade-off of 5-10% reduction in average torque due to the effective shortening of the active magnetic path.^[43]^[44]^[45] Another structural approach is optimizing the slot-to-pole combination, particularly using fractional slot-per-pole configurations, which increase the cogging torque period and thereby diminish its amplitude by disrupting the least common multiple alignment of slots and poles. For instance, a 9-slot/8-pole arrangement in PMSMs results in a cogging torque amplitude that is significantly lower—often by 50-70% compared to integer slot designs—due to the non-uniform distribution of reluctance torque harmonics. This method maintains higher winding factors and average torque while inherently lowering cogging without additional manufacturing complexity.^[46]^[47] Shaping modifications, such as adjusting the magnet pole arc to less than 180° electrical degrees (\beta < 180^\circ) or implementing slot notching, further disrupt symmetry in the air-gap flux path to cancel dominant cogging harmonics. Pole arc optimization minimizes the overlap between magnet edges and slot openings, reducing peak cogging by 20-40% in brushless DC motors, while stator or rotor slot notching—typically shallow cuts along tooth tips—alters the effective slot width to phase-shift and attenuate reluctance variations. Additionally, introducing dummy slots on the stator back iron can create phase cancellation effects, further suppressing cogging torque by up to 60% without substantially impacting the fundamental flux linkage.^[48]^[49]^[50] Alternative designs like slotless or coreless stators eliminate stator slots entirely, thereby removing the primary source of reluctance variation and achieving near-zero cogging torque in PMSMs. In slotless configurations, the windings are placed directly in the air gap, which, while requiring 20-30% higher permanent magnet volume to compensate for reduced flux density, ensures smooth torque production suitable for precision applications. Coreless variants extend this by omitting iron in the stator, further eliminating eddy currents and cogging, though at the cost of lower power density and increased material usage.

Optimization Methods

Optimization methods employ computational algorithms and advanced control techniques to refine permanent magnet synchronous motor (PMSM) designs and operations, achieving cogging torque reductions that surpass those from standalone structural changes. These approaches prioritize multi-objective goals, balancing cogging minimization with torque output and efficiency, often integrating simulation tools like finite element analysis (FEA) for validation. Genetic algorithms (GA) and particle swarm optimization (PSO) facilitate multi-objective optimization of geometric parameters such as skew angle and pole arc, aiming to minimize peak cogging torque (T_{cog peak}) while maximizing average torque (T_avg). In GA applications, electromagnetic parameters of surface-mounted PMSMs are iteratively refined, alongside improved flux uniformity.^[51] PSO variants, including territory PSO, optimize surface-mounted PMSMs by simulating particle swarms that converge on superior geometries, such as adjusted pole arcs, resulting in up to 85% cogging torque decrease and a 1.4% average torque increase.^[52] Multi-objective optimization using composite algorithms, such as multi-variable combination scanning (MVCS) with genetic algorithms, enhances outcomes in dual-stator PMSMs, achieving 90.66% cogging reduction.^[53] Harmonic injection control mitigates cogging torque via current waveform modulation in drive electronics, injecting harmonics to cancel cogging-induced pulsations. This method leverages d-q axis adjustments in PMSM controllers to generate compensatory currents that neutralize dominant cogging harmonics, reducing overall torque ripple. A fading memory Kalman filter-enhanced harmonic injection suppresses pulsations in PMSMs, with simulations showing effective cogging cancellation across operating speeds.^[54] Two-stage harmonic current injection provides precise targeting of low- and high-order harmonics in PM machines, minimizing cogging without compromising efficiency.^[55] Analytical models guide harmonic selection, ensuring injected currents align with cogging frequency components for optimal cancellation.^[56] Hybrid techniques integrate finite element method (FEM) simulations with artificial intelligence to accelerate cogging torque prototyping and compensation. Post-2020 advancements use machine learning algorithms, such as XGBoost, trained on FEM-generated datasets to model electromagnetic torque variations, enabling predictive optimization of cogging components in PMSMs. These models facilitate rapid iteration over design parameters, reducing simulation time while achieving torque smoothness. Machine learning also supports real-time cogging compensation in control systems by dynamically tuning parameters based on operational feedback, with AI algorithms adjusting for harmonic distortions during runtime.^[57]^[58] As of 2025, recent reviews emphasize advancements in cogging torque reduction for specialized permanent magnet machines, including axial-field flux-switching configurations that leverage finite element methodologies and novel topologies for enhanced performance. Emerging geometric techniques, such as unequal rotor slot arc (URSA), have demonstrated significant cogging torque mitigation in simulations and prototypes.^[59]^[60] Performance in these methods is assessed using the cogging torque factor (CTF), defined as CTF = T_{cog peak} / T_avg, where values below 1-5% indicate suitability for high-precision applications like robotics and electric vehicles. Optimized designs via PSO and GA routinely attain CTF under 2%, correlating with reduced vibration and enhanced motor reliability.^[53]^[52]

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