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Torque ripple

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Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum and minimum torque over one complete revolution, generally expressed as a percentage.

Examples

[edit]

A common example is "cogging torque" due to slight asymmetries in the magnetic field generated by the motor windings, which causes variations in the reluctance depending on the rotor position. This effect can be reduced by careful selection of the winding layout of the motor, or through the use of realtime controls to the power delivery.

References

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"Torque ripple", Emetor.

External links

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Media related to Torque ripples at Wikimedia Commons

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Torque ripple refers to the periodic fluctuations in the output torque of an electric motor as its rotor rotates, typically manifesting as the difference between the maximum and minimum torque values over one complete mechanical revolution.^[1] This phenomenon arises primarily from the interaction between the motor's back-electromotive force (EMF) and stator current, leading to harmonic variations in torque production.^[2] In permanent magnet synchronous motors (PMSMs) and brushless DC (BLDC) motors, common causes include cogging torque due to magnetic interactions between rotor magnets and stator slots, imperfect back-EMF waveforms, current ripples from inverter switching, and commutation errors.^[3] These variations are particularly pronounced in high-performance applications like electric vehicles and industrial drives, where smooth torque delivery is essential for efficiency and comfort. The effects of torque ripple extend beyond mere torque inconsistency, often resulting in undesirable vibrations, acoustic noise, and mechanical stress on motor components, which can accelerate wear and reduce overall system lifespan.^[4] In precision motion control systems, such as robotics or linear actuators, torque ripple contributes to positioning inaccuracies and velocity fluctuations, potentially compromising performance in servo-driven mechanisms.^[5] Quantitatively, torque ripple is often expressed as a percentage of the average torque, with levels below 10% considered acceptable for high-performance applications like electric propulsion systems.^[6] Mitigation strategies for torque ripple encompass both design modifications and advanced control techniques to achieve smoother operation without sacrificing power density. Machine design approaches include optimizing rotor and stator geometries, such as skewing slots or magnets, to minimize cogging effects, while control methods like field-oriented control (FOC) and direct torque control (DTC) actively shape current waveforms to counteract ripple harmonics.^[7] Emerging intelligent controls, including model predictive control and fuzzy logic, further enhance ripple suppression by adapting to dynamic operating conditions.^[7] These efforts are critical in modern electric motor applications, where reducing torque ripple improves energy efficiency, reduces NVH (noise, vibration, and harshness), and supports the growing adoption of electrification in automotive and renewable energy sectors.^[8]

Definition and Fundamentals

Definition

Torque in rotating electric machines is the twisting force that produces angular acceleration of the rotor, analogous to linear force in translational systems, and is essential for converting electrical energy into mechanical power.^[9] Torque ripple refers to the periodic fluctuation in the output torque of electric machines over one mechanical revolution, arising from non-ideal interactions in the electromagnetic field. It is typically quantified as a percentage using the ripple factor formula:

\text{Torque ripple} = \left( \frac{T_{\max} - T_{\min}}{T_{\text{avg}}} \right) \times 100\%

[(https://www.mosrac.com/resources/blog/torque-ripple.html)] where

T_{\max}

T_{\min}

, and

T_{\text{avg}}

represent the maximum, minimum, and average torque values, respectively, measured during steady-state operation. This metric captures the amplitude of the oscillatory component superimposed on the desired constant torque output. Torque ripple is distinct from cogging torque, which manifests specifically at no-load conditions due to geometric interactions between the stator and rotor, and from the average torque, which constitutes the steady, unidirectional component responsible for net mechanical work. While cogging torque contributes to overall ripple under load, the broader torque ripple encompasses both loaded and unloaded variations driven by multiple electromagnetic effects.^[9] The phenomenon of torque ripple has been recognized since the early development of AC machines in the early 20th century, particularly in induction motors where pulsating torques were observed during operation. Formal and systematic studies, however, emerged prominently in the 1980s with the advancement of permanent magnet motors for high-performance applications, as highlighted in seminal reviews of minimization techniques.

Mathematical Representation

Torque ripple is mathematically modeled as the periodic variation in electromagnetic torque output, superimposed on the average torque, and is commonly analyzed using Fourier series decomposition to capture its harmonic nature. The total torque

T(\theta)

as a function of rotor electrical position

\theta

is expressed as

T(\theta) = T_{\mathrm{avg}} + \sum_{k=1}^{\infty} T_k \sin(k\theta + \phi_k),

where

T_{\mathrm{avg}}

is the average (DC) torque component,

T_k

represents the amplitude of the

k

-th harmonic,

k

is the harmonic order, and

\phi_k

is the corresponding phase shift. This representation highlights the oscillatory ripple components arising from periodic interactions in the machine's magnetic field.^[10] In permanent magnet synchronous machines (PMSMs), the instantaneous torque equation incorporates both the permanent magnet flux and saliency effects, providing a foundation for ripple analysis under ideal sinusoidal conditions, with deviations introducing harmonics. The torque is given by

T = \frac{3}{2} p \left( \lambda_{\mathrm{pm}} i_q + (L_d - L_q) i_d i_q \right),

where

p

is the number of pole pairs,

\lambda_{\mathrm{pm}}

is the permanent magnet flux linkage,

i_d

and

i_q

are the direct- and quadrature-axis currents, and

L_d

and

L_q

are the respective inductances. Torque ripple emerges from the saliency term

(L_d - L_q) i_d i_q

when currents or inductances exhibit harmonic variations due to non-ideal machine design.^[11] Ripple amplitude can be further derived from back-electromotive force (back-EMF) harmonics, which reflect non-sinusoidal flux distributions. The phase back-EMF is modeled as a Fourier series

e(\theta) = \sum_{k=1}^{\infty} e_k \sin(k\theta),

where

e_k

is the amplitude of the

k

-th harmonic. The resulting torque ripple follows from the interaction of these harmonics with phase currents, approximated instantaneously as

T \approx \frac{e(\theta) i(\theta)}{\omega}

(with

\omega

as mechanical speed), producing ripple components at frequencies matching the back-EMF orders.^[12] Torque is expressed in newton-meters (Nm), and ripple is typically quantified as a percentage of the average or rated torque to assess relative magnitude across machines.^[5]

Causes

Cogging Torque

Cogging torque, also known as detent torque, arises from the interaction between the permanent magnets on the rotor and the slots in the stator of permanent magnet machines when no armature current is present, producing a periodic torque oscillation that persists even at standstill.^[13] This no-load reluctance torque causes the rotor to seek alignment positions of minimum magnetic reluctance, manifesting as a mechanical source of torque ripple in machines such as permanent magnet synchronous motors (PMSMs).^[14] The underlying mechanism involves fluctuations in the stored magnetic energy as the rotor rotates relative to the stator geometry, driving the system toward configurations that minimize reluctance. Mathematically, the cogging torque as a function of rotor position

\theta

is approximated by

T_{\text{cog}}(\theta) \approx -\frac{dW_m}{d\theta},

where

W_m

represents the magnetic co-energy under zero-current conditions.^[15] This energy-based derivation highlights how geometric interactions alone generate the torque without electrical excitation. Several factors determine the magnitude and characteristics of cogging torque, primarily the slot-pole combination defined by the number of stator slots

N_s

and rotor poles

2p

. The period of the cogging torque waveform is given by

360^\circ / \text{LCM}(N_s, 2p)

, where LCM denotes the least common multiple, establishing the fundamental spatial frequency of the ripple.^[16] Variations in airgap uniformity, such as due to manufacturing tolerances, further amplify the torque amplitude by altering the reluctance profile.^[17] For instance, in a 12-slot/10-pole PMSM, the LCM(12, 10) = 60 yields a cogging period of 6° mechanical.^[14]

Electromagnetic Harmonics

Electromagnetic harmonics in electric motors arise primarily from distortions in the magnetomotive force (MMF) due to stator winding distribution, non-sinusoidal back-electromotive force (back-EMF) waveforms in permanent magnet (PM) and brushless DC (BLDC) motors, and reluctance variations induced by magnetic saturation under load conditions.^[18]^[19]^[20] Stator winding distributions, such as concentrated or distributed configurations, introduce space harmonics in the MMF waveform, leading to uneven air-gap flux density and subsequent torque pulsations during operation.^[18] In PM and BLDC motors, the back-EMF often deviates from ideal sinusoidal shapes due to discrete magnet pole geometries and winding factors, generating odd harmonics that interact with stator currents to produce ripple.^[19] Magnetic saturation exacerbates these effects by altering inductance profiles nonlinearly, particularly in the rotor's d- and q-axes, which introduces additional reluctance-based torque components that amplify harmonic distortions.^[20] The electromagnetic torque in these machines can be expressed in the dq reference frame as

T_{em} = \frac{3}{2} p \left( \lambda_{pm} i_q + (L_d - L_q) i_d i_q \right)

, where

p

is the number of pole pairs,

\lambda_{pm}

is the permanent magnet flux linkage, and

i_d

i_q

are the d- and q-axis currents.^[21] Torque ripple emerges from the interaction of higher-order harmonics, notably the 5th and 7th orders, in both back-EMF and current waveforms.^[19] These harmonics cause periodic fluctuations in the torque-producing flux-current product, with the 5th and 7th components dominating due to their prominence in typical winding and magnet designs. In salient-pole machines, the reluctance torque component further contributes to ripple, given by

T_{rel} = \frac{3}{2} p (L_d - L_q) i_d i_q

, where

L_d

and

L_q

are the d- and q-axis inductances; saturation-induced variations in

L_d - L_q

enhance the oscillatory nature of this term under loaded conditions.^[21] In induction motors, rotor slot harmonics interact with stator MMF to generate torque ripple, manifesting as low-order pulsations that degrade smooth operation.^[22] For permanent magnet synchronous motors (PMSMs), the 6th harmonic from back-EMF and current distortions contributes to torque ripple without mitigation, highlighting the need for harmonic filtering in high-precision applications.^[12] Additionally, space vector modulation (SVM) in inverter drives introduces sideband harmonics around the carrier frequency, which couple with machine inductances to produce further torque oscillations, particularly at frequencies proportional to the modulation index.^[23]

Effects

Vibrations and Noise

Torque oscillations arising from torque ripple excite resonant modes in the rotor and shaft of electric motors, resulting in mechanical vibrations that propagate through the system. These vibrations occur at frequencies that are multiples of the fundamental electrical frequency

f_e = \frac{n \cdot rpm}{120}

, modulated by the harmonic orders of the torque ripple, where

n

is the number of poles and

rpm

is the rotational speed.^[5] In switched reluctance motors, for instance, the discrete nature of current pulses exacerbates these torque ripples, directly contributing to elevated vibration levels.^[24] Acoustic noise in electric motors stems primarily from these vibrations, manifesting as airborne sound generated by the interaction of oscillating components with the surrounding air. Unmitigated torque ripple can produce significant acoustic noise levels in permanent magnet synchronous motors and switched reluctance motors, particularly at operating speeds where resonances amplify the effect.^[25] Radial forces, arising from magnetic interactions across the air gap, contribute to structural deformation and sound radiation in the stator yoke.^[26] The noise spectrum typically exhibits peaks at frequencies corresponding to the torque ripple harmonics. Specific effects of torque ripple-induced vibrations include torsional oscillations that impose cyclic stresses on shaft couplings, potentially leading to material fatigue and premature failure over extended operation. In electric vehicles, these low-frequency components are often perceived as an audible "buzzing" sensation, especially at low speeds where driveline resonances are prominent and masking from other noises is minimal.^[27] Vibration amplitude is generally proportional to the torque ripple factor, which quantifies the peak-to-peak variation relative to average torque, thereby scaling the severity of mechanical disturbances.^[28] A notable case involves stepper motors, where inherently high torque ripple—often exceeding 20% of average torque—generates significant noise and vibrations that compromise smoothness, thereby limiting their use in precision applications such as robotics and CNC systems unless advanced current profiling is applied. Electromagnetic causes, including 5th-order harmonics from non-ideal windings, can intensify these vibrations in polyphase machines.^[29]

System Performance Degradation

Torque ripple in electric machines results in elevated root mean square (RMS) currents required to maintain the same average power output, leading to increased copper losses via higher I²R dissipation and an overall reduction in system efficiency.^[30]^[31] These current harmonics, inherent to torque variations, exacerbate resistive heating in the windings without contributing to net mechanical work.^[30] In servo applications, torque ripple directly induces velocity fluctuations, expressed as

\Delta \omega = \frac{\Delta T}{J}

where

\Delta \omega

is the angular speed variation,

\Delta T

is the torque ripple amplitude, and

J

is the rotor inertia; this disrupts precise position tracking and control stability.^[32] Such speed errors propagate to cumulative positioning inaccuracies, particularly at low speeds where inertia damping is insufficient.^[32] The harmonic currents associated with torque ripple generate uneven thermal distributions across motor components, promoting accelerated degradation of stator winding insulation through partial discharges and material aging.^[31] Over time, the resulting torsional vibrations induce bearing fatigue by imposing cyclic stresses, which shorten the mechanical lifespan of the drive system.^[33] In electric vehicle applications, torque ripple elevates noise, vibration, and harshness (NVH) levels, contributing to occupant discomfort and potential customer dissatisfaction that impacts market acceptance.^[34] These NVH effects are often quantified against acoustic standards such as ISO 3744 for sound power determination.^[35]

Analysis and Measurement

Simulation Techniques

Finite element analysis (FEA) serves as a cornerstone simulation technique for predicting torque ripple during the design phase of electric machines, particularly permanent magnet synchronous motors (PMSMs). This method involves numerically solving Maxwell's equations to determine the magnetic flux density

B(\theta)

as a function of the rotor angular position

\theta

, accounting for complex geometries, nonlinear material properties, and saturation effects. The electromagnetic torque

T

is subsequently computed using the virtual work principle, which equates the torque to the partial derivative of the magnetic co-energy with respect to rotor position:

T = \left. \frac{\partial W_c(\theta)}{\partial \theta} \right|_{i=const},

T=∂θ∂Wc(θ)

i=const, where $W_c(\theta, i) = \int_V \int_0^{\mathbf{H}(\theta, i)} \mathbf{B} \cdot d\mathbf{H}' \, dV$ is the co-energy (computed in FEA and differentiated numerically, e.g., via finite differences between rotor positions). This approach enables detailed visualization of flux distributions and torque waveforms, facilitating early identification of ripple sources such as cogging and harmonic interactions.^[36]^[37] Analytical models provide a computationally efficient alternative to FEA for rapid harmonic prediction in torque ripple analysis, relying on lumped parameter equivalents that simplify the machine into equivalent circuits or magnetic networks. These models approximate the air-gap flux and magnetomotive force (MMF) distributions, allowing quick estimation of torque harmonics without full spatial discretization. A key parameter in these models is the winding factor $k_w$ , which quantifies the reduction in MMF harmonics due to distributed windings and is given by $k_w = k_d k_p$ , where the distribution factor $k_d = \frac{\sin\left( \frac{q \gamma}{2} \right)}{q \sin\left( \frac{\gamma}{2} \right)}$ ( $q$ slots per pole per phase, $\gamma$ the electrical slot pitch in radians) and $k_p$ is the pitch factor; lower-order harmonics are similarly attenuated, directly influencing ripple amplitude. Such models are particularly useful in preliminary design iterations to optimize slot-pole combinations for minimal ripple.^[38]^[39] Time-stepping simulations extend FEA capabilities by incorporating transient dynamics through coupled electromagnetic-mechanical models, solving the time-dependent field equations at discrete intervals to capture evolving currents, flux linkages, and mechanical interactions. Implemented in commercial tools like ANSYS Maxwell or JMAG, these simulations model inverter-driven excitation and rotor motion, yielding time-resolved torque waveforms that reveal ripple under realistic operating conditions, such as variable speed or load transients. For instance, JMAG's time-stepping approach analyzes torque variations in induction and IPM motors by integrating circuit simulations with magnetic field solvers, enabling optimization of control parameters to suppress ripple. This method is essential for evaluating system-level effects like vibration propagation in coupled models.^[40]^[41] The harmonic balance method offers an efficient steady-state alternative, focusing on periodic solutions by balancing harmonic components in the governing equations rather than full time-domain transients. It decomposes the torque into Fourier series, solving for each harmonic $T_k$ via coefficients derived from assumed sinusoidal flux and current waveforms, which reduces computational cost for high-speed analyses while accurately predicting dominant ripple frequencies. This technique is particularly effective for machines with periodic geometries, avoiding the need for extensive time steps. Validation of these simulation techniques typically compares predicted torque ripple against experimental measurements from dynamometer tests, demonstrating accuracies exceeding 95% for low-order harmonics (e.g., 5th and 7th), with discrepancies often attributable to unmodeled manufacturing tolerances.^[42]^[36]

Experimental Methods

Experimental methods for measuring torque ripple in electric machines typically involve hardware-based setups to capture real-time torque variations, back-electromotive force (back-EMF) waveforms, vibrations, and associated noise under controlled operating conditions. These techniques validate theoretical models and quantify ripple amplitudes, often achieving resolutions sufficient for precision applications. Common setups use data acquisition systems to record signals at high sampling rates, enabling post-processing for ripple computation. Dynamometer testing remains a primary approach for direct torque measurement, where the motor under test is coupled to a hysteresis brake dynamometer to apply a stable load independent of speed. Torque is sensed using strain gauge-based transducers mounted on the shaft, offering accuracies around ±0.1 Nm for small to medium motors, with signals digitized via oscilloscopes or DAQ modules to capture instantaneous waveforms. Ripple is computed as the peak-to-peak variation or via Fourier analysis of the torque trace, typically at steady-state speeds, allowing correlation with operating parameters like current and rotor position.^[43]^[44]^[45] Back-EMF measurement provides an indirect assessment of torque ripple by capturing open-circuit voltage waveforms, which reflect harmonic distortions contributing to torque pulsations. Hall effect sensors detect rotor position to synchronize sampling, while the phase voltages are recorded using high-resolution ADCs during no-load rotation. Fast Fourier Transform (FFT) analysis extracts harmonic components from the back-EMF signal, correlating them to torque ripple orders; for instance, fifth and seventh harmonics often dominate in permanent magnet synchronous motors. This method is particularly useful for isolating electromagnetic causes without load influences.^[46]^[47] Vibration analysis complements direct torque measurements by detecting mechanical effects of ripple through accelerometers affixed to the motor housing. Triaxial piezoelectric accelerometers, integrated with systems like NI DAQ modules, record acceleration in radial and axial directions at sampling rates exceeding 10 kHz to capture ripple-induced frequencies. Spectrum analysis via FFT identifies components synchronous with electrical orders, such as multiples of pole pairs, quantifying vibration amplitudes that scale with torque ripple severity. This technique is essential for assessing structural resonances in assembled systems.^[48]^[49] Noise measurement evaluates acoustic signatures linked to torque ripple using microphone arrays arranged per ISO 1680 standards for airborne noise from rotating electrical machines. Precision-grade microphones capture sound pressure levels in a semi-anechoic environment, with data processed to determine sound power and correlate decibel peaks to specific torque orders, often at frequencies tied to slot harmonics. This method adheres to engineering accuracy (grade 2), revealing ripple contributions to tonal noise in applications like electric vehicles.^[50]^[51] Standardized procedures, such as those in IEEE 115 for synchronous machine testing, guide ripple measurements by specifying no-load and load conditions for precision drives to minimize vibrations. These protocols ensure reproducibility across laboratories, integrating torque, vibration, and noise data for comprehensive validation.^[52]

Mitigation

Design Modifications

Design modifications to the physical structure of permanent magnet synchronous machines (PMSMs) aim to minimize torque ripple by altering the magnetic field distribution and reducing harmonic interactions at the source. These hardware changes, such as skewing and winding configurations, directly address cogging torque and electromagnetic asymmetries without relying on post-manufacture control adjustments. Stator or rotor skewing involves an axial shift of the slots or magnets by one slot pitch, which averages the reluctance variations over the machine length and can significantly reduce the cogging torque amplitude, often approaching zero for the fundamental component when skewed by one slot pitch. This technique effectively cancels higher-order harmonics in the cogging torque waveform, though it increases the end-winding length and complicates winding assembly.^[53]^[54] Fractional slot windings, where the slot-per-pole-per-phase value

q = N_s / (2 p m)

is non-integer (

m

being the number of phases and

p

the pole pairs), inherently minimize magnetomotive force (MMF) harmonics that contribute to torque ripple. For instance, a 9-slot/8-pole configuration in PMSMs achieves lower torque ripple compared to integer-slot designs due to improved harmonic cancellation and higher winding factors.^[55]^[56] Optimization of pole-slot combinations, such as 12 slots with 10 poles, selects configurations that increase the least common multiple of slots and poles, thereby reducing the order and magnitude of cogging torque harmonics. Dummy slots can further balance reluctance asymmetries, particularly to compensate for manufacturing tolerances, leading to smoother torque profiles without significant average torque loss.^[57]^[58] Magnet shaping techniques, including sinusoidal or V-shaped permanent magnets, smooth the back-electromotive force (back-EMF) waveform by reducing spatial harmonics. Sinusoidal shaping minimizes low-order harmonics in the air-gap flux density, while V-shaped interiors in PMSMs can decrease torque ripple by around 40% through better flux distribution and reluctance torque enhancement.^[59] Airgap notching introduces shallow slots in the stator back-iron to modulate reluctance variations, effectively canceling specific harmonic components that cause torque pulsations. This method achieves significant torque ripple reductions by altering the magnetic permeance without substantially impacting the average torque output.^[60]

Control Algorithms

Control algorithms for mitigating torque ripple in permanent magnet synchronous motors (PMSMs) primarily involve real-time electrical compensation through current or voltage modulation, leveraging the motor's dynamic model to counteract harmonic distortions during operation. These methods enhance baseline performance from motor design by injecting corrective signals in the control loop, often integrated with vector control frameworks. They are particularly vital in applications requiring smooth torque delivery, such as electric vehicles, where ripple can induce vibrations exceeding acceptable limits without active suppression.^[61] One widely adopted technique is harmonic current injection, which compensates for torque harmonics by superimposing specific harmonic components onto the fundamental current references in the dq-frame. The compensating current is calculated as

i_h = -\frac{T_h}{\lambda_{pm} \cdot k}

, where

T_h

represents the harmonic torque component to be canceled,

\lambda_{pm}

is the permanent magnet flux linkage, and

k

is the torque-current coupling factor derived from the motor model. This approach targets dominant 6th-order harmonics arising from inverter switching or back-EMF distortions, achieving up to 70% reduction in ripple amplitude without significantly increasing current magnitude.^[61]^[62] Enhancements to field-oriented control (FOC) further minimize torque ripple through advanced predictive or deadbeat controllers that modulate the q-axis current (

i_q

) based on a precise motor model. Deadbeat predictive control, for instance, predicts and enforces zero current error within one sampling period by compensating for delays and cross-coupling effects, particularly effective at high speeds where traditional FOC exhibits up to 15% torque deviation due to parameter mismatches. These model-based modulations reduce ripple by dynamically adjusting

i_q

references to align with instantaneous torque demands, outperforming standard FOC in dynamic response while maintaining stability across speed ranges.^[63]^[64] Direct torque control (DTC) suppresses flux and torque ripples by refining hysteresis bands around reference values, which directly influences switching vector selection to avoid excessive deviations. Adjusting band widths—typically narrowing them adaptively via sliding mode techniques—limits torque excursions to within 5% of rated value while mitigating low-order harmonics from variable switching frequencies. This results in smoother operation at low speeds, where conventional DTC can produce ripples exceeding 10%, and reduces overall current THD by optimizing inverter commutations.^[65] For scenarios with periodic ripple, such as repeating operational cycles in servo drives, iterative learning control (ILC) iteratively refines reference currents by learning from previous iterations' errors. The algorithm updates the input currents as

i_{k+1} = i_k + \Gamma e_k

, where

\Gamma

is a learning gain matrix tuned for convergence, and

e_k

is the torque error from the k-th iteration, effectively attenuating harmonics like the 6th and 12th orders over cycles. Experimental validations show ILC significantly reducing peak-to-peak torque ripple after a few iterations, with minimal computational overhead suitable for real-time implementation.^[66]^[67] Advanced model predictive control (MPC) optimizes control actions over a finite prediction horizon, evaluating multiple voltage vectors to select the one minimizing a cost function that penalizes torque ripple alongside current errors. In EV PMSM drives, MPC horizons of 2-3 steps enable precise torque tracking, achieving ripple levels below 2% under varying loads by incorporating constraints on switching frequency and overmodulation. This method excels in handling nonlinearities, providing superior suppression compared to hysteresis-based schemes, with reported reductions up to 60% in harmonic content.^[68]^[69] Recent developments (as of 2025) include AI-optimized controls, such as sparrow search algorithm-enhanced ILC, for improved ripple suppression under parameter uncertainties, and data-driven approaches using online identification of flux harmonics for adaptive compensation without precise motor models.^[70]^[71]

Applications

Electric Vehicles

In electric vehicles (EVs), permanent magnet synchronous motors (PMSMs) and interior permanent magnet synchronous motors (IPMSMs) dominate traction applications due to their high efficiency, power density, and suitability for power ratings of 100–300 kW, enabling responsive acceleration and extended range.^[72] These motors demand exceptionally smooth torque delivery to ensure passenger comfort and vehicle stability, as torque ripple—typically arising from back-EMF harmonics and cogging effects—generates audible noise and vibrations that propagate through the chassis.^[73] Additionally, ripple induces mechanical stress on the battery pack and powertrain components, potentially accelerating degradation and reducing overall system reliability in dynamic driving scenarios.^[33] A primary challenge in EV traction is low-speed cogging torque, which manifests as uneven motion during urban stop-and-go driving, exacerbating NVH issues and compromising drivability.^[74] Harmonic torque ripple further complicates regenerative braking, where fluctuating torque can lead to instability in energy recovery and control response, particularly under variable load conditions.^[72] For instance, the 2012 Nissan Leaf employs a 48-slot/8-pole IPMSM topology, which inherently limits torque ripple to around 7.78% through distributed windings that suppress higher-order harmonics, though further reductions are achieved via rotor optimizations.^[75] Mitigation in EVs integrates motor design with advanced control strategies, such as combining multi-slot/pole configurations like 48-slot/8-pole with field-oriented control (FOC) to actively compensate for ripple harmonics while adhering to automotive NVH targets for vibration levels below perceptible thresholds.^[76] This approach not only minimizes passenger discomfort but also enhances braking stability and efficiency. As of 2025, the shift to silicon carbide (SiC)-based inverters enables switching frequencies above 20 kHz, effectively reducing current ripple and associated torque pulsations by minimizing inverter-induced harmonics.

Industrial Systems

In industrial applications, torque ripple is a significant concern for induction and synchronous motors operating in the 1-100 kW power range, where these machines drive stationary equipment such as pumps, fans, and robotic systems. These motors are commonly used in servo-driven automation setups, where even minor torque variations can compromise positional accuracy and operational smoothness. For instance, in precision manufacturing lines, torque ripple introduces unwanted fluctuations that degrade the synchrony required for high-speed assembly processes.^[77]^[78] Key challenges arise from harmonic-induced speed pulsations, particularly in conveyor belt systems, where torque ripple leads to uneven motion and product misalignment during transport. In such setups, the periodic torque variations cause the belt to accelerate and decelerate subtly, resulting in cumulative positioning errors that affect downstream processes like packaging or sorting. Similarly, cogging torque in stepper motors, which manifests as discrete torque dips during rotor-stator alignment, hinders precise positioning in industrial tasks such as CNC machining or pick-and-place operations, amplifying vibrations and reducing system reliability.^[79]^[80]^[81] Practical examples illustrate effective management of torque ripple in these contexts. In ABB systems, direct torque control (DTC) is employed to achieve low torque ripple and torque repeatability as low as 1%, enabling smooth trajectory tracking for assembly and welding tasks by directly regulating stator flux and torque without intermediate current loops.^[82]^[83] For pump applications, variable frequency drives (VFDs) integrated with vector control minimize ripple by decoupling torque and flux components, allowing precise speed adjustment while suppressing harmonic distortions that could otherwise cause flow instabilities in water treatment or HVAC systems.^[84] Mitigation strategies tailored to industrial demands include design modifications like notched airgaps in reluctance motors, which unevenly distribute magnetic flux to counteract cogging harmonics and reduce peak-to-peak torque variations by up to 20% without sacrificing average output. For cyclic loads in equipment such as printing presses, iterative learning control (ILC) algorithms adaptively compensate for repeatable torque disturbances over successive operations, iteratively refining control inputs to suppress ripple and maintain consistent sheet registration. These approaches align with performance standards like IEC 60034-1, ensuring minimal losses and vibrations in demanding environments.^[85]^[86]^[67]

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Torque ripple

Definition and Fundamentals

Definition

Mathematical Representation

Causes

Cogging Torque

Electromagnetic Harmonics

Effects

Vibrations and Noise

System Performance Degradation

Analysis and Measurement

Simulation Techniques

References

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