Cogging Decoded: A Thorough UK Guide to Cogging, Its Causes and How to Minimise Its Impact

Cogging torque is a familiar foe for engineers working with brushless DC motors, stepper drives and permanent magnet machines. It appears as a periodic sticking or jolt in the rotor position, especially at standstill or low speeds, and can complicate control, reduce smoothness and elevate acoustic noise. This article unpacks what Cogging is, why it happens, how it manifests across motor types, and, crucially, how to reduce or manage it in practical designs. Whether you are designing a high-precision servo, a compact actuator, or a consumer motor, understanding cogging will help you optimise performance and reliability.

What is Cogging?

Cogging is a magnetically generated torque ripple caused by the interaction between the stator teeth and the permanent magnets on the rotor. As the rotor turns, the magnetic attraction and reluctance vary with position, producing a torque that can either assist or resist motion momentarily. In other words, cogging torque arises when the magnetic fields in the machine prefer certain angular positions, creating a repeating “bump” as the rotor passes those positions. This effect is independent of electrical excitation and is most noticeable at standstill or low speeds, where drive control has limited options to smooth the motion.

Causes of Cogging

Tooth–Pole Interaction

The most fundamental cause is the periodic alignment of rotor magnets with stator slots. When the number of stator slots and rotor poles share a common divisor, certain positions become energetically favourable, and the rotor tends to “lock” into these positions. The resulting cogging torque is periodic, repeating with a mechanical angle equal to the greatest common divisor of slots and poles. Designers often exploit this relationship to predict cogging and tailor the architecture to minimise it.

Slot Openings and Winding Layout

Slot openings, slot fill, and winding distribution influence the magnetic reluctance landscape. Nonuniform slot shapes or inconsistent winding packing can exacerbate local variations in flux density, increasing cogging. Even subtle manufacturing tolerances in the slot geometry or the magnet segment boundaries can amplify the effect, particularly in compact devices where the air gap is small and the magnetic circuit is tight.

Air Gap Variations

The air gap between stator and rotor is a major determinant of cogging magnitude. A highly uniform air gap across the circumference reduces uneven magnetic attraction, while small gaps or eccentricities can magnify cogging. Manufacturing tolerances in rotor balancing or stator alignment can introduce eccentricity that makes cogging more pronounced in practice.

Magnetic Material and Geometry Choices

The choice of magnets (for example, neodymium-iron-boron) and permanent magnet spacing, along with stator slot design, governs how strongly the rotor magnets interact with the slot teeth. Highly anisotropic materials or magnets with high energy density can intensify the magnetic ripple that becomes cogging torque. Conversely, careful geometry, such as pole and slot pairing and skewing techniques, can dampen the effect.

Manufacturing Tolerances

Even well engineered designs are subject to tolerances in machining, stamping, and assembly. Tiny deviations in tooth width, slot depth, or magnet alignment can accumulate to a noticeable cogging harmonic. Quality control and precision manufacturing play a critical role in keeping cogging within acceptable bounds.

Cogging in Different Motor Types

Permanent Magnet Synchronous Motors (PMSM)

PMSMs rely on permanent magnets on the rotor and a synchronous drive field from the stator. Cogging in PMSMs is often more noticeable at standstill or very low speeds because the PWM drive cannot easily modulate torque when no back‑EMF is present. In high‑precision servo applications, cogging can degrade position accuracy and increase vibration. Mitigation requires a combination of mechanical design and control strategies.

Stepper Motors

Stepper motors are particularly susceptible to cogging because they move in discrete steps. The cogging torque interacts with the intended stepping sequence, potentially causing missed steps or resonance. Engineers frequently implement mechanical and electrical strategies to improve microstepping performance and maintain smooth motion across the rated speed range.

Brushless DC Motors (BLDC)

BLDC machines typically aim for smooth torque with wide speed ranges. Cogging in BLDCs manifests as stickiness at low speeds or during starting. In high‑duty or high‑precision BLDC drives, designers counteract cogging with skewed windings, staggered slots, and drive control strategies that pre‑empt the torque ripple during ramping.

Measuring Cogging

Torque Ripple Signatures

Cogging torque can be measured as a standing torque ripple or as a profiling of torque versus rotor position at no electrical excitation. A torque sensor or a calibrated test bench can quantify peak cogging values and their periodicity. This data helps compare different mechanical or electrical designs, guiding the choice of remedies.

Standstill and Low-Speed Testing

Because cogging is most evident near standstill, many engineers perform standstill tests with the motor lightly clamped or loaded to observe the torque ripple when the rotor is held in a fixed position and then released. Dynamic tests at low speeds can reveal how cogging interacts with drive electronics and control algorithms.

Strategies to Reduce Cogging

Mechanical and Geometric Techniques

• Skewing: Offsetting the stator slots or rotor magnets relative to each other by a deliberate angle reduces alignment coincidences, smoothing the torque profile.
• Optimised slot–pole combinations: Selecting slot counts and pole counts that minimise the coterminous harmonics lowers the cogging amplitude.
• Non‑uniform slot openings: Subtle variations in slot opening shapes and widths can disrupt regular cogging harmonics.
• Pole smoothness and magnet segmentation: Refining magnet block dimensions and segmentation improves uniform magnetic flux distribution.
• Air-gap tuning: Ensuring consistent air gaps across all teeth and poles helps reduce localized reluctance variations.

Electrical and Winding Techniques

• Winding layout and skew: Skewed windings distribute the magnetic field more evenly and dampen cogging harmonics.
• Controlled slot fill: Uniform copper distribution and careful winding pack reduce irregular flux concentrations.
• Advanced insulation and materials: Using high‑quality insulation minimises degradation that could affect the flux path over time.

Control and Drive Strategies

• Ramp profiles: Gentle current ramping during start‑up and cut‑back at low speeds can mask cogging by avoiding abrupt torque changes.
• Microstepping and current shaping: In stepper and BLDC drives, microstepping and sine‑wave current profiles smooth the torque output and suppress perceived cogging.
• Torque ripple compensation: Predictive control or feed‑forward techniques can pre‑empt the known cogging profile to maintain smoother motion.
• Active damping: Modern controllers implement closed‑loop torque damping at low speeds to counteract stickiness and jitter caused by cogging.

Material and Manufacturing Considerations

• Material selection: Choosing magnets with stable temperature and low coercivity loss helps maintain a stable cogging profile across operating conditions.
• Quality control: Tight tolerances in stamping, magnetisation, and assembly reduce the real‑world mismatch that makes cogging worse.
• Quality assurance of skew and alignment: Verifying the physical skew angles and alignment during assembly ensures the theoretical benefits are realised in practice.

Cogging Torque and System Performance

Cogging torque does not only affect smoothness; it can influence control bandwidth, precision, and reliability. In servo or high‑precision systems, cogging can limit the achievable resolution and degrade the repeatability of positioning. In consumer devices, accelerated wear or audible noise can result from ongoing cogging effects. Therefore, engineers often trade off between the lowest possible cogging and other design costs such as weight, size, efficiency, and manufacturing complexity.

Practical Guidelines for Engineers

Selecting a Motor for a Given Application

When choosing between motor options, consider the acceptable level of cogging for your application. High‑speed machines with robust drive controls may tolerate modest cogging if overall efficiency and system inertia are advantageous. For precision positioning, favour designs with proven cogging reduction features, including skew, optimised slot/pole counts, and reliable manufacturing processes.

Integrating with Drives and Control Systems

Drive electronics and control strategies play a crucial role in mitigating cogging. A well‑tuned control loop, appropriate ramping, and, where applicable, torque ripple compensation, can substantially reduce the practical impact of cogging. Don’t underestimate the value of characterising a motor on your specific drive chain, as the interaction with electronics can alter the cogging seen in theory.

Design Trade‑Offs

Many cogging reduction techniques come with trade‑offs in cost, efficiency, or power density. Skewing and advanced winding designs may increase manufacturing complexity and price. However, the payoff is often a quieter, smoother motor with better low‑speed performance, which in many applications justifies the investment.

Case Illustrations: Real‑World Scenarios

• A compact PMSM used in a CNC spindle exhibited noticeable low‑speed jitter. By applying a combination of skewed stator slots, redesigned magnet segmentation, and a refined air‑gap tolerance, the cogging torque was reduced by a factor of two without system downtime being extended.
• A high‑torque stepper motor for a 3D printer encountered start‑up hesitation. Implementing microstepping with a sine‑approximation drive curve and careful slot optimisation significantly improved smoothness and positioning accuracy at low speeds.
• A BLDC actuator in an automated valve system showed reduced audible noise after relocating from uniform slot geometry to a staggered, skewed arrangement, combined with drive‑side torque ripple compensation at low rpm.

The Future: Trends in Cogging Reduction

Advances in materials science and manufacturing continue to refine how cogging is addressed. Emerging approaches include advanced magnetic materials with lower temperature sensitivity, improved additive manufacturing for precise rotor and stator geometries, and adaptive control algorithms that learn and compensate for cogging in real time. As drives become smarter, the boundary between mechanical design and control theory grows tighter, enabling more effective suppression of cogging without sacrificing efficiency or power density.

Conclusion

Cogging is a fundamental characteristic of many permanent magnet machines, arising from the magnet–slot interaction, air‑gap dynamics, and the geometry of the motor. While it cannot be eliminated entirely in all designs, it can be substantially reduced through a thoughtful combination of mechanical design choices, winding and slot configurations, and sophisticated drive strategies. For engineers and designers aiming for smooth, predictable motion, a deliberate focus on cogging from the earliest stages of development is essential. With the right balance of skew, slot/pole optimisation, and intelligent control, Cogging can be tamed, delivering quieter operation, higher positional accuracy, and better overall machine performance.