Monthly Archives: July 2022

What is Cogging Torque?

As the primary feature and the focus of ThinGap’s product lineup, a lot has been spoken about zero cogging torque in the product lineup. Cogging torque is the interaction between the permanent magnets in the rotor, and the slots of the stator. Because the rotor magnets are attracted to the stator teeth, the torque required to move the rotor changes with the relative position of the rotor to the stator. In other words, additional energy is required to break the position dependent detent torque to advance the motor.

Cogging torque adds harmonic content to the torque-versus-angle curves for each phase. When driven by a sinusoidal current, harmonic content of the torque-versus-angle curves cause undesirable variations in torque production that interfere with smooth motor rotation. Additionally, phase imbalance common in a traditional slotted motor can further exacerbate the issue. The result is a phenomenon known as torque-ripple which is a variation of torque production with position.

There are a few issues when trying to integrate a traditional cogging motor into an application that requires precision actuation. To mitigate cogging torque in a slotted motor, it is possible to design the lamination stack with a rotational skew, so the magnetic field of the stator is diagonal to that of the magnetic field of the rotor. Alternatively, the magnets can be designed with a skew. Other techniques include using different slot-pole combinations and compensating for cogging and phase imbalance in the drive electronics.

However, when integrating slotted motors into systems for precision actuation, there will always be issues. Due to the nature of manufacturing, there is always variance between two seemingly identical motors, which can have an unpredictable impact on the cogging profile and phase balance. Furthermore, solutions to minimize cogging torque typically reduce the total output of the motor. The sunk cost of trying to mitigate cogging torque on both motor costs and development labor is better spent on the correct solution: slotless, coggless, brushless DC motors from ThinGap.


The benefit of ThinGap slotless and ironless motor designs is that they eliminate cogging torque by design. Their uniquely constructed phase windings produce a balanced phase amplitude and angle relationships between phases. This results in less than 1% total harmonic distortion of the back EMF waveform which minimizes torque ripple when paired with sinusoidal drive electronics.

A graph depicting normalized torque-vs-angle curves, with 1% phase imbalance.

ThinGap’s slotless motor architecture with wave-wound coils prevents cogging since the air-core rotor has no slots. Wound around a flat bobbin with a basket weave style winding, ThinGap’s composite air core motor kits have no iron. The thin nature of the coils produces very high copper packing and a motor with a large through hole, up to 80% of the outer diameter. In addition, ThinGap’s motor kits are available with Halbach array rotors, optimizing the magnetic flux for the highest torque density in a slotless motor.

To learn more about zero-cogging motion solutions, please contact us.

ThinGap’s TG Series for Flywheel Applications

For many years, ThinGap motors have been successfully used in a variety of flywheel applications, ranging from gyro-stabilization in boats and satellites, to momentum storage for renewable energy. ThinGap’s TG Series of slotless motor kits are ideal for flywheel applications because of their highly efficient lightweight composite stator, optimal balance between torque and inertia, with negligible rotational losses, and zero cogging that is critical to achieving smooth motion at high speed.

A photo montage detailing an exploded ThinGap TG motor kit, a sailing yacht, and a satellite above the earth, with the caption "Flywheel Momentum Storage"

Boats of all sizes stand to benefit from gyroscopic stabilization. Due to Newton’s third law of motion – every action carrying an equal and opposite reaction, a gyroscopes motion can be used to stabilize a ship or boat in heavy seas. A marine gyro-stabilizer is a large, spinning flywheel housed in the depths of the ship, near the keel. As the flywheel spins, computers on the bridge orient the flywheel against the ships current rolling motion, cancelling it out. ThinGap’s high efficiency motors make an excellent flywheel stabilizer of all sizes and can withstand harsh marine environments.

Another application where gyroscopic flywheels are used to change orientation is with satellites and their use of Reaction Wheel Assemblies. Yet another case of Newton’s Third Law, when a reaction wheel is spun up inside a satellite, the spacecraft turns in the opposite direction of the flywheel’s rotation, with this being used to steer and orient spacecraft in orbit. Because motion in space is multi-axial, each satellite carries multiple reaction wheels in an assembly to freely rotate. Since 2015, ThinGap has shipped thousands of kits for reaction wheel assemblies in large commercial and Government funded constellations, and in 2022, spun off their reaction wheel assembly motor kits to a separate product line.

An emerging use for highly efficient slotless motor kits is in momentum storage. Pioneered more than a decade ago for motorsports applications, excess or available energy is transferred to a motor that spins up a flywheel, whereby storing that energy in a rotating mass as a mechanical battery, instead of as chemical energy. Advantages of retaining the energy in a momentum flywheel application includes high power density compared with conventional batteries for quicker charge and discharge. Future applications of momentum storage could even extend to emerging markets such as renewable energy.

ThinGap’s ironless stator puts all the magnetics, the heaviest part of the motor, in the rotor. This maximizes the inertia for a given weight and size requirement, meaning the necessary flywheel mass can be reduced, and sometimes fully incorporated into the rotor, resulting in a lighter weight package for the same momentum storage capacity. Due to no iron saturation in the stator, ThinGap’s peak torque capacity is much higher for a similar weight motor, giving a dynamic response significantly better than the competition at a lighter weight.

To learn more about ThinGap’s TG Series of motor kits, click here.