The Benefits of Low Cogging Torque

Torque plays a key role in choosing between radial flux and axial flux motors. Axial flux motors are often preferred in electric and hybrid applications. Cogging torque plays a critical role in motor performance. 

Matrishvan Raval, Turntide Technologies head of product, discussed cogging torque, the challenges it presents, and the benefits of low cogging torque. 

What Is Cogging Torque?

The electromagnetic fields within a motor interact with each. The two primary electromagnetic field sources are the magnetic rotor and the stator, which has electrical wires through which current flows. Both have their own electromagnetic fields with a cross coupling between them. Cogging torque is the result of these electromagnetic field interactions with discontinuity in that movement. 

“If you’ve ever turned a motor by hand, you’ll feel it turning in discrete steps,” Raval said. “If you turn it a bit faster, there’s resistance in those detents and a shaking movement of the motor.” 

The relationship between torque ripple and cogging torque is that cogging torque is typically because of inherent electromagnetic design optimizations, or lack thereof, in the motor itself.

“Torque ripple—more of a resultant condition—has cogging torque as a component,” he said. “There are other control features—the inverter control and overall mechanical construction of the powertrain itself that can lead to a torque ripple.

“Torque ripple is the summative effect. Cogging torque is one of the elements that go into it along with the mechanical powertrain construction [and] control techniques from the inverter.”

The two different winding topologies, distributed and concentrated, affect the severity of torque ripple and cogging torque. Concentrated windings are simpler to manufacture and automate, but this type results in higher cogging torque. 

Distributed windings—though more difficult to manufacture and automate—allow for harmonics optimization. This leads to greater efficiency because it eliminates wasteful torque ripple and cogging torque. It also results in lower acoustic noise levels and less vibration.

Challenges Caused by High Cogging Torque

Most applications require smooth motor functioning, especially for precision operation and applications that require low noise.

“If you don’t have smooth operation of the motor and low cogging torque, you experience a lot of vibration and jerkiness in any precision motor,” Raval said. “In addition, motion requirements that the application might have would not get fulfilled. 

Cogging torque challenges the precision control of a motor and induces vibration and jerkiness in the application.

“If you want to move at a specific speed without inducing high vibration or there’s an actuator that the motor is driving and it needs to have precision motion control, then cogging works against that precision.”

If the application is in an environment with high vibration like construction and mining, reducing equipment vibration is more important. 

“The different vibrations start to resonate with each other and, overall, affect the motor’s structural integrity and the platform in which the motor is installed,” Raval said. 

Any application that requires a smooth noise, vibration, and harshness (NVH) profile will require a motor with low cogging torque. To ensure that the motor’s cogging torque is low, “we typically rely on the motor’s electromagnetic design,” Raval said. “That’s the motor’s magnetic material. It also selects the slot pole geometry optimizations.”

Motors with a certain numeric combination feature a few stator slots and rotor magnet poles as an electromagnetic design feature. 

“That aids the smoother correlation of the stator and rotor magnetic fields. Then you end up with lower cogging torque,” Raval said, adding that, without those optimizations in place, higher cogging torque will be induced when the motor is operating.

“It’s a selling point for the original equipment or vehicle manufacturer to have a motor with inherently low cogging torque.” 

Benefits of Low Cogging Torque 

Precision control of the motor is another benefit of low cogging torque.

“There are applications that would require motion in very discrete steps,” Raval said. “The low rpm operation of most motors tends to be challenging because of inherent motor or electromagnetic design.

“If you have low cogging torque in your motor, then even at lower rpms, you’re able to have more precise control of motor movement.”

Factors transcending motor design also affecting cogging torque, including inverter control. Special techniques in inverter control like current injection can be used to reduce torque ripple and cogging torque. Instead of just a motor consideration, cogging torque ends up being a system-level consideration, according to Raval. 

“If you have a segmented armature design of stators, that heavily affects cogging torque. The segmentation itself introduces discontinuity, leading to higher cogging torque.” 

Raval said that some motor manufacturers have a segmented armature design that claims to have higher torque or power density, “but the NVH performance of those motors would be terrible compared to a motor that’s been optimized to not have cogging torque.”

Cogging Torque May Led to Damage

Because cogging torque introduces higher vibration and noise profiles, there is a chance for greater acoustic noise. 

“The same electromagnetic parameters and slot pole geometry optimizations that lead to a lower cogging torque also led to lower acoustic noise,” Raval said.

Motors can be noisy without those optimizations.

“That is the distinct line of a motor because of the harmonics arising from the stator slots and rotor poles. They have not been optimized and have resonances. That’s when you start hearing the motor whine,” Raval said. 

The side benefit of lower cogging torque is a highly optimized electromagnetic design that leads to a lower acoustic signature and noise profile. Benefits include smoother operation, low noise, low vibration, and low jerkiness. Most applications—especially passenger vehicles—call for this optimization.

The Future of the Technology 

“The gold standard is fully optimized designs with little or no cogging torque,” Raval said. “There will be incremental gains in future designs. Three to five years ago, we had few customers asking for low acoustic noise files. They now require this as a central feature.”

Some motor manufacturers, including Turntide, are jumping in, not just with electromagnetic design optimization, but control techniques helping to reduce cogging torque.