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Cogging Torque and Torque Ripple: What You Need to Know

15 Jun 2021
Kollmorgen Experts

Many applications require very high torque and smooth motion at relatively low motor speeds. Electro-optical/infrared systems are a prime example.

EO/IR imaging is used for military defense in a wide variety of land, air, sea and space applications. These systems provide situational awareness and targeting under all lighting conditions, including day and night, fog, smoke, blowing sand and dirt. And they must acquire and maintain an absolutely steady image while counteracting motion due to vehicle acceleration/deceleration, vibration, road shock, air turbulence and other unpredictable forces.

High torque is required to move the imaging gimbal quickly in response to relatively massive inertial forces. Smooth motion is required to provide the clearest possible image as the gimbal moves and settles into a continuous succession of new positions. Relatively low-speed motors are required to keep the system light and responsive, without the need for gearboxes or other transmission components that add complexity, compliance and backlash to the system.

The Technology Dilemma

For any application that must meet similar requirements, engineers confront a dilemma—often compounded by misconceptions about motor technology and capabilities.

Maximum available torque and torque density can best be achieved using conventional brushless motors with a slotted lamination design. Yet these motors inherently exhibit cogging torque, which might appear to affect the smoothness of operation at low speeds. Slotless motors are not subject to cogging torque, yet they may still exhibit torque ripple when energized and cannot deliver the same torque per volume as a conventional motor.

So which technology should you choose for low-speed, high-torque applications? And how can you ensure the smoothest possible motion? Let’s take a closer look at the relevant concepts.

Cogging Torque

Cogging torque is caused largely by the attraction between permanent magnets mounted on the rotor and the steel teeth of the stator laminations. Cogging can be physically felt as an intermittent “jerking” motion when you rotate the shaft of a conventional brushless motor. Slotless motors do not exhibit this property. In an unenergized state, the rotor can rotate freely because the permanent magnets are not attracted to the nonmagnetic stator coils.

While cogging torque can contribute to torque ripple, it’s important to note that the electromagnetic properties of an energized motor differ considerably from an unenergized motor, and the cogging that you feel with your fingers does not translate directly to cogging that can be “felt” by the load in motion.

Torque Ripple

Torque ripple is uneven torque production throughout the rotation of a rotor in an energized motor, caused by variances in the electromagnetic fields and their interactions between the rotor and stator. Cogging torque in a conventional motor contributes to these variances.

Cogging torque influences torque ripple in a conventional motor.

However, it is important to note that all electric motors—including slotless motors—exhibit torque ripple. This is primarily due not to cogging, but to armature reaction. As the current level rises, flux in the magnetic circuit shifts, introducing harmonics into the motor torque constant waveform. This effect is most pronounced in slotless motors at medium to high current.

Armature reaction induces harmonic distortion in electromagnetic waveforms, causing torque ripple in a slotless motor.

The claim that slotless motors don’t exhibit torque ripple is a myth. At high speeds, the inertia of the rotor and load tend to “smooth out” any torque ripple. However in low-speed applications, torque ripple can cause unwanted fluctuations in the driven load whether using a conventional or a slotless motor. For low-speed applications that depend on perfectly smooth motion, this is an important issue that needs to be addressed in the overall system design.

For example, in EO/IR systems, torque ripple can compromise the quality of visual data, affecting targeting accuracy in an offensive application or even compromising soldier safety in a defensive application.

Conventional Slotted Lamination Design or Slotless?

Marketers of slotless motors may point to the lack of cogging torque as a desirable feature for applications such as EO/IR. But cogging torque is not generally presented as a specification for servo motors, and with good reason. The behavior of the energized motor is what matters mostand,as mentioned before, all motors including slotless designs are subject to torque ripple.

For EO/IR applications, conventional brushless motors are generally the better choice. Given the same size motor, a conventional slotted lamination motor delivers higher available torque per volume. This is particularly important at the low speeds (typically under 500 rpm) required in many applications, such as our EO/IR example.

A conventional motor generates significantly greater constant and peak torque compared to a slotless motor of equivalent volume.

See our blog post, “Conventional and Slotless Motors: What You Need to Know,” for a deeper dive into these brushless motor designs and their particular application strengths.

Mitigating Torque Ripple

Assuming you have selected a conventional brushless motor to deliver maximum torque density for your low-speed application, what can you do to minimize torque ripple? Since cogging torque can contribute to ripple torque, one tool is to modify the motor to reduce cogging torque. We discuss this approach in “Conventional and Slotless Motors: What You Need to Know.”

These design variations do tend to reduce torque and torque density to a greater or lesser degree, however, and can’t completely eliminate cogging torque. Whether or not a modified motor design is used, the most powerful tool for minimizing torque ripple is the control system, including high-resolution feedback, high-bandwidth control loops and advanced, load-disturbance-canceling drive algorithms.

A high-resolution feedback signal of the torque/force being applied to the load throughout the rotation of the rotor can be brought to a summation point with the drive’s command signal, canceling the effects of torque ripple to maintain smooth acceleration and velocity. This can be compared to the 180° phase-reversal technique used to remove unwanted ambient noise in noise-canceling headphones. Advanced drives of the kind typically used in EO/IR applications can provide this torque ripple mitigation.

Likewise, when running in velocity and/or position loop mode, the velocity loop can reject torque ripple by summing the motor’s feedback signal with its command signal. When the velocity loop bandwidth is high enough to dominate the primary contributors to torque ripple, cogging torque should have no effect, while velocity ripple can be almost completely rejected.

High-resolution feedback, high-bandwidth control loops and advanced, load-disturbance-canceling drive algorithms can eliminate most torque ripple.

Putting It All Together

Between choosing the right motor technology for the application, using a modified motor design when appropriate, and applying control technologies to mitigate torque ripple, it’s possible to achieve the right balance of torque density and smoothness in a perfect-fit motor for virtually any application.

In the case of a low-speed, high-torque, super-smooth application like EO/IR, that means choosing a conventional high-torque motor—possibly with certain design modifications—to minimize cogging, and applying the appropriate feedback and control technologies to significantly reduce torque ripple. Other types of applications will have their own, very specific requirements.

Kollmorgen has the standard and modified products, the industry-specific application know-how, and the commitment to collaborative success you need to meet practically any motion requirement. Ready to discover what your application is capable of?

About the Author

Kollmorgen Experts

Kollmorgen Experts

This blog was a collaborative effort among a team of motion and automation experts here at Kollmorgen, including engineers, customer service and design experts. Wherever you are in your project, we’re here to help.

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