What is Brushless DC Motor Cogging and How Do I Get Rid Of It?

A common issue, especially with sensorless brushless DC motors

‘Cogging’ is a general term (possibly even a colloquial term) used to describe a brushless motor appearing jumpy or jittery. It is much more prevalent at lower speeds than higher as high speeds tend to smooth out the motor considerably. An example of ‘cogging’ can be seen in the video below in the unoptimised brushless motor and controller example

Note how the motor stutters and jumps at the startup phase – rather like an engine on a cold day without enough choke on. Eventually, as the speed increases, the motor begins to smooth out and the issue is no longer prevalent. However, it can cause numerous potential issues in practice, issues at startup are common as can be seen in the video. This can even lead to entirely unreliable starting in certain cases, where the motor will start 6 times out of 10 and the other 4 it will judder, cog and then not start. 

What are the options for eliminating cogging?

There are several potential solutions for reducing or eliminating cogging which are all worth exploring. Some are more relevant to certain applications than others but it is going them.

  1. Firstly, it has to be noted that sensorless motors are much more likely to have this issue than sensored motors. Therefore, a simple fix may be to look at introducing a sensored version of the same motor rather than a sensorless motor. However, we understand that in a number of applications this may not be possible for technical and / or financial reasons. If this is the case then please read on as there are a number of potential options to reduce cogging with sensorless motors.
  2. Switching to a brushless motor with more poles – the more poles a motor has the smoother it will perform at lower speeds (where most cogging occurs). This will typically make startup easier and reduce cogging.
  3. If you are using a sensorless motor then a trapezoidal type controller will have less chance of starting up as required than will other types of controller. We’d suggest you try to use a sinusoidal brushless motor controller – sinusoidal controllers perform better at lower speeds and hence are likely to reduce cogging.
  4. Check current settings. One common cause of cogging is that the startup current settings are not set appropriately. This is especially true in applications where startup occurs under load. Increasing the power going into the motor at startup can give the motor the ‘extra kick’ that it needs to avoid cogging.
  5. Use a carefully optimised brushless motor controller. We regularly optimise our brushless motor controllers for a range of customers to help address the challenges of specific applications. As can be seen in the video above, this can lead to a significant benefits and can entirely eliminate cogging from the startup and running of the motor. 

How do I know which solution is best for my application?

There are certain solutions which may be completely impractical for your application – whether it be a cost or technical issue. For example, higher pole count motors or sinusoidal controllers typically (though not always) increase the cost of a project. Equally, sensored options are typically more costly than sensorless. 

As with many things in motor control engineering, the answer is typically bespoke to you and your project. If you need incredibly smooth startup every time then it is likely that a fully closed loop sensored system will be the best option. However, if you need reliable startup rather than smooth startup it is perfectly possible to optimise a low pole count sensorless brushless DC motor controller to deliver exactly the performance you need.

Solutions such as optimising a trapezoidal brushless DC controller like the ZDBL15 are very low cost and (depending on the required volume we can even carry this out at no cost). This method is highly reliable because it tailors the controller not just to the motor you have but also to the specific demands of the application.

If in doubt – give us a call – we’re happy to help

As mentioned above, finding the right answer to resolving a cogging issue is often application specific. We would always encourage you to have a quick look at our optimised options as these are increasingly popular due to the time and cost savings available by doing it. 

However, we do always encourage you to get in touch with us if you are having this issue or would like to discuss the best option for your particular application. You can click here for contact details for us and if you have any questions please ask.

Category: Brushless Motor Controller FAQs
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