EMC Testing of Brushless DC Motor Controllers to ensure successful performance and compatibility
EMC testing is a crucial part of any motor controller development, especially those that are going into applications such as automotive, medical or related applications. Where this is the case there are a range of key performance and safety criteria and certifications that must be met. These vary depending on the specific industry but tend to focus on similar overall characteristics.
It is important to remember that any EMC test for a final approval must involve all of the exact components that will be used in the final production version of a particular product. This is quite simply because it can often be the case that different components can have a significant impact on each other when they are put together. A spinning motor will be emitting EMC radiation, a controller will also be emitting some EMC radiation. However, what is sometimes overlooked is the fact that the two can impact heavily on each other on occasion to make the problem even worse. This is often the case where certain frequencies can become particularly problematic.
A simple analogy of this type of problem concerns the opera singer and the glass that smashed. The reason for the glass smashing in this case (if it even happened!) was that the singer’s voice hit the resonant frequency of the particular glass and did so with such amplitude that it vibrated the glass so vigorously that it smashed. In the same way, certain components can be particularly vulnerable to certain frequencies.
So what's involved in an EMC test?
All EMC tests take place in an anechoic chamber. This chamber has been especially setup to prevent any outside EMC interference in order to enable accurate measurement of the components in the particular test rig. That said, there will always be a level of background radiation from the Earth that cannot be eliminated from the chamber and its readings. Below is an example of this type of chamber with an aerial setup to pickup EMC emitted from the test setup.
The key aim of the test in a motor control sense is to analyse the controller and it’s wider setup being used in its actual setup and being used in the broadest possible range of activities in order to identify any problem areas that may exist. As outlined at the start of the article, it may well be the case that a controller and motor combination can function perfectly in 99% of it’s performance but has a big problem area at one frequency point (perhaps at one speed in the motors operation). Where this is identified there are a number of potential solutions that can be implemented to remove this as an issue.
What can be done if a problem is identified?
Where a problem is identified by the test, there are a number of potential solutions that can be implemented. The first is to look at how the controller is isolated – either by a casing or by other methods. This can make a significant difference to emitted EMC by reflecting this back into the casing. Additional solutions such as potting the controller can also help to dissipate these issues by helping to dissipate heat (cooler electronics will perform better and is therefore likely to emit EMC).
In addition to these methods there are changes that can be made to PWM frequencies, board layouts, isolation between the control circuits and ground plane on the board and the addition of ferrites to absorb certain frequencies or reduce it to a level that is adequate for the test to be a success. The picture below shows an example of ferrites being added in testing to see how they impact on performance.
EMC Regulations are critical to my project's success - what can I do?
There are a number of industries and applications where EMC testing is hugely important in shaping the way in which controllers are designed. If your project is in one of these areas then we can certainly help.
There are several important strands to approaching the problem which must all be recognised in order to ensure success. The first is that there are certain clear design principles which must be followed in order to reduce EMC, all of which are relatively simple to implement and which are included on all of our stock controllers.
The second major area that needs to be understood and followed is to identify the chosen solution as early as possible in terms of motors and other related electronics and to ensure that these are then used in all of the testing. We do not carry out EMC tests in house but work closely with a number of partners throughout the design process to identify potential problems areas and design the solutions in to the new controller, rather than simply testing once everything is already done. This approach can identify problems early and can also point to the solution quickly, thereby saving money and time.
The final area is to think about being flexible and open minded with component selection and other options. It’s easy for us to say this but there are certain, often lower cost, types of motors that can be hugely problematic in terms of EMC, where similarly spec’d higher quality build options can be a lot better in this area.
In conclusion: not an issue if adequate planning is done
EMC testing can be one of the most problematic parts of a product design or it can be a straightforward, incorporated part of the design process. The difference will be determined by how well the project is scheduled or planned, the level of testing carried out and how quickly key component choices are made.