Inverter Manufacturer Recommendations
Roxburgh EMC's Product Manager, Tom, discusses the questions behind filter selection for inverters.
Engineers phoning our technical line will often ask, “what filter can I use to achieve compliance with X EMC standard in my application” The unwelcome answer is, “well it depends”. Whilst professionals in our industry are too polite to accuse me of making things unnecessarily complicated I often feel they wonder why cannot have a simple answer.
One only has to move a few kHz down the frequency spectrum and there are many products that offer guaranteed performance. Similarly, you can refer to an inverter user manual and find a wealth of information about filters, motor cable lengths and limits you will meet.
Here I take a moment to explain how these inverter manufacturer’s recommendations are often arrived at and how that fits with my seemingly less helpful prevarications.
At Roxburgh EMC we are lucky enough to work with some of the world’s most pre-eminent inverter manufacturers. As part of the development process, the drive manufacturer will want to specify EMC filters and test their performance. However, every drive application is different and it is not possible to test every permutation of motor lead, drive setting, power level and installation style. This means we have to start compromising and generalising and the following questions need to be answered.
What Motor Cable Length to Test?
Conducted emissions generally get worse as motor cables increase in length. For example, a test passed with a 10m motor cable might be failed with a 25m cable. Inverter manufacturers tend to have an idea of the most likely applications for their drives and hence the most likely motor cable lengths. Usually we test a few round numbers up to 50m though have in the past gone up to 1200m when asked to.
Which Drives to Test?
Most drive ranges have many different products with slightly different ratings. They mostly use the same technology so the theory is that it does not make sense to test every separate item so we select a representative sample. Usually this will be the largest power rating in each frame size as most drive ranges have many fewer frame sizes than individual products.
What Limit Lines?
The main European limit lines of interest are C1 (domestic and light industrial) and C2 (industrial). Ideally, every motor lead length would meet C1 with the recommended filter, however, inverters are noisy and the longer the lead the less likely that is to be achieved. This is why you might often see inverter specification such as 10m C1 and 50m C2 for a particular filter rather than specify a very high performance filter, which can achieve C1 for both lengths.
What Carrier Frequency?
Most inverters come with a default carrier frequency, which is relatively low, and the option to increase it if necessary. The higher the carrier frequency the more intense the noise so should we test all inverters at the highest possible carrier frequency and make the limits harder and more expensive to meet? Alternatively, do we test at the default (which is what most people will use) and supply a more cost effective solution. Different manufacturers chose different routes.
How to Connect the Inverter?
Here the question is easier to answer. We use CISPR standards for equipment and methods and IEC standards for generic application and specific product groups. We need a ground plane, a known impedance stabilised power supply, screened and terminated motor cables. All of these ideal test specifications may be difficult to include into a real world application.
Therefore, when the back of the inverter user guide says that for a 25m motor cable you can achieve C1 limit lines, they actually mean that is the case for an ideal installation, running at a particular carrier frequency that is not necessarily tested using your particular drive model.
We have worked with many drive manufacturers over the years, helping to develop robust test plans and timelines and often developing specific new filter ranges to meet the application demands. However, as with everything EMC, there are no guarantees. Best practise and best theory does go a long way to give the best chance to pass the all-important practical tests.
Chat to us if you require advice with filter selection.