Cutting Through the Noise
Product Manager Tom Downing discusses how to select the correct filter for Power Drive Systems.
For such a seemingly simple topic of using combinations of passive components to reduce potentially harmful radio frequency noise being put onto the supply, things can rapidly become complicated. Here I am concentrating on selecting the correct filter for Power Drive System (PDS) through six fairly simple questions:
1) What regulations do you need to meet?
Usually for a Power Drive System (PDS) the relevant European standard is EN/IEC 61800-3. This specifies four separate limit lines C1, C2, C3 and C4. C1 being the lowest and hardest to meet and C4 the highest and easiest. If you are having trouble deciding which is relevant to your application we can help, but for the sake of brevity I am assuming you already know. In most applications C1 and C2 are the most appropriate. C1 is often referred to as domestic and C2 industrial. That is a little misleading as C1 actually applies to domestic and light industrial installations so any factory that does not have its own power supply to which no domestic dwellings are connected should be included in category C1.
Other application standards will also call back to these four limit lines. Once the correct limit is selected you can begin thinking about how to go about limiting conducted emissions through filtering.
Many smaller drives have inbuilt filters. As space is limited the internal filter performance is also limited and the manufacturer will usually set some parameters related to motor cable lengths. For example they may say that the internal filter is good for 2m (motor cable) and C1 or 5m C2 as long as you use screened cable and correct installation etc. If for example you need 3m motor cables and C1 limits then you will need to consider an external filter.
The good news is that often the capacitance to ground of the internal filter can be disconnected so you only need to consider the earth leakage from the external filter.
External filters have their own rule of thumb limitations such as C1 50m and C2 100m which we can advise dependant on the specific model. It is best to do a test to confirm the system is behaving as expected, we can also help with this using either our fully calibrated pre-compliance laboratory where we have motors and cables already available in combinations up to 200m or if preferred we can bring our mobile test equipment to your UK facility.
2) What is the Voltage rating of the power supply?
All EMC filters will work at Voltages lower than their specified maximum but should not be used above. The type of power supply is also important as an alternate (typically lower) rating is usually provided for IT networks.
3) What is the power supply frequency?
Most EMC filters will work just as well at frequencies from DC to 60Hz. If the power supply is 400Hz suitable filters are available but there is less choice. Some manufacturers will be able to provide de-rating advice for use of standard filters with 400 Hz applications.
4) What is the maximum continuous operating current and ambient temperature?
A filter should be sized with a current rating above the continuous operating current of the system. Most filters specify this at an ambient of 40°C or 50°C. If you plan to exceed the ambient temperature rating of the filter you can proceed with the appropriate derating. Most manufacturers will help you calculate this. Some regulatory territories, such as UL, will demand extra testing to confirm the derating is sufficient. If you expect an inrush or transient over current most filters have generic specifications to account for this. Roxburgh EMC filters quote 150% of rating for 1 minute in 10 but this can be translated into higher current for shorter durations – we can help with these calculations.
5) What are the earth leakage requirements of your application?
Dependant on the type of installation a residual current device, RCD, will need to be fitted. This will cut the power when earth leakage breaches a certain point – typically 30mA, 100mA or 300mA. As most EMC filters contain capacitors connected between line and ground they can increase the total earth leakage of the system. Filter datasheets tend to quote typical earth leakage based on assumed parameters that should help the user make an informed decision. Many filter manufacturers offer reduced leakage models to help where leakage requirements are low.
6) What space do you have available and what terminations are required?
EMC filters are produced with huge variety of packages and termination styles. There are tall thin types and squat wide types. Dependant on rating there are styles with touch proof terminals, busbars, studs, fastons and flying leads. Many drives manufacturers have teamed up with filter producers to offer bespoke “footprint” filters which fit neatly underneath the drive. Roxburgh EMC already offer a wide variety of products which can be seen at https://www.dem-uk.com/roxburgh/ and we also routinely customise our products to meet application requirements.
This will be simple stuff to all of you expert engineers out there but hopefully it is useful to someone who is approaching these issues for the first time. We have our own experts here at Roxburgh EMC, so there is always someone available and willing to discuss any EMC conundrums you may have on your mind.