Interference supression for small D.C. Motors

Interference suppression for small D.C. Motors

A lot has been written on this forum regarding decoupling ICs and the suppression of interference from small electric motors. The standard 220nF capacitor across the motor brushes seems to work in most cases...but not all. It strikes me that the vast majority of threads deal only with capacitative suppression; inductive suppression is not often mentioned. With this in mind I opened the casing on a Scalextric Digital set to see how the professionals deal with the spectacular level of interference from the often rather rough motors engaging with the track through sparky pieces of copper braid. Apart from the expected ceramic capacitors across the tracks, I found no less than eleven inductive devices that appear to be there to suppress interference. I have photographed the board and marked up the inductors.



There are ferrite beads (a) slipped over the power output leads to the tracks and to the opto-switchs that detect the passing vehicles. There are ferrite chokes (b) on the power outputs to the tracks. There are inductors (c) with three turns of enamelled copper wire in the leads to each hand controller. Eleven inductors in all!

I have the impression from this forum that not a lot of PICAXE users are employing inductors as well as capacitors to suppress interference, but it seems industry does. Before I go and buy a sack-load of ferrite, would some of our experts provide a commentary on this subject? It would be most appreciated.

To simplify the "quote and reply" function of this forum, may I ask for comments in these specific areas:

Hi,

Its refreshing to see such efforts in dealing with interference suppression. Most dc motors without supressors
make pretty good spark transmitters, and as for some pc power supplies...
I think ferrite beads on picaxe leads would attenuate outgoing vhf-uhf interference, and to a lesser degree the reverse.
The only time I've ever seen inductors in motor leads were on a cassette deck motor. As for calculating the value needed,
I would think there are quite a few variables. Inductance of motor winding, self resonance of motor winding, and any in-line inductors.
Thanks for showing the photo.

regards john

Thanks, John, for your comments; I was surprised to find only one response to this topic! Eleven inductors to separate a microcontroller from a couple of slot cars and so little comment? Perhaps I posted at a bad time of the week. Capacitors seem to pervade this topic when it comes to interference on various power or logic lines - buckets of expertise seems to be on this forum, but what about the use of inductors, folks? I would really appreciate a little more guidance... Hippy, Dippy, Westy, Technical... and all the other venerable experts; do you have any advice?

http://reprap.org/wiki/Nophead's_Extruder_Tweaks#Interference_suppressor

Quite a bit of info under "interference suppressor"

The reason inductive suppression is not often referenced here is simple.
If somebody is asking it's they don't know, more often than not they will also fail to understand the reasons given but will be able to follow the suggestions given and will usually acheive the desired result. With capacitive solutions it's about using the appropriate type of capacitor and putting it in the correct place. When it comes to inductive solutions, much much more information is required about the details of the type of noise generated and will often involve the requirement of 'scope plots, detailed layout schematics and far more information than is ever likely to be forthcomming.

The bulk of simplistic interferance problems are CAUSED by inductance in tracks, cabling and even the components themselves. Slapping a few capacitors in not quite the right place can make things worse but in most cases is unlikely to. Slapping a few inductors (or even ferrite beads) in a few cables will in most cases make things a lot worse. For example, if you were to fit ferrites in your motor leads without fitting a cap across the motor, you would increase the noise and not reduce it. If you fit them near the drive output rather than the motor you will again increase the noise generated.

Ferrite beads are more to do with the reduction of very high frequency (but low energy) noise which is more about radiated noise and is required more to meet legislative compliance than it is to do with preventing glitches that might upset your PICAXE. So, although ferrites play an important part, they will not very often solve the type of problem encountered by newbies here.

Moving on to inductors. These have been mentioned where appropriate but it's not a topic for the faint hearted. You need to consider not only the inductance but also the frequency, current and core material. Again for those that understand the reasoning they probably don't need to ask, for those that don't, any response is likely to cause further confusion.

Finally, not many hobiest part bins contain ferrite beads or even inductors so suggesting their use will result in the need to buy parts and advice on what and where to get.

So, long story short, from my part, I avoid suggesting the use of inductors because I'm lazy and don't want to enter into long threads about how/what/where and above all, I don't want to risk causing extra expense that may easily result in making things worse. It's a bit of a catch 22. If you know, you don't need to ask. If you need to ask, the answer is difficult to explain outside a classroom environment.

I was always of the impression that inductors were for the voltage portion and capacitors for the current portion of any waveform. Given capacitors pass ac and not dc and inductors pass dc and not ac.

Bluejets, thank you very much for a spot-on reference. The link refers to a clear step-by-step analysis with an oscilloscope of the interference from a motor that is commonly used by hobbyists. The author shows the impact of capacitors and then the additional benefit of ferrites in the removal of interference, albeit on a somewhat empirical basis.
The interference that was suppressed was causing issues with I2C communications and the 'scope shows the superimposition of the interference on the I2C signal before and after ferrites. The combination of capacitors and ferrites solved the problem. That is precisely the sort of information that I was seeking.

Beaniebots, thank you for a further discourse upon the subject.

I'm with BB on this. Capacitors can be treated pretty much like plug-and-play, whilst inductors (and ferrites) can be far more complicated, requiring equipment and mathematics beyond this forums (and Picaxes) intended demographic. Of course they have their place, and it can be fascinating stuff, but most folk here would rather just see their Picaxe do something cool, than spend hours over a hot CRO and calculator.

Of course, if you can find a way to provide simplified inductor based solutions to specific questions, I'm sure everyone could benefit. (Not least me!)