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[dreadnought72]

Hi there!

I've got four electric motors in my boat, situated close together - but not touching - and each driving one shaft. From a magnetic point of view, each motor is affected by its neighbours, and I'm wondering if this means that radio interference will be higher than one motor running alone.

Now, I've read on this forum that suppression capacitors don't necessarily have to be situated right on the motor cans. Given the space constraints, that would be ideal. Therefore, can anyone see any issues with:

• Electrically connecting all four motor cans together to make a common rail.
• Connecting this cable to all four shafts.
• Connecting capacitors from all four +ve and -ve cables to this cable.
• Connecting capacitors between each pair of +ve/-ve cables.

The capacitor distance from the motors will be around 10cm, on "plenty thick" power cables.

Any thoughts or advice would be most welcome.

Andy

[malcolmfrary]

The motors, being probably metal canned, should each be capable of restraining their own magnetic field to a level that does not affect their neighbours.  Should.
The commoning bus rail connecting cans and shafts should do no harm, but equally might not be doing any good, either.
By + and - I assume you mean motor  + and -.  The capacitors you mention are a GOOD THING.  The capacitors accross the motor + and - are also a GOOD THING. 
Putting them 4" away I am less certain about.  If it is the only place to mount them, fair enough, but the basic prinpcipal of supression is that the supressor should be as near to the source as possible.  If you must have the 4" run, make it a twisted pair, so that any interfering signal has the chance of cancelling out.  If the cancelling does not hapen, you will have made a nice little aerial to transmit interference, and this will be assisted by the capacitor, rather than reduced.
Funny subject, interference.

[dreadnought72]

The motors, being probably metal canned, should each be capable of restraining their own magnetic field to a level that does not affect their neighbours.  Should.

As it stands, it's obvious each motors' magnetism is pulling/pushing on nearby motors, but they're all bolted in, and any forces are from the static magnets, which won't change when in operation. So I shan't worry about this.

The commoning bus rail connecting cans and shafts should do no harm, but equally might not be doing any good, either.

Ah. I was rather hoping for a "go right ahead, it'll all be ok." sort-of-answer. Or even a "don't under any circumstances."

Putting them 4" away I am less certain about.  If it is the only place to mount them, fair enough, but the basic prinpcipal of supression is that the supressor should be as near to the source as possible.  If you must have the 4" run, make it a twisted pair, so that any interfering signal has the chance of cancelling out.  If the cancelling does not hapen, you will have made a nice little aerial to transmit interference, and this will be assisted by the capacitor, rather than reduced.

Can I Faraday-cage the motors/wiring/ESCs up to the battery leads? Would that be beneficial?

Andy, almost wishing he'd not asked...

[Circlip]

Dreadnought, Have you seen the size of suppresion capacitors normally used? Even 100V disc ceramics are probably smaller that the interconnecting wire you're thinking of using? In the good old days we used to be able to get MUMETAL screens to help with magnetic isolation, not sure if this now applies with EC'S, even if we can still get it. Question to the learned, would flux rings on the motors help/

                      Ian

[RipSlider]

Not sure flux rings would do a huge amount. Might help if you were using Dreadnaughts concept of running the capacitors away from the motors.

Dreadnaught:

Things to think about
-Supression capacitors are TINY
-Putthing them in place *is* generating a faraday cage, one made out of the motor housing.
-if you use a common rail, then you'll need a exceptionally large and very well cooled ESC. You might end up spending £150+ to get away with not using a few capacitors on the motors.
-you can always take a brushless approach, which will greatly reduce interfence, and is still cheaper than the huge ESC you'll need in your original plan.


Steve

[Stavros]

Contact FLJ for the best advice

Stavros

[dreadnought72]

Thanks for all the input...

Ripslider - I shall fit indie capacitors, since I can do this before I mount the motors, and all the evidence is saying "do it".

With regards to Faraday cages, I can probably get the receiver over a metre away from all the power electrics, so I'm not sure that's going to be necessity (especially with suppressed motors).

Brushless? Would be lovely - but I've bought-&-paid-for what I have. I'm (not unhappily) stuck with them, and the two ESC's are ready to be tested and fitted. The common rail I mentioned was for the grounding of the motor cans and prop shafts - it shouldn't (should it?) be affected at all by the power cables - but I'll leave this following the advice I've been given.

Many thanks to all!

Andy

[Circlip]

Steve, to make sure we're batting on the same wicket, cos i know your into Puters an things, I'm not talking about Ferrite rings? Somewhere I have a motor with a sprung on metal sleeve which when taken off shows an horrendous magnetic field round it, but when slid back on is negligible? This is the type I was meaning, perhaps MY terminology of flux rings is wrong. Not trying to score points, just trying to clarify?
 
          Ian

[wombat]

Flux rings are used as part of the design of the motor - not for magnetic screening but to boost the magnetic field around the armature. If you remove them you will get a horrendous leakage of magnetic field as you have just broken the magnetic circuit. The motor designers try to prevent magnetic leakage as it reduces the field strength around the armature reducing the the performance of the motor and requiring them to fit more powerful (and expensive) magnets.

Mumetal could be used for magnetic screening, but it is expensive, difficult to work correctly (for example if memory serves correct it needs to be annealed in a hydrogen atmosphere) and if you don't work it correctly it can end up almost useless. And there isn't really any point.

For the suppression of the motors use ceramic capacitors with 100V rating mount them as close as possible to the motor - the further you place them from the motor the less effective they will be. 220nF across the terminals, 100nF from each terminal to the can.

When connecting up the motors, avoid using a common rail - better to use a "Star" topology where the current paths for the individual motors end up at a common point - preferably the terminals of the battery. What you don't want is any form of a loop.

Wom

[Circlip]

Yep, Googled flux rings and they came up with an INTERNAL ring with the mags fastened to it, not what i was thinking,Have been up in the archives trying to seek info in what I've seen in past model books to no avail. It will probably hit me later, but I'm sure of what i have in the motor sleeve.

[DickyD]

Hi Circlip
The Mtroniks  - Vision600 Marine motor has the sleeve that you are talkng about and they call it a slip ring. 

[Circlip]

Just been out to the -10deg storeroom and found the little s3d, it's an ex cordless drill motor, still got trigger fastened to it and it's sooo suprising the difference the sleeve makes, thanks for info.

[malcolmfrary]

When setting up, there are a few things that must be done like the provision of capacitors as near as the brushes as possible and using reasonable motors (some motors should just never be used anywhere near a radio) and keeping the low level signal wires such as the ESC control leads and aerial as separated from the power wiring as possible
But many steps beyond these could have effects either for good or bad, and should only be done when a problem is noted, and then one at a time, as nobody is inclined to undo a precaution to cure a problem.  Belt and braces on self supporting trousers is OK as long as the belt is around the waist and not the knees.

[Telstar]

Hi
I would like to reinforce Malcolm's comments . When dealing with interference you should ALWAYS fit suppression at near as possible to the source of the interference, in this case at the brushes of each motor. Malcolm's remark about wires between the source and suppressors acting as an Ariel is so true.
As far as the magnetic field of the motors being screened, this should not be a problem, the motors have permenant magnets thus effectively a 'DC' field, which will have little or no interaction with anything else.
I have found that 'earthing' all the metal work (motor cases, propshafts etc ) can in many cases cause as many problems as it cures, I always try to keep it simple  Suppress at source

cheers Tom

[david.harrison]

Guys, I have written an interesting article on RFI suppression that has now been published in two North American magazines.

Its' on my web site at :

http://modelsolutions.ca/articles/RFIArticleMarch2006.pdf

Any supression component should be wired as close to the source as possible, otherwise, as has already been pointed out, the connecting wires will themselves act as interference radiators!!

I also recommend the use of transient voltage supressors (also called transorbs) for large motors causing a lot of interference. I don't know if you can get these in the U.K., but we sell those also.

Cheers