What size capacitor to use?

[david cuthbertsonParticipant @davidcuthbertson76633]

I have a question for the electronic technicians out there. I have a pair of 6v to 12v motors ponied together. Powering them are a pair of 7.2v 1800ma batteries with a harness that connects them to an msonic esc. The motors currently have .1 microfarad capacitors soldered from the motor terminals to the motor cans. The problem is this: when the model gets about 200' away, the signal from the receiver seems to be getting interference from motor arcing. Close in the problem doesn't occur. Should I solder in larger capacitors or add them in other locations?

Thanks in advance.

Dave

[david cuthbertsonParticipant @davidcuthbertson76633]

[Malcolm FraryParticipant @malcolmfrary95515]

Provided that the capacitors are in working order, and not just bits of wire vanishing into a ceramic or plastic blob, they should be OK. I would, however, add another across the motor terminals. All capacitors should have their leads as short as possible, this to stop the capacitor lead acting as a transmitter aerial.

The actual value of the capacitors, within very broad limits, is fairly unimportant – what does matter is the type, which needs very good RF performance and high DC resistance. Fortunately, disc ceramics are among the cheapest components available.

For what is worth, I've never used capacitor to case – I hold to the view that if the single one across the motor terminals doesn't do the job, I need to pay attention to the motor by cleaning the comm and possibly doing something with the brushes. On that occasion, mending the motor with a new one worked best.

Did I mention keeping the capacitor leads short?

If the motor leads are more than a few cm long, twist the pair. The idea is that any interference that is being radiated by the wires will be equal and opposite, cancelling out. Unless one of the tag to case capacitors is duff. In which case the supression arrangement becomes an interference enhancer.

[Dave MilbournParticipant @davemilbourn48782]

Solder a 0.22uF ceramic across the motor terminals for a start. Next try using a separate 4-cell pack for the receiver and disconnecting the BEC of the ESC. I've long held the view that taking the Rx operating voltage from the ESC is the very dirtiest place to do it, with every risk of RF noise flashing across to the receiver circuit from the motors. If you're using one of the cheap 2.4GHz sets it's also just possible that your model is at or near the extent of its range – even at 200 feet.
DM

[david cuthbertsonParticipant @davidcuthbertson76633]

Malcolm and Dave,

Thanks for your tips. I'm going to try your suggestions and see what happens. (A facetious friend suggested not running the boat beyond 100' away.) I'll keep you posted.

[david cuthbertsonParticipant @davidcuthbertson76633]

Malcolm and Dave,

Thanks for your tips. I'm going to try your suggestions and see what happens. (A facetious friend suggested not running the boat beyond 100' away.) I'll keep you posted.

[TelstarParticipant @telstar]

I would endorse the previous posts also, mount the RX antenna (aerial) as far from the motor/wiring as practical.

Malcolm "For what is worth, I've never used capacitor to case – I hold to the view that if the single one across the motor terminals doesn't do the job, I need to pay attention to the motor"

This is very true, fitting Caps. to the motor case (Ground) does little or no good, in most cases since the motor case is not connected to ground. I don't fit them either.

Tom

[TelstarParticipant @telstar]

I would endorse the previous posts also, mount the RX antenna (aerial) as far from the motor/wiring as practical.

Malcolm "For what is worth, I've never used capacitor to case – I hold to the view that if the single one across the motor terminals doesn't do the job, I need to pay attention to the motor"

This is very true, fitting Caps. to the motor case (Ground) does little or no good, in most cases since the motor case is not connected to ground. I don't fit them either.

Tom

[Southern SailorParticipant @southernsailor76583]

Hi from the deep south. I have a similar question. I have a Speed 600 motor with a 5 nanofarad capacitor across the terminals and a 1 nanofarad from each terminal to case. I want to add capacitors to a Speed 450 motor. The question I have is whether the size of motor makes any difference. From what Malcoplm says, the answer probably is that it does not make any difference. Would that be a fair assumption? Thanks. Brian

[TelstarParticipant @telstar]

Hi Brian Yes, that is a fair assumption. The size and type of capacitor is selected to suppress certain interference, and is not dependant on the motor. Only stipulation, Fit as close to the motor as possable.

Cheers Tom

[Southern SailorParticipant @southernsailor76583]

Thanks Tom. Interesting that with the advent of brushless motors, the existance of such things as suppressors is becoming scarce in this neck of the woods, and I had to go to an electronics supplier to source them, the hobby shops no longer stocking such things. I must say that going brushless is quite tempting when looking for a suitable motor in new installations for a number of reasons, efficiency (increasing the length of run) and power (smaller and lighter motor) being two of them. A whole new ball game. Cheers. Brian

[Southern SailorParticipant @southernsailor76583]

Hi again from the deep south. Malcolm you mentioned twisting the motor leads. Is this the same as having them running parallel or is twisting the requirement that you are suggesting? I am using two core flex hence the question. Cheers. Brian

[Dave MilbournParticipant @davemilbourn48782]

Here is chapter and verse on motor suppression from an acknowledged expert on model boat electronics

**LINK**

……..and yes – it's the twisting that matters

Dave M

[Malcolm FraryParticipant @malcolmfrary95515]

If two wires in the same circuit are separate, they will both act as transmitting aerials, and whatever is doing the receiving will be further away from one than the other, so the unwanted signal will not cancel. A flat twin is better than separate wire, since the wires are kept close together, but there is still the constant difference equal to the thickness of the insulation. Not much, but the difference is there. Twisting ensures that on average the two wires are the same distance from the receiving wire. Having said that, with the length of wiring runs in normal size model boats, the difference between flat twin and twisted will be minimal – if there is a problem that fitting a capacitor does not fix, the basic problem is the motor being in need of TLC or early retirement. The twist does not need to be more that about one turn per inch – I have seen them twisted like a model airlane rubber motor – this is more than likely to damage the insulation thus creating more problems than it solves.

Dave's link should be required reading.

[Southern SailorParticipant @southernsailor76583]

Thanks Dave and Malcolm. Twisting it is then. Who was it – Chubby Checker (Sp??) I think? Cheers. Brian

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