Capacitors on a motor

The brushes in the electric motor can produce an electrical arc (like a spark plug). This arcing, in turn, can cause radio interference which is not good for R/C operation. The small capacitors act as shock absorbers and kill the RF noise. For this reason I would caution against removing the caps. Since they don't operate under very stressed conditions, I don't see any reason to replace them unless they are damaged as a result of a crash.

I hope this helps.

On Sun, 24 Dec 2006 13:29:59 -0500, "BCRandy" wrote in :

Noise suppression? In other words, do they help keep the motor from generating random radio signals?

Just a Wild Amateur Guess. ;o)

Beats me. I wouldn't expect them to go bad in ordinary use--but that's another WAG.

Marty

If you were about to convert a motor to RC use, how would you go about figuring what the values of capacitors should be, that you are going to add?

Seems I am motor rich, and would like to experiment with making one or two fly.

The noise reduction capacitor is normally determined simply by the function of the noise to be reduced and has no bearing on whether it is used for RC or just general noise reduction.

Here's a fun experiment for you. Run an unprotected and unfiltered motor next to an AM radio tuned to a dead spot. By dead, I mean no station. There will always be a faint hiss in a dead spot. Run your motor near the radio and listen to the noise it makes in terms of radio frequency noise, not the whine of the motor itself but what you hear in the speaker as a result of the motor being on.

Now add a ceramic capacitor of the value .01 microfarad rated at 50 volts or 100 volts across the motor terminals. Smaller is better on noise capacitors and be warned that you should never use a polarized capacitor (electrolytic or tantalum are examples of polarized capacitors) on a motor. Make a note of the difference in noise levels. Now try the same with a .1 microfarad of the same rating and see if you can tell the difference. You can do that experiment with hundreds of values of capacitors until the cows come home and the results should not differ significantly no matter what value used. Thus, the de facto standards of .01 or .1 are the most often selected for the convenience and price. They are used in billions of noise reducing operations and are found everywhere they sell electronic components in bags of 100+ for pennies each.

As mentioned earlier, the spark is a noise generator but that can't be stopped or eliminated by a capacitor no matter how big or how powerful. The reason is that the spark is a necessary byproduct of a motor's brushes and emits its own field. Marconi knew this and the very first radio transmissions across the Atlantic were just noise from gigantic sparks. This brush noise fact of life led to the development of the brushless motor which uses pure induction to move the rotor. It's the effects of the collapsing induction field that the noise capacitor reduces in both cases, not spark noise. As the motor rotates away from the point where the electromotive power is applied, the field in the coils collapses rapidly and can emit a field of energy around the coil as it does. That capacitor acts as a shunt for this field and effectively kills it allowing your radio some level of safety from the motor.

To fully reduce the motor noise as much as is possible, the easiest method is to have a good ground throughout your electronic motor subsystem and ensure that your motor case is grounded within that system. A screw from the case and a wire to the negative terminal of your battery is normally all that is required. Also, you may want to cover the gaps that you have in the motor where you can see the brushes contacting the motor armature. You can add a Faraday type shield which will cut the emissions a great deal. To find a good and nearly free one, look for large cables that have ground mesh inside them. Remove the mesh, expand it so that plenty of air can pass through and slip it over your motor so that the mesh covers the holes through which the brushes are visible. Be sure to ground this mesh and secure it with a clamp of a plastic wire tie and you'll have reduced your noise as much as is possible without spending a fortune.

Other tips would be to use ferrite cores and put a single loop of the positive motor wire through the loop. Here's a large file link that describes the technical nature of what a ferrite core can do for you.

Disclaimer: I realize that this is a very technical issue but I tried to simplify it for anyone to read. The engineers among us will realize that I did not go into the level of explanation that would confuse the hobbiest. I did that on purpose so if you want to discuss theory in greater depth, do it without me.

-- Ray

"Ray Haddad" wrote

Thanks! This post goes into the "keeper" file. I hope I don't crash my computer and lose it, somehow!

As far as the level of detail you went into, this was just about right. Any deeper, and you would have been doing it without ME, too!

Now to find a reasonably priced speed controller, or a kit for one. Anyone got any recommendations?

Interference suppression only, don't remove, and no, they are low stressed parts and will never break electrically, though they may snap in a crash.

between 10nF andf 100nF plate type capacitors are ideal. Don't use tubular..they have significant series inductance. Normally ceramic is best.

For brushed? I've got some that I won't use, the cut off is too low for Li-pos, for free(might need to solder 'em on) Gimme your snail mail. mk mjmwcs at htcomp dot net

as far as specs. one came out of a firebat, 10-15amps- I'll check mk

"MK" wrote

Thanks!

On the way, on back channels.

Very helpful! Thanks to all who responded.

Mad. You don;t have a low throttle cutoff on glo engines.

Since going LIPOS I have never USED the LVC.

It was never there to protect bateris..it was there to make sure you could still dead stick in, in an emergency.

In my experience, guys like MK aren't all that rare; they just don't leave a big footprint, really tread lightly and don't demand recognition. In any case, they are the people who really define what I like to think of as real human beings.

Here's to you, Mk. Merry Christmas.

Harlan

"H Davis" wrote

Yep. What he said!

In the last year I have received so much free stuff and given so much away to others..

There are a few people who take and don't give..but in the scale and old timer arena, its mostly a question of 'well I bought that X years ago,.and now I am into Y..so you might as well have it'..

| > For brushed? I've got some that I won't use, the cut off is too | > low for Li-pos, for free(might need to solder 'em on) ...

| Since going LIPOS I have never USED the LVC.

Ok, that's fine, but you're not everybody.

Personally, for LiPos, if the LVC is there, I'll use it. If not, I don't worry about it. Having a LiPo cell go from 4.2 volts (full) to

That may have been true before the advent of LiPos for R/C (well, it was *certainly* true, but the LVC did also help to protect the pack from reverse charging the weakest cell for poorly matched cells) but with LiPo packs today the LVC is most certainly there for both reasons

-- both to 1) keep the battery pack from going below 3.0 volts/cell AND 2) make sure you have enough power to do a deadstick landing.