Two or Three Capaciters? - RC Groups
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Sep 13, 2001, 04:50 AM
Professional Goofball
OCModels's Avatar

Two or Three Capaciters?

I have always soldered on three capacitors to all my 280, 300 and 400 motors and noticed that the latest 400 motor I bought only came with two capacitors in its bag. I have seen other motors on other planes using only two and wanted to know what if anything is the difference between two and three capacitors. Do I need to add a third capacitor, or is two sufficient enough to suppress the noise from the motor?

Jeffrey Park
Orange County Aeronauts
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Sep 13, 2001, 05:12 AM
Registered User
swami's Avatar
I would say 3 capacitors.
Usually Speed 400 comes with 2 capacitors installed.
(The 2 wires coming out of the casing at the poles)
I install 1 between the poles too.
It's a 470nF/250V capacitor.
Graupner order nr: 3588
(2 cap. / 2US $)

Sep 13, 2001, 06:45 AM
Registered User
From the FAQ (link at top of page)
"How many capacitors do I need on the motor and what values ?"

"It is a good idea to fit at least 2 capacitors to most brushed motors to cut down on the radio interference which the motor may generate. The same capacitors can be used for all brushed motors, the value does not change with the size or power of the motor. The capacitors should be soldered from each motor terminal to the motor case. For extra security against interference you can also fit a third capacitor between the 2 motor terminals."

You may well get away without the 3rd capacitor, but if you get interference then fit it before trying anything else. I usually fit all 3, but I don't pay a dollar each for them, more like 10c.

Sep 13, 2001, 09:42 AM
Strange Plane Central
frankenfoamy's Avatar
Some ESC have a Schotki diode accross the motor leads.
The can effectively replace the cross terminal capacitor if the motor leads are short.

I routinely fly a S400 motor with no suppression aside from the diode in the SIRIUS GFS ESC.

My motor leads are less than one inch.

(no internal suppression either)
Sep 13, 2001, 08:31 PM
Registered User
In some applications it might be okay to use two caps or even one. Personally I view it as very cheap insurance and always use three capacitors. When I add my own caps I use 0.1f caps. My logic is I'd rather not have to track an interference problems down to insufficient suppression capacitors even though it may be that I wouldn't have had a problem in the first place. Except for the miniscule weight they do no harm whatsoever. Some have claimed they cut RPM by a few hundred to 2,000, but I have seen no evidence of this whatsoever on the Speed 280 and 400 motors, MagMayhems and an Atomic Force that I've played with.
Sep 14, 2001, 10:13 AM
Registered User
cyhyam's Avatar
I have always flown with 2 caps and have never had a problem even with a variety of receivers. In fact if you look inside a servo you will find that the motors only have 2 caps. 1 from each brush to the case. So if the third one was necessary anyone flying with servos would be having problems. On the other side of things though, the third cap is inexpensive and doesn't add much weight.
Sep 14, 2001, 11:35 AM
Superhero, Third Class
astroboy's Avatar
Does that 3rd capacitor cause any power loss?

Sep 14, 2001, 12:50 PM
Registered User
Originally posted by cyhyam
So if the third one was necessary anyone flying with servos would be having problems.
Or at least anyone with servos turning at 12000 rpm and pulling 10A or more would be having problems I think the electrical conditions are slightly different.

Some people have said that adding the 3rd capacitor loses some revs or power but no-one can explain how that might happen. Many of us have conducted our own tests and found it not to be the case. Obviously it's very easy to test, take motor with 2 caps and check revs, add 3rd cap, check revs again, preferably on a fixed power supply but using the same battery within a few minutes is probably near enough.

Sep 15, 2001, 12:18 AM
Registered User
FWIW I have had bad troubles when I left the cap between the terminals off on a Spd400.

I think they are noisy little things and need all the help they can get.

As a retired electonics guy, a .01 uf capacitor is not going to affect rpm in any way.

All it does is short out the high frequency noise right at the terminals and keep it from going any further. A cap is just two metal plates, separated by a small distance. To Direct current it looks like nothing is there. To high frequency spikes ( like from sparks ) it looks like a short.
Which is why it affects the noise without affecting the motor power.

If you put a cap on big enough to affect the power, it still wouldn't be a problem 'cause it would be so heavy no plane could lift it!!


Sep 15, 2001, 11:19 PM
Registered User

Capacitors on Motors

When a motor changes poles there is an ever so brief time that the current of the motor is open circuited.This causes the sparking of the motor much like a coil on a spark plug.
The Capacitor "catches the energy" of that brief instant of time and keeps high frequency currents down to a minimum.

it would be better to use a Cap across the motor leads I wouldnt use an electrolytic unless it is rated for switching regulator use as the internal series impedance of a larger cap like that might not be doing anything at all but getting a nice ride.

.01uF caps are a good compromise but it really depends on the system as a whole.Some times it's necessary to use a decade cap arrangement to get the job done

.1uf .01uf .001uf all in parallel
Sep 16, 2001, 04:17 AM
Registered User
Please don't try to use electrolytic capacitors for interference suppression. For one thing there is never any need for capacitors with such high values as electrolytics and for another they explode far too easily.

Ceramic capacitors are the ones you want. They work far better than any other type at the frequencies we're interested in.

Sep 16, 2001, 06:48 PM
Tacoma, WA, USA
William A's Avatar
FYI, alot of S400 motors come with 2 caps allready installed inside the end bell.
Sep 17, 2001, 03:44 AM
Professional Goofball
OCModels's Avatar
Say I have one of these motor and I add three more capacitors? What would this do if anything? How can I tell if my S400 already has the capacitors? Why after a couple years I never knew this.... I'll have to figure out that last one myself.

Thanks for all the great responses,

Jeffrey Park
Orange County Aeronauts
Sep 17, 2001, 05:22 AM
Registered User
Adding extra capacitors does no harm. S400s with the internal caps have a thin solid wire poking through each terminal when new (and sometime you can see the caps through the cooling holes).

AFAIK it's only Graupner who fit the internal caps, so that's real Speed 400s not other motors with "400" in the name. But I could be wrong.

Sep 18, 2001, 11:14 AM
rebmeM roineS
MrBungle's Avatar
400 sized motors made by Johnson (Graupner's are Mabuchi) are fitted with caps internally, straight out of the Johnson factory.
The 400 sized Johnson motors also have larger cooling slots than the Graupner/Mabuchi's.