Supressing an electric motor

I have almost completed building an electric on-board starter for my Biglift glider tug with Moki 180. I am using a standard 05 type electric motor to power the device and would
like to know how I go about suppressing any electrical noise that will be generated when I activate the starter. One usually uses capacitors soldered across the electric motor's terminals, but how does one calculate the correct value of the capacitor ?? How actually does the capacitor suppress the noise and is there a better way ?? Another aspect that interests me is how capacitors assist electric motors with start up torque. If one of you guys can give me a short exposition on this or refer me to some source, I would be most grateful.
Regards Andy in South Africa
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Andy wrote:

Its not crirtical. Its there to short teh 72Mhz, but leabeve teh 8Khz or so un marked...pleanty of difference between the two.
Normally about 10-50nF is ideal, use small ceramics, soldered right across teh brushes and between each brush and teh motor case.

It short circuits teh high frequency radio noise from the brush sparks, before it can get into the motor wires and radiate as radio signals.
Other thigs to do are
(i) Schottky diode across the motor. This helps clamp flyback spikes.
(ii) Keep motor wires very very short, and twssit together.
(iii) Keep radio as far away from the motor and ESC as the wires allow.
(iv) sometimes and extar capacitor right across teh receiver + and - helps too.
(v) sometimes ferrite beads or chokes in teh servo wires help.

They don't in our applications. I suspect you are thinging of AC motors and the starting capacitor. That doesn't help start up torque - its there to make sure the motor starts in the right direction on a single phase AC motor, but thats a whole other story.

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go to the end of this page: http://www.webx.dk/rc/sp540/sp540.htm

Biglift
would
does
motors
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OK, Mr. Sherrer, I have to ask; had you already sometime in the past completed that investigation, or did you "whip it up" that day since your reply is dated 8 hours after the post? Either way, your work is impressive for clearly (and in a simple manner) demonstrating to me for the first time in 20 years why installing capacitors are a must on electric motors. All of these years my avionics buddies have never been able to explain to a former jet engine mechanic like me what the capacitors actually do as effectively as you just did.
Thank-you from a different Andy.....than the orginal post.

how
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Thanks guys for the info. Its much appreciated. Regards Andy in SA
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Andy Z wrote:

Yes, nice scope traces. Fiest time I had seen what a Schottky does tho its what I guessed amnd borne out by my practical experiments - i.e. knocks off a fair bit of stiff that teh caps don't.

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Hi Thomas, Natural and all,
The Natural Philosopher wrote:

Thomas, you don't state you're using a Schottky, the 1N5401 is a normal diode, rated at 3A. Have you noticed a difference between normal and Schottky diodes? Did you/can you make scope traces for Schottky's?
Met vriendelijke groet ;-) Ron van Sommeren near Nijmegen, the Netherlands
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Ron van Sommeren wrote:

Shottkys are simply faster, and have less forward drop. So they do a better job with RF.
A normal diode takes a bit of time before it starts to clamp, so you get some spikes. Shottkies clamp earlier. A normal power diode is, for example, fine at 50/60hz for rectifying mains, but shottkies are widely used in switched mode supplies running at several (tens?) of Khz.
Since our controllers run at those sorts of speeds, schottkies are better, but any good diode is likeley to be better than none.

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