rshazi

Hiya all,
I get capacitors with the electric motors in my ARF kits. I know they store charge, but how does that work in relation to electric motors on airplanes? Do I have to solder them onto the motors? Thanks

Ed

The capacitors are meant to suppress the spark that is developed across your motor brushes. Solder one between the terminals of each motor. Without the capacitor, sparking will cause radio interference.

rshazi

I see. Thanks Jim! Say, it won't hurt if I don't, right?

Steve Lewin

One capacitor should be soldered from each motor terminal to the motor case. If there are 3 the third one goes across the 2 motor terminals.

They suppress radio interference. Without them you may suffer all sorts of radio glitches etc. If you are already flying without them and having no trouble then you may not need them but they are very cheap insurance against radio problems.

Steve

DeadMeat

Using capacitors is good for you motor.

I have to go waaaaay back to my electronics theory, but here is what capacitors actually do.

Capacitors are used in DC circuits as line filters. In a perfect DC circuit your voltage stays constant. However a lot of DC circuits are not clean like that. The voltage fluctuates up and down. The capacitor is a miniature storage cell. When voltage starts rising, it resists voltage change until it reaches capacity. When voltage drops, it resists the voltage drop until it is discharged.

Now how does that apply to a DC motor? When you go from off to full throttle the voltage jumps from 0 to full power. This could not be good for the guts of your motor. The capacitor resists the sudden surge of juice and brings the power up more gradually. Same goes for when you throttle back. Basically the capacitor is there to protect your motor from sudden power surges.

Tom

Steve Lewin

Sorry Tom but while your theory of how capacitors work is sort of correct that's not at all what they do on our electric motors. For a start our motors don't run off true DC they use a switched (PWM) current, usually switched at around 1 to 3KHz.

The capacitors we use are very low values (around 0.01uF) and have no practical storage capability. If you think back again you'll perhaps remember that the impedance of a capacitor is inversely proportional to frequency. At high frequencies they look like a short circuit, at DC they look like an open circuit. The purpose of the suppression caps is to act as low impedance shunts at radio frequencies and effectively short the RF interference from the sparking brushes to ground.

Steve

DeadMeat

You are right Steve.

The last electric model I built used a pot for a speed controller. I forgot about them newfangled electronic contollers. :stupid:

And yes they are pulse wave controllers. The unfortunate bi-product of using a oscillating or pulsing circuit in this case is RFI. And that isn't limited to the arcing brushes. The motor wiring emits RF as well. This is why you want your controller and motor wiring as far away from you RX as possible. Also keep the motor wires as short as possible. OH. And don't cross any of the motor wires with your antenna or a servo wire. You will pick up stray currents.

Thanks Steve. I needed to dust of some of my old electronics books.

Tom