what is the function of the small capacitor straddling motor leads in electronic devices?

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lemonie7 years ago

Motors are inductors, if you pump energy into them you store some of it as magnetic-field. If you stop, the field-collapses and the inductor tries to spit the energy-out in the opposite direction.
On a motor-commutator this can cause arcing (sparks) which generate broad-spectrum radio (it interferes with TV, Radio, Phones, etc). Like Steve' said Its a "suppressor" it helps stop radio interference, because it conducts the energy without arcing. Capacitors don't conduct DC, but if polarities are switching they will conduct.

.  This may be picking nits, but caps don't conduct, they store.
Yes, but they also discharge. AC goes through them.

. Electrons go in and electrons go out, but electrons to not go through. Ever see an ampacity rating for a capacitor?
Strictly speaking AC carries no (net) current in either direction, so you might argue that copper doesn't conduct AC either?

. In a cap, the electrons go in and out at the same point and don't travel through - think inflating/deflating a balloon.
Udon6 years ago
Aren't caps also used over motors to improve power factor?
Or does that only apply to big AC motors?
Re-design7 years ago
It may serve several functions.  Listed in order of expectation.

1. If it's an audio device it us usually there to filter out rf interference created by the sparking of the brushes of a dc motor.

2. Or it may be there to kill the inductive spike caused when the motor turns off.

3.  If it's an ac motor it is a starting capacitor that helps the motor start and then gets turn out of the circuit.
Its a "suppressor" it helps stop radio interference being generated by the motor. In fact, its usually better to put a resistor and capacitor in series across the motor, and what looks like a cap is actually an RC network.