1185Views9Replies

Author Options:

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

9 Replies

user
lemonieBest Answer (author)2010-07-20


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.

L

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)lemonie2010-07-20

.  This may be picking nits, but caps don't conduct, they store.

Select as Best AnswerUndo Best Answer

user
lemonie (author)NachoMahma2010-07-21

Yes, but they also discharge. AC goes through them.

L

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)lemonie2010-07-21

. Electrons go in and electrons go out, but electrons to not go through. Ever see an ampacity rating for a capacitor?

Select as Best AnswerUndo Best Answer

user
lemonie (author)NachoMahma2010-07-22

Strictly speaking AC carries no (net) current in either direction, so you might argue that copper doesn't conduct AC either?

L

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)lemonie2010-07-22

. In a cap, the electrons go in and out at the same point and don't travel through - think inflating/deflating a balloon.

Select as Best AnswerUndo Best Answer

user
Udon (author)2010-10-28

Aren't caps also used over motors to improve power factor?
Or does that only apply to big AC motors?

Select as Best AnswerUndo Best Answer

user
Re-design (author)2010-07-20

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.

Select as Best AnswerUndo Best Answer

user
steveastrouk (author)2010-07-20

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.

Select as Best AnswerUndo Best Answer