Oct 28, 2011, 08:29 PM Better then Sliced Bread! United States, CA, Arcata Joined Oct 2011 2,562 Posts Discussion Question about motors, why do they put resistors on them? I was looking at my motors, and I noticed that all of them have capacitors soldered between the positive and negative terminals, why is this and why would it not cause a short circuit? Last edited by NorCalMatCat; Oct 28, 2011 at 08:32 PM. Reason: It is actually a capacitor not a resistor.
 Oct 28, 2011, 08:32 PM Better then Sliced Bread! United States, CA, Arcata Joined Oct 2011 2,562 Posts Oops, just realized it is actually a capacitor, I edited above to reflect that, but I can't edit the post title I guess. Latest blog entry: Let'sFPV NEW 12v UBEC for FPV
 Oct 28, 2011, 08:35 PM CP heli ≠ 3D heli United States, NJ, Point Pleasant Beach Joined Mar 2009 1,674 Posts You're talking about a DC motor. There's something to do with feedback, a DC motor is also a generator and speed controllers don't like to be fed current through output wires.
 Oct 28, 2011, 08:47 PM S.A.D. member United States, TX, Round Rock Joined Dec 2004 9,022 Posts Brushed motors use capacitors to reduce the arcing in the commutator. Without these capacitors the arcing creates a lot of radio interference which screws up the receiver. The brushless motors we use are also DC motors but they do not have mechanical commutators. The commutation (change of polarity) is done electronically in the ESC.
Oct 28, 2011, 08:52 PM
CP heli ≠ 3D heli
United States, NJ, Point Pleasant Beach
Joined Mar 2009
1,674 Posts
Quote:
 Originally Posted by ivanc Brushed motors use capacitors to reduce the arcing in the commutator. Without these capacitors the arcing creates a lot of radio interference which screws up the receiver. The brushless motors we use are also DC motors but they do not have mechanical commutators. The commutation (change of polarity) is done electronically in the ESC.
Technically, they are three phase AC motors, we just run them on DC, hence the speed controller. It's easy to confuse the two, and not knowing it won't make a difference anyway.

Now, I'm thinking of a crash I had with my first RC plane ever. About a decade ago I had a motor-glider with a Graupner 400 in it. When building it, I never soldered those capacitors on, one day it dove nose first into dirt, that's probably why. I miss that thing.
Oct 29, 2011, 12:25 PM
Rocket Programmer
United States, CO, Golden
Joined Jul 2007
25,040 Posts
Quote:
 Originally Posted by Gedexas Technically, they are three phase AC motors, we just run them on DC, hence the speed controller. It's easy to confuse the two, and not knowing it won't make a difference anyway. Now, I'm thinking of a crash I had with my first RC plane ever. About a decade ago I had a motor-glider with a Graupner 400 in it. When building it, I never soldered those capacitors on, one day it dove nose first into dirt, that's probably why. I miss that thing.
I thought it was a brushless DC motor that we use? It has electronic commutation as mentioned above, but the current only goes one way. The commutation doesn't change the direction. Well maybe... in a star-winding?

Quote:
 Brushless DC motors (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are electric motors powered by direct-current (DC) electricity and having electronic commutation systems, rather than mechanical commutators and brushes
http://en.wikipedia.org/wiki/Brushle...electric_motor

Regardless from that - the capacitors are to eliminate radio interference.
 Oct 29, 2011, 01:08 PM CP heli ≠ 3D heli United States, NJ, Point Pleasant Beach Joined Mar 2009 1,674 Posts A little off topic, regarding the brushless AC vs DC discussion. It's the fact that we use DC power source that makes these motors DC. If you look at the motor part only and ignore the speed controller, our brushless motors look identical to a common AC fan motor, with the only exeption of some (not all) AC motors put magnets on the inside of windings, as opposed to the outside. The other difference is: we don't feed these outrunners with a true three phase current, our speed controllers approximate the three phases and instead of a sine wave, our speed controllers feed a "on-off" current to only two of the wires at a time. That's also why our outrunners make so much noise. If you look at a power generator turbine closely, it's basically a three phase motor, except it's driven by steam. http://boomeria.org/physicslectures/...phasemotor.jpg Edit: To put it short, it's all in the definition.
 Oct 31, 2011, 10:26 PM S.A.D. member United States, TX, Round Rock Joined Dec 2004 9,022 Posts Off-topic, deleted. Sorry.