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Old Oct 28, 2011, 07:29 PM
Better then Sliced Bread!
NorCalMatCat's Avatar
United States, CA, Arcata
Joined Oct 2011
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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?
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Last edited by NorCalMatCat; Oct 28, 2011 at 07:32 PM. Reason: It is actually a capacitor not a resistor.
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Old Oct 28, 2011, 07:32 PM
Better then Sliced Bread!
NorCalMatCat's Avatar
United States, CA, Arcata
Joined Oct 2011
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Oops, just realized it is actually a capacitor, I edited above to reflect that, but I can't edit the post title I guess.
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Old Oct 28, 2011, 07:35 PM
CP heli ≠ 3D heli
Gedexas's Avatar
United States, NJ, Point Pleasant Beach
Joined Mar 2009
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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.
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Old Oct 28, 2011, 07:47 PM
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ivanc's Avatar
United States, TX, Round Rock
Joined Dec 2004
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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.
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Old Oct 28, 2011, 07:52 PM
CP heli ≠ 3D heli
Gedexas's Avatar
United States, NJ, Point Pleasant Beach
Joined Mar 2009
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Quote:
Originally Posted by ivanc View Post
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.
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Old Oct 29, 2011, 11:25 AM
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jasmine2501's Avatar
United States, AZ, Mesa
Joined Jul 2007
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Quote:
Originally Posted by Gedexas View Post
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.
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Old Oct 29, 2011, 12:08 PM
CP heli ≠ 3D heli
Gedexas's Avatar
United States, NJ, Point Pleasant Beach
Joined Mar 2009
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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.
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Old Oct 31, 2011, 09:26 PM
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United States, TX, Round Rock
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Off-topic, deleted. Sorry.
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