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# On commutators in generators...

#1
07-22-07, 09:16 PM
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On commutators in generators...

I would just like to clarify the purpose of the commutator/brush assembly..

It serves two purposes, correct? Converting AC to DC. But also something more fundamental...

Conducting electricity from the armature coil to the output. Yes?

So if you're just using AC from the generator (like just plugging a TV into it) with no need for DC, the commutator brushes are still doing something. Yes?

#2
07-23-07, 08:40 AM
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I'll take a stab at this one...
The brushes are simply to provide a path from the spinning rotor, stator, to the stationary armature. Generators produce AC voltage which is converted to pulsating DC, usually by use of a diode to block one half of the sine wave.
Some AC generators, such as most alternators, require an external voltage to excite the field. The output leads come off of the armature, and is where the current is produced.

#3
07-23-07, 11:16 AM
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hmm

So, the commutator is always converting AC to DC, even if you're using the generator for AC? (Like plugging a TV into it?) And how would that work, since the TV requires AC?

I'm confused.

#4
07-23-07, 12:23 PM
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http://en.wikipedia.org/wiki/Commutator_(electric)

I am not certain what configuration any certain type of gen or motor may use. I relate a commutator more with a DC Motor such as a starter motor than a generator. I am not sure that it practical to get AC from a DC generator.

Since AC is a desirable output, and brushes are not needed in a generator to produce AC, it would make sense that most generators would be of this type. As I mentioned to convert AC to DC simply requires blocking one of the alternating cycles. AC constantly alternates poles from + to -, - to +. A diode is an electrical gate that only allows current to flow in one direction. It will allow say, + to - but not - to +, therefore the current flow is only in one direction however, it will "pulse" and no current will flow during the blocked cycle. This cycle happens several times per second. In US households 60Hz, is 60 times/second

#5
07-23-07, 01:38 PM
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I read that wiki article and many others before coming here. They are all quite vague, surprisingly.

The bottom line is that a normal honda generator, the kind with AC outlets on it, has a commutator with brushes. If the brushes don't work, the generator will not produce power of any kind. I know this because I had a Honda and although the motor ran fine, it wasn't producing electricity. It turned out the commutator brushes were messed up, not connecting with the rotor. They had to be filed even and reinstalled to make contact. The AC outlets were dead until I did this.

That's why all these online explanations confuse me. They contend that the commutator is for converting AC into DC. I was not using DC. Nevertheless, I needed the commutator and its damn brushes. Argh.

#6
07-23-07, 01:55 PM
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I've been out of school for way too long and don't have practical engineering experience to say for sure.
Something comes to mind about the permanent magnets exciting the field which would be DC current. If there is no field there is no current produced.

#7
07-24-07, 09:21 AM
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Most generators use the brushes to get AC voltage to the outlets from the rotor slip rings. The cycles, 50 or 60 are determined by the engine speed, which in most cases is about 3600 rpms but Onan had some low rpm engines that were used in Motor Homes, etc. that ran about 1800 rpms for noise suppression and still produced 60 cycles. To get DC output a diode bridge rectifier is used to get a positive dc voltage from the AC output. There are only a very few brushless generators that a consumer will find on the market and the rotor will be a permenant magnet and more expensive.

#8
07-24-07, 02:57 PM
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ah ha

Excellent. That's what I wanted to know for sure. Internet lore on this subject is extraordinarily pathetic and incomplete. And it's such a simple thing - getting power from the spinning part, to the outlet. Haha. Unbelievable.

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