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I just took apart our washing machine to replace the carbon brushes.

Was wondering why the machine stopped working when the brushes were worn down?

Also if the engine makes a wuring noise after you put it together, does that mean that the brushes have been put in the wrong way?

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I marked this down because I though the questioner could answer it with just a little research of his/her own. I then decided that the guidelines did not really endorse this, but found I could up rate it back to 0 only to 1. Given the two choices I went with -1. If an admin can remove my rating and comment then it is fine with me. –  russ_hensel Sep 15 '11 at 23:25
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I don't think this question deserves a downvote. The question may be basic, but it also seems in line with the user's knowledge of the subject. I gave it one up vote to counter the down vote. –  Olin Lathrop Sep 15 '11 at 23:32
    
I thought it was a good question. MOST (not all) of the questions asked here would benefit from a bit more basic research. The question about The whirring noise (often caused by the mew brushes bedding in) will not be answered by many basic answer sites. +1. –  Russell McMahon Sep 16 '11 at 6:51
    
Shiraz - you need to read up on DC motor theory to understand what the purpose of brushes are for. More importantly - the brushes allow the commutator to do an important job. Look for DC motor commutator to see what it does and how. –  Russell McMahon Sep 16 '11 at 6:54
    
Thanks for all the help, the machine is up and running again. –  Shiraz Bhaiji Sep 16 '11 at 13:56

2 Answers 2

up vote 8 down vote accepted

The brushes transfer the electricity from outside the motor to the spinning winding in the center of the motor. They undergo quite a lot of friction, and after a while wear out. Carbon is used as it is a reasonable conductor, and is soft enough to wear down instead of wearing down the "comutator" - the ring the brushes press against. Brushes are designed to be replaceable in large motors because of this.

When brushes are first put into a motor they won't be the exact same shape and size as the commutator (which may have had a small amount of wear) and thus won't make a perfect smooth contact. With time the brushes will wear to fit the commutator perfectly. The noise you hear is probably the brushes undergoing this initial shaping wear and will stop soon enough.

Also, the brushes are located inside a tube-like recess, and are pushed against the commutator using small springs. If the noise doesn't stop after a while a small amount of grease to help stop vibration in the brushes may be in order. make sure of the thermal properties of the grease though, as brushes are liable to get rather hot.

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The purpose of carbon brushes is to reduce wear on the commutator, compared with what metal brushes would do. In many cases, carbon brushes can be made long enough that they will not wear out while a device would otherwise be usable; when they do wear out, they can be readily replaced. If one were to use metal brushes instead, then by the time the motor had been used enough that carbon brushes would have worn out, the commutator would probably be just about shot.

The situation is somewhat analogous to break pads versus rotors. One could construct brakes with steel pads and rotors, but rotors would wear out very quickly.

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Thanks for the answer, but why are brushes needed at all? –  Shiraz Bhaiji Sep 15 '11 at 22:50
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How else is electricity supposed to get to the armature? –  supercat Sep 15 '11 at 22:53
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@supercat Well there are brushless motors which supposedly do not have brushes... –  AndrejaKo Sep 15 '11 at 22:56
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@AndrejaKo: The cheapest and easiest way to make a motor that can run off DC is to put coils on the armature and use a commutator and brushes to repeatedly switch the direction of current in the coils. The cheapest way to make a powerful motor that can run off wall current is to do likewise (there are cheap ways of making small motors which use only the AC line cycling to move the magnetic field, but they have very poor starting torque--not good in a washing machine). –  supercat Sep 16 '11 at 1:49

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