What is the purpose of carbon brushes in an electric motor

  • 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?

    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.

    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.

    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.

    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.

    Thanks for all the help, the machine is up and running again.

  • Majenko

    Majenko Correct answer

    9 years ago

    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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