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.
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.
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.
@supercat Well there are brushless motors which supposedly do not have brushes...
@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).
@AndrejaKo Those are induction motor. More specifically they are squirrel cage induction motors. They run on AC supply.
@ab29007: Induction motors require a rotating magnetic field to get started. If three-phase power is available, turning on all three phases will produce such a field easily even on startup. If three-phase power isn't available, it will be necessary to delay the current in one of the phases, and the cost of doing that will depend upon the amount of current--and thus torque--required. Getting high starting torque is much more expensive than getting high running torque.
@supercat For the same capacity, the DC motors areuch expensive than AC motors. Because of commutator, brushes and winding design. As for getting high starting torque, variable frequency drive may be expensive but we need to use starters in DC motors too. So the cost may end up being equal. But the cost of maintained and repair still tops of in favor of induction motor. Also my comment was in regard to your comment where you said there are brush less motors. So I just felt the need to clarify that there are no brushless DC motors.
@ab29007: Electronic motor controls have gotten cheaper over the last decade, but there are many price/performance/endurance points where brush motors are and will continue to be dominant. The only time DC brush motors require any special starting mechanism circuitry will be if a full-power cold start would overload either the motor or the supply. Otherwise, they simply work.
@supercat I think it all comes down to the power rating of the machine. AC machines have smooth and efficient operation but efficiency comes into play only at higher power ratings. For low power and frequent sop and starts, DC is preferred. Since initial setup cost will take over.